The ecology of marine fishes is a broad topic that may be addressed only in general terms here. The important factors for consideration are few, however. Quite simply, fishes interact with their physical environment; with other organisms, such as plants, invertebrates, reptiles, and mammals; and with other fishes. How and why these interactions occur is the focus of this discussion. In particular, we focus on aspects of the following: community ecology, population ecology, life history and reproductive ecology, habitat use, special habitats and adaptations, and feeding ecology.
Communities, assemblages, guilds, and niches
A community consists of all the organisms present and interacting within a given area. For example, a coral reef community consists of corals, benthic algae, phytoplankton, zooplankton (both demersal and pelagic), various micro- and macro-invertebrates, fishes, reptiles, marine birds, and marine mammals. There are numerous links, in terms of both habitat and trophic relationships, between members of each group.
Fishes and other organisms occur in assemblages within a community. A fish assemblage is composed of all the species populations within the community. Assemblages have order and structure, and both are maintained by interactions between species within the assemblage and with assemblages of other kinds of organisms within the community.
Within an assemblage are groups or species of fishes with similar patterns of resource use. These are called guilds. Although many guilds may consist of members that are taxonomically related to one another, membership is determined by ecological factors. For example, a guild of obligate Pocillopora eydouxi coral-dwelling fish species could include one or more hawkfishes (Neocirrhites armatus, Paracirrhites arcatus, and Paracirrhites forsteri—Cirrhitidae), a coral croucher (Caracanthus maculatus—Caracanthidae), a scorpionfish (Sebastapistes cyanostigma—Scorpaenidae), a goby (Paragobiodon species—Gobiidae), and a damselfish (Dascyllus reticulatus— Pomacentridae). In this example, only the hawkfishes are closely related to one another, although this relationship is not necessary for guild membership. Another example of a guild would be all those species that browse benthic algae, pluck zooplankton from the water column, or hide beneath the sand. Furthermore, juveniles and adults of the same species might not be members of the same guild. For example, juveniles of numerous species may shelter in mangrove roots, but as adults some of those species are found living in association with corals.
The place a fish has within the community or assemblage is its niche. A niche simply defines habitat, microhabitat, and physical parameters within the two, as well as diet and feeding strategies, symbiotic relationships (if any), and other functional roles (i.e., its role in predator-prey interactions). Thus, within the guild of obligate coral-dwelling fishes described earlier, we would find coral crouchers living deep within the branches of the coral and feeding upon coral-dwelling microcrustaceans or passing zooplankton, while in another niche, the larger freckled hawkfish, Paracirrhites forsteri, would be perched on the outer branches of the same coral and ambushing smaller fishes or crustaceans that passed close by.
Considerable debate has taken place over the way in which fish assemblages, particularly those of reef fishes, are ordered and structured. This debate is centered on questions of how highly diverse assemblages are maintained, how so many species can coexist, what limits diversity and abundance, whether composition and structure is temporally and spatially predictable, and whether the processes involved are uniform across geographical scales. Essentially, assemblage structure was thought to be the outcome of deterministic or stochastic processes. Deterministic processes emphasize fine-scale ecological niches that encompass interactions, such as competition, cooperation, predator-prey, and so on between species. Larvae settling onto a given site of the reef would recruit successfully and become established only in the absence of con-specific adults in favored niches or in vacant niches otherwise. Stochastic processes are random, in that successful recruitment is dependent on chance. Actually, both kinds of processes operate on assemblage structure, and their relative importance is felt on different temporal and spatial scales.
Population structure is dependent upon rates of reproduction and survivorship among individuals within the population. Population structure is also influenced by rates of migration into and out of the population. Recruitment of larvae, both locally produced and from distant sources, is another major factor. One hypothesis about the effects of recruitment on population size and structure is the recruitment-limitation hypothesis. In this case, the number of adults per unit area is limited by the number of larvae available for recruitment. (This hypothesis also has been proposed to explain assemblage structure.) The rates of these various factors vary annually, and the overall structure of a given population may be denoted by year classes (ages) or cohorts. Some year classes are stronger or larger than others. This difference has implications for the management of populations under exploitation by fisheries, because if the largest or most successful year class of a given population reaches maximum age and subsequent year classes are not so successful, overfishing occurs, and the population could be in danger of collapsing.
With some species, age structure also may reflect size structure within a population. Thus, a population of a species with indeterminate growth may consist of different-sized fishes at different levels of abundance for each size class. In the case of many reef fishes, however, size becomes a poor indicator of age, because growth tapers off after a few years or less, depending on the species and certain environmental factors. Growth rates of individuals within populations determine how much biomass is produced for a given population during a given unit of time.
Natural mortality affects population structure too. Starvation; disease; predation on eggs, larvae, juveniles, and adults; cannibalism; and old age all contribute to natural mortality. Mortality caused by fishing is additive, and total mortality for any given population under exploitation is a matter of concern for fisheries and conservation managers.
The genetic structure of marine fish populations is determined by gene flow within and between populations of the same species. Interpopulation gene flow is dependent upon the level of connectivity between two populations. Populations that are relatively close together geographically and served by the same current regime are more likely to have higher levels of gene flow compared with distant populations. In contrast, isolated populations are more likely to diverge over time. Geographic or ecological variation in characters may result. If this variation is great and reproductive isolation occurs, speciation (the creation of new species) may ensue.
