Ecology

Introduction

The ecology of invertebrates consists of all the external factors acting upon that organism. These factors may be either physical or biological. The physical or abiotic environment consists of the nonliving aspects of an organism's surroundings, including temperature; salinity; pH (a measurement of acidity or alkalinity); exposure to sunlight; ocean currents; wave action; and the type and size of sediment particles. The biotic environment consists of living organisms and the ways in which they interact with one another.

Invertebrate species have colonized all types of aquatic habitats. For example, sponges of the class Calcarea are restricted to firm substrates. They are also restricted by physical factors that affect their skeletons, limiting their habitats to shallow zones. Hexactinellida sponges colonize soft surfaces; they prefer to live in deep water. Demosponges can live on such different substrates as rock, unstable shell, sand, and mud; in some cases they burrow into calcareous material. They are found in a variety of underwater habitats ranging from upper intertidal to hadal depths (below 20,000 ft or 6,100m). The ecological dominance of the Demospongiae reflects their diversity in form, structure, reproductive capabilities and physiological adaptation. Cnidarians and ctenophores are mostly marine; however, a few groups have successfully made their way into freshwater habitats.

Most lower metazoans are either sessile polyps, which means that they are attached at the base to the surface that they live on, or planktonic carnivores. Some, however, employ suspension feeding and many species harbor symbiotic intracellular algae that supply them with energy. Hydroids, scyphozoans, and anthozoans live in seas around the globe, from polar to tropical oceans. Most lower metazoans, however, live in coastal waters.

Physical factors

Light

Sunlight has an important role in both terrestrial and marine environments, powering the process of photosynthesis that provides energy either directly or indirectly to nearly all forms of life on earth. The diel, or 24-hour cyclical migrations of epipelagic species, are at least in part active responses to changing light levels. Epipelagic refers to the upper levels of the ocean that are penetrated by enough sunlight for photosynthesis to occur. Aurelia aurita approaches the surface during the day, at both midday and midnight, or only at night, and becomes scattered throughout the water column at night or during the sunlit days. Diel migrations probably do not occur in the bathypelagic zone (about 3,280–6,562 ft or 1–2 km); migration in the mesopelagic zone (about 656–3,280 ft or 200–1,000 m) depends on the levels of available light in that zone. Sunlight is necessary for vision as well as photosynthesis. Many animals rely on their vision to capture prey, avoid predation, and communicate with one another.

Bioluminescence is a type of visible light produced by marine animals such as scyphozoans, hydrozoans, ctenophores, squids, thaliaceans, and fishes. It may be used for counterillumination or as ventral camouflage. Another possibility is that bioluminescence is a useful defense mechanism against potential predators.

Turbidity

Turbidity refers to the cloudiness of sea water caused by the suspension of sediment particles and organic matter. High concentrations of suspended particles in the water over offshore coral reefs are considered a stress factor for coral colonies because they reduce the amount of light for photosynthesis and smother coral tissues. Nevertheless, many reefs with large growths of coral are found in relatively turbid waters, such as the fringe reefs around the inshore continental islands in the Great Barrier Reef lagoon. This finding suggests that turbid water is not necessarily harmful to coral. Fine, suspended particles provide a large surface area for colonization by microorganisms that produce organic nutrients. By limiting light penetration, turbid water also limits the distribution of both benthic algae and phytoplankton, which are at the base of the web.

Temperature

All lower metazoans are ectothermic (sometimes referred to as "cold-blooded"), which means that they retain the same temperture as their surroundings. Because of this restraint; invertebrate physiology has evolved to operate in a specific temperature range for each species. Most organisms can tolerate only a narrow range of temperatures; changes above or below this critical range disrupt their metabolism, resulting in a lowered rate of reproduction, injury, or even death. Since temperatures change less rapidly in the open sea than in shallow waters, species in shallow waters can tolerate a wider range of temperature than deep water species. Temperature often influences the distribution, reproduction, and morphology (form and structure) of these organisms. Colonies of Obelia geniculata and Silicularia bilabiata living in cold water develop long, branching hydrocauli (stalks), whereas colonies of these species living in warm waters have short stems with few branches. Gametogenesis in the hydrozoan Sertularella miuresis begins when the temperature reaches 50°F (10°C) and stops when it reaches 64°F (18°C). Coryne tubulosa reproduces asexually at around 57°F (14°C), but produces medusae when the temperature falls to 35°F (2°C). The acclimation temperature of Chrysaora quinquecirrha polyps is about 51°F (10.50°