Competition exists if two or more fishes require the same resource and the abundance of that resource is limiting within a given area. Competition between fishes of the same species is termed "intraspecific," whereas competition between different species of fishes or between a fish and another organism, such as a sea urchin, using the same algal resource, is deemed "interspecific." Intraspecific competition is an important factor contributing to the success of one individual over another within a population of the same species. This success may be measured ultimately by the proportional reproductive contribution to the population that one individual makes. Interspecific competition is important for determining the structure, and hence diversity, of an assemblage of fishes.
The relative success of a population of one species over another at securing a vital resource, such as microhabitat or food, determines which species persists and which does not. If so, then how can many fish assemblages be so diverse? Usually, competition is reduced or avoided altogether by resource partitioning among species that live together in as state known as "sympatry." In our coral-dwelling fish example, we see that coral crouchers and hawkfishes avoid competition for the same coral and food resources by living in different parts of the coral and eating different kinds of food. An example of a situation wherein competition probably functions is the case of two fish species that have identical food or habitat requirements but live apart in spatially or geographically distinct areas in a state known as "allopatry." If two or more allopatric species with the same ecological requirements come together, there probably will be two outcomes. The first is that only one species will "win out" and continue to use the contested resource while the other(s) will fail to become established. The second is that all of the species in question will become established, because there will be a shift in resource utilization, sometimes quite dramatic and including rapid morphological changes relevant to the resources available, with only one species using the original resource while the others adapt to using different resources.
The interaction between predators and prey in a given assemblage affects prey in many ways but also may have implications for the population of predators. With respect to prey, predators can cause mortality or injury, with obvious negative consequences. Or the steady influence exerted on prey species by predators results in changes in the way prey utilize habitat or food resources so as to avoid predation. These changes have a profound effect on how the prey population reproduces and sustains itself.
The size of a population of prey species affects the ability of the predator to influence that population. Thus, the number of
predators in a population may increase in direct proportion to an increase in the size of a prey population. This is an example of a density-dependent response that is compensatory; that is, the predator compensates for the increased number of prey by increasing its own numbers. If the prey population becomes too large and the predator population does not keep pace, the ability to influence the size of the prey population is reduced. In other words, there is safety in numbers. This form of density-dependent response by the prey population is depensatory, which is defined as a decrease in the relative risk of predation or impact upon the population by predation because of an increase in prey numbers.
What happens to the predator population when the prey population disappears? This is an intriguing question, especially with respect to coral reef systems that have been affected negatively by natural or anthropogenic (caused by man) habitat destruction. For example, groupers (Serranidae: Epinephelinae) like to feed on their cousins, the fairy basslets (Serranidae: Anthiinae). Fairy basslets tend to recruit, as post-larvae, to corals. If a coral bleaching episode kills off the corals on a given reef, the fairy basslets have nowhere to recruit. In time, the population of fairy basslets will decline, and there will be few or none left for the groupers to eat. Will the grouper population decline, or will its members simply switch to another kind of prey and get by? Conversely, what happens to the fairy basslet population if the grouper population is reduced greatly by overfishing? Will the fairy basslet population increase significantly in size, or will some other factor come into play? These are important questions that require further attention in the study of reef fish assemblage interactions.
Life history and reproductive ecology
Marine fishes, like their freshwater counterparts, possess a number of life history and reproductive traits and strategies that enable them to live and reproduce successfully under a variety of environmental conditions. These traits and strategies may vary geographically, historically, and hence phylogenetically within or between species. Many of the most important traits and strategies are discussed here.
Body size varies among marine fishes, with both the largest, the whale shark (Rhincodon typus, Rhincodontidae), and the smallest, a dimunitive goby (Gobiidae), existing in the same environment. Different body sizes confer distinct advantages. Large body size is favorable for species that swim in the water column. Large size conveys greater protection against predation by all except the largest predators and also allows for greater storage of energy and longer and faster swimming abilities. The latter comes at a cost to reproductive effort, however, because energy that would be available for reproductive activities is required instead for somatic (body) growth. Small body size, on the other hand, allows for greater access to benthic shelter and the potential utilization of a wider spectrum of food items, but at a greater risk of predation. Naturally, there are exceptions to these examples. Small-sized baitfishes, such as anchovies (Engraulidae) or reef herrings (Clupeidae), swim openly in the water column, whereas large-sized morays (Muraenidae) or wolf eels (Anarhichadidae) are associated closely with benthic shelter. Within species, larger body size conveys distinct advantages in terms of territory size, the acquisition of mates, and reproductive success.
Age and size at maturation in marine fishes also vary. Generally speaking, a marine fish that matures at an early age and at a smaller body size has a greater opportunity to reproduce before dying but usually has relatively low fecundity and smaller eggs. However, for pelagic spawning fishes, the acts of courtship and spawning expose the participants to predation risk from lurking predators. Also, energy diverted toward reproductive effort typically means that growth is slower in these fishes. In contrast, older and larger fishes invest in growth, delay reproduction, and have a greater risk of death before reproduction first occurs. On the upside, the larger female fish are more fecund and produce either more eggs or larger eggs; the larger male fish produce more sperm and, within species, may have more opportunities to mate compared with smaller fishes. The relationship between age and size in marine fishes has long been thought to be linear for most species. Recent studies of numerous reef fishes, however, have shown that growth in many species may be rapid at first but tapers off after a few years, yet these fishes may live for several more years. Thus, body size cannot be used to predict age in these fishes.
Sex ratios may vary both within and between species of marine fishes. One reason may be the population size within a given area, and another may be the age of those individuals that make up the population. The sex ratio is important in relation to effective mating opportunities and the development of a mating system. Marine fishes may be gonochoristic, in that the sex is determined genetically and they begin life either as a male or as a female. A variation on this theme is known as "environmental sex determination." In this case, the sex is determined by some environmental factor, such as seasonal water temperatures. Thus, females of a given species are produced during one time of year at a given temperature, whereas males are produced later in the season at a different temperature.
Marine fishes also may be hermaphroditic, in that they are capable of changing from one sex to another and, in some species, back again. Alternatively, they may function as both a female and a male either sequentially or simultaneously. Protogynous sex change occurs when a female changes her sex to become male. Males generally are larger than females in this system. Protandrous sex change takes place when a male changes his sex and becomes a female. In this case, females are larger than males. The former strategy is more common than the latter. Control of sex change is largely social in relation to mating system dynamics, but age also may be a factor in many species. A third variation has been described for some highly site-attached species, such as coral-dwelling gobies of the genus Paragobiodon (Gobiidae). Here, a larger female changes sex, becomes a male, and realizes greater fitness by spawning with smaller resident females. If, however, a larger male joins this new male, the new male will be forced to compete with the larger male for access to the resident females and probably will lose. Thus, it will forfeit mating opportunities as well. The sex-changed male will change sex again, reverting back to being a female, but will still realize some measure of fitness by staying on and spawning with the larger male.
Sequential hermaphroditism occurs in some species, such as the hamlets, Hypolectus (Serranidae), in that a mating pair switches sexual roles during long bouts of courtship and spawning. First, one fish spawns eggs that are fertilized by the second fish. Afterward, the second fish spawns eggs that are fertilized by the first fish. Simultaneous hermaphroditism occurs when an individual is capable of producing both eggs and sperm at the same time. This less common strategy is practiced largely by fishes that dwell in deep waters (such as many members of the order Aulopiformes), where the probability of encountering a mate is relatively low. Some species, such as numerous wrasses (Labridae) and parrotfishes (Scaridae), have a dual strategy, in that males and females are determined genetically (primary phase) but females can undergo protogynous sex change and become males (terminal phase).
The number of mating partners a fish has during the course of a breeding season is known as the "mating system." Generally, marine fishes are monogamous, polygamous, or promiscuous. Monogamy consists of a single pair that may join together only for spawning but also may share a common territory or home range and remain together for one or more seasons. Polygamy occurs in two principal forms, polygyny and polyandry. Polygynous groups vary in form. For instance, a single male mates with two or more females, and mating may occur in a socially controlled group. Alternatively, males may form leks with other males in a specific area for the purpose of displaying to and attracting several females for spawning. Males also may defend nests or spawning sites within fixed territories and mate with two or more females in succession. In polyandry, females mate with more than one male over the course of a season. For some species, such as anenomefishes (Pomacentridae), a single female in an anenome exerts control over and spawns with two or more resident males and, through social interaction, delays the growth and maturation of additional males that also may reside there. Promiscuity occurs when males and females spawn together, with little or no mate choice.
There is some plasticity in the mating system in relation to local population size. For example, if a population of the humphead wrasse (Cheilinus undulatus, Labridae) is relatively large at a given locality, it will form a polygynous spawning aggregation. If the population level is quite low, however, it may reproduce in a single-male polygynous mating group. Similarly, the obligate coral-dwelling longnose hawkfish, Oxycirrhites typus (Cirrhitidae), is polygynous if the coral in which it dwells is large enough or near enough to neighboring corals to support a male plus two or more females. If the coral is capable of supporting only the male and one female, the pair is facultatively monogamous.
Sexual dimorphism in size or color pattern usually is found in polygamous and, to a lesser extent, some promiscuous species but seldom in monogamous species. Larger size or more distinct color patterns confer advantages for attracting mates and maintaining relationships with them. Regardless of the mating system used by a given species, if mates of one species are difficult to find, an individual may choose to spawn with a closely related species that is more common, and hybrids might result. Typically, these hybrids do not produce viable offspring should they have an opportunity to mate.
Marine fishes spawn eggs with external fertilization, lay eggs after internal fertilization, or have internal fertilization with the release of fully developed young. There are at least four different modes of spawning and external fertilization. Demersal spawning includes the deposition and fertilization of eggs in nests or directly on the substrate; in pouches, such as those of male pipefishes and seahorses (Syngnathidae); or by oral brooding, such as in cardinalfishes (Apogonidae), in which the eggs are deposited and cared for within the mouth cavity of a parent. Pelagic spawning is the release of eggs and their subsequent fertilization at the peak of an ascent into the water column by a pair or spawning group. Numerous species of marine fishes spawn in this manner. Fishes that spawn pelagically in the water column but have eggs that sink to the bottom are known as egg-scatters. On the other hand, fishes that spawn pelagic eggs close to the bottom are known as benthic egg broadcasts. In contrast, some species, such as skates (Rajidae), are oviparous and have internal fertilization but deposit egg cases that develop and hatch externally. Live bearers have internal fertilization of eggs, and then the eggs develop inside the mother before the young are released. There are two forms of this trait. When eggs develop with nutrients contained in the yolk sac but without nourishment from the mother, it is called "ovoviviparity." Stingrays (Dasyatidae), for example, have this reproductive trait. "Viviparity" is when the young receive nourishment from the mother during their development. An example would be the tiger shark (Galeocerdo cuvier, Carcharhinidae).
The timing of spawning or breeding also varies; it may occur at dawn, dusk, during daylight, or at night. Factors include light level, tidal state, mating system, and reproductive mode. Spawning or breeding frequency and seasonal duration vary within species, because of local environmental conditions, and between species, because of phylogenetic differences. Frequency of spawning or breeding is controlled by physiological and phylogenetic constraints that limit the production of eggs or the ability to brood young. Other factors include lunar periodicity and access to mates. Seasonality is highly pronounced and is dependent upon annual variation in water temperature, the number of hours of daylight, and a host of other factors. On tropical reefs, where temperatures are generally warm and stable throughout the year, some hawkfishes (Cirrhitidae) court and spawn daily all year long. The same species at higher latitudes are limited to spawning only during warmer months. Groupers (Serranidae) that form spawning aggregations in the tropics or warm temperate regions, on the other hand, may spawn only once or twice a year in relation to lunar phase. Fishes living in cold temperate regions may be limited to spawning only when there is a shift in season, such as from winter to spring or summer to autumn, whereas others spawn strictly during the warmer summer months. Whether spawning or breeding frequency and seasonality favor adults or their progeny is a subject of considerable interest.
Fecundity or clutch size varies with species, body size, egg size, age, spawning frequency within a season, and latitude in relation to both the length of the season and the water temperature. Generally, there is a positive relationship between body size and egg number. Larger fishes produce more eggs compared with smaller fishes. The relationship is not always so neat, however. Because fecundity can be partitioned into three kinds—batch, seasonal, or lifetime—it is possible for smaller fishes to have relatively greater fecundity than larger fishes. For example, a smaller fish that spawns one or more batches of eggs per night for the course of a spawning season that could last all year in the tropics might have greater fecundity seasonally or over the course of its lifetime than a larger fish that spawns just once during a relatively short season and then dies.
Egg and larval sizes vary between species and also may vary within species, depending on body size, latitude, or other geographical or environmental factors. Generally speaking, large eggs mean that a greater amount of resources has been devoted to their production, and the result is large larvae, better equipped for survival. The advantage of smaller eggs is that more can be produced per unit time compared with larger eggs, and thus there are more opportunities to produce viable young. The drawback to small egg size is that, with fewer resources made available for development, the larvae also will be small and less equipped for survival.
Parental care in fishes includes the investment a parent makes before spawning or breeding, or prezygotic parental care, and the investment made after spawning or breeding, or postzygotic care. An example of the former strategy is nest building, while the latter includes nest guarding, oral incubation, pouch brooding, or internal brooding. Postzygotic parental care of eggs and larvae is practiced extensively by freshwater fishes but far less so by marine fishes. Among marine fishes, postzygotic parental care of eggs means that small, cheaply produced eggs could be afforded a benefit that increases their chance of survival. Ironically, most species that practice parental care in the marine environment tend to have eggs that are much larger than those produced by species that lack parental care. Pelagic spawning, egg scattering, and benthic broadcast spawning are practiced by a majority of marine species, and they do not engage in parental care.
The duration of time between egg fertilization and hatching varies with egg size. Small eggs spawned pelagically usually hatch rapidly compared with larger eggs that require tending. Similarly, eggs fertilized internally require a longer gestation time before the young emerge from the mother. As most marine species have pelagic larvae, the amount of time spent drifting passively or swimming weakly in the water column varies with species and also with environmental circumstances. Larval life duration depends on the growth rate of the larvae, which in turn is dependent upon its energy stores, rate of metabolism, and ability to feed before settling. Rapid growth to a larger size dictates that energy requirements and metabolism will be high, and thus the need to feed more often will be greater, or else starvation will occur, and death will result. Exposure to predation also is greater, and most larvae fall victim to predators or the effects of starvation before settlement takes place. Small larvae with short larval life durations and poor dispersal capabilities are more likely to settle and recruit locally. Short larval life duration means less risk from predation, because rapid settlement into favorable habitats can be accomplished. If settlement does not occur and the larvae are carried out to sea, however, they, too, will die from predation or starvation. Size and the growth rate potential do not always influence dispersal capabilities of larvae, however. Larvae of many tropical reef species, for instance, may be adapted to long larval life and hence possess long-distance dispersal capabilities. These same larvae, however, may be caught or trapped by local oceanographic conditions and disperse only short distances before settling into suitable habitats.
Lifetime reproductive effort typically is defined in two ways. Fishes are either semelparous or iteroparous. Semelparous species usually reproduce only once in a lifetime, with a spawning event that may be quite large, depending upon the species. Examples include the freshwater eels (Anguillidae) and various salmons (Salmonidae), although some individuals of the latter group may survive and return to spawn again. Iteroparous species spawn or breed frequently during their lives, either in the course of a single season or over many seasons, depending upon the species. Examples include groupers (Serranidae), hawkfishes (Cirrhitidae), or parrot-fishes (Scaridae).
Marine fishes live virtually everywhere in the world's oceans. In general, marine fishes may be found at the upper limit of the intertidal zone down to the bathypelagic realm several thousand meters deep, from freshwater in the upper portions of an estuary system to the hypersaline waters of now landlocked bodies of water or shallow flats in arid regions, and from balmy tropical reefs to intensely cold polar seas. Their distribution in these diverse habitats is made possible by behavioral, anatomical, and physiological adaptations that meet the specific or unique demands of those habitats. The classification of habitats is complex and is the subject of considerable interest if not outright debate. This review adopts a simple approach and considers habitat in relation to patterns of zonation based on depth and substrate.
Marine fishes that live on the bottom or in association with some form of structure on the bottom, such as a rock or submerged mangrove root, are considered to be benthic or demersal species. Those that swim up into the water column, whether in a shallow estuary or bay, in the open ocean, or in the deep open ocean, are considered to be pelagic species. Some species are both, in that they live in close association with the bottom but frequently are found in the water column, either foraging or moving over a wider area away from shelter. These species are considered to be benthopelagic species. With respect to depth, these kinds of fishes can be found across a wide range.
Among benthic fishes, certain species are specialized to live—as juveniles, adults, or both—in shallow tide pools or splash zones at depths of less than 1.5 ft, or 0.5 m. Examples of tide-pool fishes include certain morays (Muraenidae), marine sculpins (Cottidae), blennies (Blenniidae), and gobies (Gobiidae) that shelter within the confines of the pool and may endure the effects of a falling tide. The clingfishes (Gobiesocidae) are well adapted to life in both tide pools and the splash zone above them. Their ventral fins, modified into effective sucking discs, allow them to cling to stone walls above tide pools that are immersed only by the splash of waves. Some clingfishes, and indeed other tide-pool species, are especially well adapted to avoid desiccation and thermal extremes. Benthopelagic fishes in tide pools, such as damselfishes (Pomacentridae) or kuhlias (Kuhliidae), swim about in the depths of the tide pool. Generally, these kinds of fishes are less tolerant of the effects of low water levels or temperature shifts and must migrate out of the tide pool with the falling tide, only to return again as the tide floods.
Tidal effects also are pronounced in estuaries, mangroves, sea-grass flats, algal and shallow kelp beds, coral reef flats, and other kinds of flats dominated by mud, sand, rubble, cobble, larger rocks, or living organisms, such as oysters. In estuaries, benthopelagic and pelagic fishes move upstream with a flooding tide, often well into freshwater, and then move back downstream with a falling tide. These fishes are termed "euryhaline" species, in that they are tolerant of a wide range of salinities. Fishes living among mangrove roots, inshore sea-grass flats, or algal beds, whether in or adjacent to an estuary, are adapted similarly. Benthopelagic and shallow pelagic species, such as halfbeaks (Hemirhamphidae) or mullets (Mugilidae), simply move off their various flats with the falling tide. Benthic species often seek shelter in holes, under rocks in depressions, and in deeper pools, or they may migrate to adjacent channels.
The effects of tide upon the fishes' habitats are less pronounced in the subtidal zone. Here, the various habitats are submerged constantly. Tidal effects typically are limited to patterns of current flow and what may be carried to and from this zone with the current. Thus, food, in the form of prey moving off a shallow flat in the intertidal zone with the falling tide, may be brought into the subtidal zone. Similarly, sediments from intertidal habitats move with the tide, creating turbid conditions in deeper water. Subtidal habitats are quite diverse as well and include coral and rocky reefs, sea grasses, algal and kelp beds, and deeper flats of sand, mud, rubble, rocks and boulders, and hard bottom or pavement. Some of these flats may be dominated by certain kinds of organisms, such as sponges, soft corals, or oysters.
Some habitats have pronounced levels of zonation as well. Coral reefs are a good example. Typically, there are three kinds of coral reefs: fringing reefs, where the reef is adjacent to a shoreline; barrier reefs, where the reef is well offshore and usually runs parallel to the adjacent landmass; and atolls, which are reefs that grow and emerge as a landmass, typically a sea mount, sinks beneath it over time. Barrier reefs and atolls usually have lagoons. Fringing reefs, being much narrower, have back troughs or some other form of channel on the reef flat within the intertidal zone. Regardless, seaward from the edge of the reef, one would find the reef front or spur-and-groove zone, one or more reef terraces or benches, and the reef slope. The reef front may be a shallow wall that drops directly from the reef margin to the first terrace. Fishes living here generally are pelagic or benthopelagic, although a number of benthic species may be found among emerging corals, in holes, or around rocks. Alternatively, the spur-and-groove zone extends outward from the reef margin in a pattern resembling a human hand. The "fingers" of the hand represent spurs of coralline rock that extend outward from the face of the reef. Live corals resistant to the effects of wave action may grow upon these spurs. The spaces between the fingers represent the grooves, which are nothing more than surge channels between the spurs. These grooves are shallow at the reef face and deeper as the first terrace is approached, and the bottom of the channels consists of coral rock pavement, boulders, dead coral rubble, sand, or live corals.
Often, a complex network of holes, caves, and tunnels exists within this zone, with direct connections to the reef flat above. Elongated spur-and-groove zones, especially those that extend well out onto the deeper first terrace, are indicative of the effects of the rise and fall of sea levels historically. Numerous species of benthic, benthopelagic, and pelagic species are found in the spur-and-groove zone. For example, blennies and damselfishes utilize holes or corals on the spurs or in the grooves. Morays, lionfishes (Scorpaenidae), squirrelfishes and soldierfishes (Holocentridae), and sweepers (Pempheridae) employ the network of caves and tunnels within this zone. Certain hawkfishes (Cirrhitidae) or groupers (Serranidae) may perch on corals or hide on ledges or next to rocks and ambush passing prey. Reef herrings (Clupeidae) may swim in the water column above and flee the approach of predatory trevallys (Carangidae) that patrol this zone right to the reef margin.
Below the spur-and-groove zone is the reef terrace or bench; there may be one or more of these, depending on local geological history and changes in sea level. Coral development on the terrace, independent of geographic variation, is dependent upon the degree of exposure to wave action. In somewhat protected areas, the diversity and abundance of corals may be relatively high, whereas in areas exposed to heavy wave action and scouring, the diversity and abundance of corals may be low, and coral pavement predominates. Regardless, fishes in the terrace zone utilize what is available to provide shelter, food, and mating sites. Benthic fishes, such as damselfishes, hawkfishes, and scorpionfishes, make use of corals. Benthic species, such as sandperches (Pinguipedidae), blennies, and gobies, employ holes, sand-filled depressions, boulders, and pavement. Benthopelagic species, such as snappers (Lutjanidae), goatfishes (Mullidae), butterflyfishes (Chaetodontidae), angelfishes (Pomacanthidae), wrasses (Labridae), parrotfishes (Scaridae), and filefishes (Monacanthidae), move about home ranges or defend territories. Pelagic species, such as gray sharks (Carcharhinidae), trevallys, and barracudas (Sphyraenidae), patrol the terrace in search of prey.
Deep slope and wall habitats generally occur below the reef terrace. In some places, such as atolls, the transition between the spur-and-groove zone and the wall or deep slope occurs without the presence of a reef terrace. In other places, one or more terraces are present before the deep slope begins, and the slope may separate two terraces from each other. Wall and deep slope habitats often are characterized by the presence of various corals, sea fans and black corals, sponges, hydrozoans, and numerous other benthic invertebrates. These offer shelter and food to innumerable species of small, benthic fishes. The face of the wall or slope may be eroded with numerous holes and caves that provide shelter for many diurnal species, such as groupers and dottybacks (Pseudochromidae), and nocturnal fishes, such as bigeyes (Priacanthidae), soldierfishes, and squirrelfishes. Off the face of the wall or slope are found hovering fishes, such as certain butterflyfishes, angelfishes, damselfishes, and fairy basslets (Serranidae), that feed upon plankton in the water column.
In reef systems with lagoons, some of the same general habitat types may be present. At the back side of a barrier reef flat or in a pass connecting the outer reef with the lagoon, there often is a slope or wall that drops down into the depths of the lagoon. These habitats generally are protected, although they may be subject to intense tidal currents, may have a rich community of benthic invertebrates, and, correspondingly, support a wide variety of species. Damselfishes, butterflyfishes, angelfishes, wrasses, and triggerfishes (Balistidae) may hover in the water column but seek shelter in or along the wall or slope as necessary. As the slope gives way to sand or rubble, garden eels (Congridae), sanddivers (Trichonotidae), gobies, and peacock soles (Soleidae) are visible, but if threatened, they will rush into holes or bury themselves in the sand. Shallow portions of the lagoon often have patch reefs or coral bommies (large, isolated coral heads) that function effectively as islands in a sea of sand and rubble. These islands provide structure and, no matter how small, attract a remarkable number of species. Lagoons also may have sea-grass beds in shallower areas and a corresponding suite of species, such as juvenile emperor fishes (Lethrinidae), snappers, goatfishes, and parrotfishes.
A temperate region analog of the coral reef is the kelp forest. Kelp is a marine plant that may grow as long as 65.6 ft (20 m). It provides a dense jungle that is utilized by numerous temperate species and, depending upon depth and proximity to kelp, may offer different microhabitats to members of the same fish family. As such, different species will be adapted to the surface, mid-reaches, and base of the kelp and to the water column surrounding it.
Among fishes, location is everything. The exact place where a fish lives is known as its "microhabitat." Fishes, especially small fishes, have remarkable plasticity in what they adapt to as a home. For example, scorpionfishes, coral crouchers, hawkfishes, damselfishes, wrasses, gobies, and numerous other species live within or atop the branches of corals. Moray eels, jawfishes (Opistognathidae), blennies, and gobies, among others, inhabit holes. Other water column–dwelling species, such as some triggerfishes, seek shelter in holes as well. Some species, such as pipefishes (Syngnathidae) and clingfishes, are specialized for living in crinoids and sea urchins. Similarly, various seahorses, hawkfishes, and gobies are specialized for life on sea fans and black corals. Pipefishes, seahorses, some juvenile wrasses, and filefishes mimic the leaves of sea grasses or fleshy algae. Blennies and many other small benthic species have adapted to life in empty seashells and worm tubes. Cling-fishes, gobies, blennies, and labrisomids (Labrisomidae) live in sponges. Even the sand serves as a distinctive microhabitat. Snake eels (Ophichthidae) burrow under the sand and seldom emerge, except at night. Stonefishes (Scorpaenidae) and stargazers (Uranoscopidae) lie buried beneath the sand and ambush passing prey. There are numerous other examples of benthic fishes that utilize natural and man-made structures as microhabitats.
The pelagic zone is divided into different zones relative to depth as well. The epipelagic zone ranges from the surface down to a depth of 656 ft (200 m). Between 656 and 3,281 ft (200–1,000 m) is the mesopelagic zone, followed by the bathypelagic zone at 3,281–13,123 ft (1,000–4,000 m), the abyssal zone at 13,123–19,685 ft (4,000–6,000 m), and the hadal zone below 19,685 ft (6,000 m). Most pelagic fishes occur in the epipelagic (more than 1,000 species) and mesopelagic and bathypelagic zones (about 1,000 species combined).
The epipelagic zone is the limit at which photosynthesis takes place. Phytoplankton occur there and form the basis for a food chain that consists of consumers ranging from zoo-plankton to blue whales. Fishes of the epipelagic zone have bodies that are streamlined, to allow for greater speed in the pursuit of prey or the evasion of predators. Many epipelagic species, such as dolphinfishes (Coryphaenidae), tunas (Scombridae), and marlins (Istiophoridae), make seasonal migrations to feed and mate. Speed often is essential, and many species have independently evolved the ability known as countercurrent exchange, which effectively turns them into warm-blooded organisms. This trait, found in mackeral sharks and tunas, among others, is especially useful in cooler waters. Species associated more with inshore waters, such as striped bass (Moronidae) and bluefishes (Pomatomidae), also make seasonal migrations to track prey movements and to reproduce. Reef-associated species, such as trevallys and barracudas may migrate for spawning, but the distances traveled are far less.
Many epipelagic species are denoted by their silvery, bluish, or greenish blue body coloration, which makes them difficult to see in open water and thus decreases the risk of predation. This coloration also benefits predators, allowing them to approach prey fishes more easily. Large predators, such as marlins and many tunas, are more darkly colored, however, and dolphinfishes are among the most brightly colored species in the epipelagic zone. Some species of epipelagic fishes are especially adapted for life at the surface. Specializations may include enlarged pectoral and caudal fins that may be used to facilitate escape, modified snouts that allow for greater feeding efficiency on the surface, or enlarged eyes that promote detection of potential predators and prey at the water-air interface. Predators include the needlefishes (Belonidae), which are capable of sudden, powerful bursts of speed that allow them to jump repeatedly out of the water in pursuit of prey. Prey species have adapted to escape this pursuit. Halfbeaks and ballyhoos also may make repeated jumps to evade predators. Perhaps the most famous example of aerial evasion is that of the flyingfishes (Exocoetidae), whose modified pectoral fins resemble wings and whose modified caudal fin rudders are capable, when touching the surface of the water while the fish is airborne, of supplying additional thrust. Flyingfishes can travel up to 1,312 ft (400 m) in a single flight at a speed of more than 43.5 mi (70 km) per hour and can make flights repeatedly in succession. Pelagic fishes not particularly adapted for flight often seek shelter beneath flotsam, under jellyfishes, or attached to other larger fishes. The remoras (Echeneidae), which attach themselves to sharks, rays, billfishes, whales, and even ships, are able to hitchhike around the epipelagic zone more or less under the protection of their hosts.
Those zones below the epipelagic have confounding physical and biological factors, the effects of which escalate with depth. Increasing water pressure, decreasing water temperature, little or no light penetration, seemingly vast spatial distributions, and the patchy distribution of food resources all heavily influence which fishes live where and how. There is a remarkable diversity in species, however, and, because many of these factors have similar effects upon unrelated species, there is also a extraordinary similarity in characters that have evolved through convergent evolution. Fishes of these zones may be large (more than 6.6 ft, or 2 m) or small (less than 2 in, or 5 cm), yet they possess large or elongated mouths and dagger-like teeth for grabbing prey. Others have tubular eyes that augment the efficiency of light detection. Conversely, many species lack functional eyes entirely and rely upon other senses. Numerous species possess photophores (light-emitting organs) to attract both prey and mates. They also may have modified dorsal fin rays or chin barbels, often with photophores, that are used to attract prey. Many species have thin bones and specialized proteins that allow for gas regulation and neutral buoyancy in the absence of swim bladders. Fishes distributed in relatively shallow mesopelagic waters at higher latitudes often are found at greater depths in the tropics. This phenomenon, known as tropical submergence, allows these species to expand their geographical distribution while remaining in cooler and more comfortable water temperatures. Pelagic fishes of these zones also migrate, but the direction is more vertical than horizontal. At night many species rise hundreds, if not thousands of meters in depth, some to the surface, before returning downward during daylight hours. These vertical migrations usually track the movements of prey during a 24-hour cycle. Other fishes, especially such deep benthopelagic and benthic species as the tripodfishes (Ipnopidae), never make the migration and depend solely on what they encounter or what "rains" down upon them from the water column above.
Special habitats and adaptations
As indicated earlier, marine fishes are specially adapted to living in extreme environments. Deep-water pelagic and benthopelagic fishes, as illustrated earlier, are prime examples of adaptation to extremes. There are numerous shallow-water examples, too. Some marine fishes, such as the reef cuskfishes (Ophidiidae), have adapted to the dim world of tunnels and caves beneath the reef front or spur-and-groove zone but emerge at night to hunt small fishes and invertebrates. Polar fishes have adapted to extremely cold water temperatures that may fall below 32°F (0°C). (It is a curious trick of physics that saltwater does not freeze at this temperature.) For example, the icefishes (Nototheniidae and others in the suborder Notothenioidei) of Antarctica and the Southern Ocean have evolved a specialized protein that acts as an antifreeze that prevents these fishes from freezing. These species have evolved to fill various niches too. Some, such as Trematomus nicolai, are benthic, whereas others, such as Trematomus loennbergii, are benthopelagic in deeper water; still others, such as Pagothenia borchgrevinki, are pelagic and swim and feed just beneath the ice.
Fishes also have adapted to aerial exposure. Examples include clingfishes and blennies in the upper reaches of the intertidal zone and mudskippers (Gobiidae) of the genus Periophthalmus, which retain water in their gill cavities and are capable of hopping and skipping across mud flats, rubble flats, and among the branches of mangroves. Marine fishes have adapted to hypersaline conditions as well. In arid regions, back bays, estuary sloughs, tide pools, and now-landlocked seas, all tend to have salinity levels far higher than that of seawater. Some species of fishes, such as clingfishes and gobies, have evolved mechanisms that allow them to regulate their osmotic pressure under these conditions. There is a limit, however. The landlocked Dead Sea in the Middle East, with a salinity in excess of 200 parts per thousand (ppt), versus an average of 36 ppt in seawater, is simply too salty for fishes to survive. Conversely, some seas, such as the Baltic in northern Europe, have such low salinity levels in some areas that such freshwater species as the pike Esox lucius (Esocidae) may coexist with euryhaline marine species.
Species that migrate between marine and freshwater, or vice versa, during both juvenile and adult phases of their respective life cycles have managed to conquer the problems associated with different salinity levels and osmotic regulation. For example, anadromous fishes, such as the salmons and sea trout (Salmonidae), live most of their adult lives in the ocean but migrate up a river or stream (often the one in which they were born) to spawn. Their juveniles live for part of their life cycle in freshwater before moving downstream and out to sea. (Some populations may become landlocked, however.) Catadromous fishes, such as freshwater eels (Anguilidae), live their adult lives in freshwater but migrate well out to sea to spawn and die. Their juveniles often return to their natal streams to begin their adult lives. Amphidromous fishes, such as some gobies (Gobiidae) and sleepers (Eleotridae), also live their adult lives in freshwater and spawn there as well. Their eggs and larvae are carried out to sea, however, and the post-larvae migrate back up stream, sometimes against formidable barriers, to begin their lives as adults. Other amphidromous species are born in saltwater but have young that migrate into freshwater to grow and then return to saltwater to grow more and to reproduce as adults.
Marine fishes have a wide range of diets and methods of feeding. They may be divided generally into herbivores, carnivores, detritivores, and omnivores. Herbivores are those that feed upon plants and plant materials. They do so by grazing or browsing upon benthic algae, sea grasses, or other plant life. Other herbivores may use specialized gill rakers to strain phytoplankton from the water column. Some species, especially certain damselfishes (Pomacentridae), act as farmers of the benthic algae they consume. For instance, they may kill a patch of coral that subsequently is used as a substrate for benthic algae to recruit upon. The farmer fishes then tend the algae, removing unwanted species and feeding upon desired ones, at the same time that they defend the algal patch against other herbivores. Other herbivores, such as parrot-fishes (Scaridae), may simply bite off and crush corals in order to strain the symbiotic zooxanthellae algae resident within the coral polyp.
Carnivores feed upon a great variety of animals. Zooplanktivores strain or pluck zooplankton from the water column. Corallivores excise or pluck polyps from their coral skeletons; alternatively, they may crush the corals with strong teeth and strain the polyps through their gill rakers. During coral spawning season, numerous fish species, especially butterflyfishes (Chaetodontidae) and damselfishes, feed upon coral eggs as they float upward into the water column. Others may be specialized to pluck or nip the tips of anenomes, hydrozoans, or other coral-like organisms. Some fishes may be generalists when feeding upon invertebrates, but others are highly specialized for taking only certain kinds. Thus, some fishes specialize in microinvertebrates, such as diminutive worms, crabs, shrimps, or mollusks, whereas others target macroinvertebrates, such as squids, octopuses, lobsters, or large crabs.
Invertebrate prey may be benthic, such as clams, oysters, and tunicates, or they may be pelagic, such as squids and swimming shrimp. Prey may be sifted from sand or rubble, crushed, grabbed, bitten, or swallowed whole. Some fishes are able to feed on prey items that may be difficult, if not dangerous, to consume. Some triggerfishes (Balistidae) can bite and crush sea urchins bearing venomous spines without apparent damage to themselves. Similarly, the humphead wrasse (Cheilinus undulatus, Labridae) can feed on adult crown-ofthorns starfish without being damaged by this organism's strong, venom-tipped spines. Pelagic macroinvertebrates may include jellyfishes that are consumed by molas or ocean sunfishes (Molidae) as they drift in the water column. Prey also may reside out of water. For example, archerfishes (Toxotidae) are specialized for feeding upon insects by shooting a stream of water at them so as to knock them down from mangrove branches or other forms of structure; the fallen insect then is consumed on the surface.
Piscivores feed upon fishes exclusively, although many species also vary their diet by consuming invertebrates. These predators actively hunt, chase, herd, grasp, stun, club, shock, ambush, bite, or engulf other fishes. Various small species are specialized for feeding on fish scales or skin, either as juveniles or as adults. Others, such as cleanerfishes (certain Labridae, Gobiidae, Chaetodontidae, and so on), have evolved to remove ectoparasites or damaged tissue from "client" fishes that visit their cleaning stations. Still other species are specialized as parasites that feed upon host fishes. Host fish species include potential predators (e.g., sharks, moray eels, needlefishes, groupers, snappers) as well as numerous other species that are active during daylight hours (e.g., butterfly-fishes, angelfishes, damselfishes, wrasses, parrotfishes). Larger fishes, especially sharks, may feed on floating sea birds, reptiles, and mammals in addition to fishes. Detritivores sift detritus from the bottom and strain it through their gill rakers. Omnivores eat both plant and animal material as adults. Many species of fishes undergo ontogenetic shifts in diet and feeding methods, meaning that their diet and feeding methods change with age and growth. Species that feed upon phytoplankton or zooplankton as juveniles may switch to fishes or large invertebrates as adults.
Through their trophic interactions, fishes have important direct and indirect effects upon the structure of the communities in which they live. They can influence, among other factors, rates of productivity and biomass turnover, nutrient cycling, sediment production, shifts in water quality, shifts in food web composition, or shifts in species composition and relative abundance within a given assemblage.
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Terry J. Donaldson, PhD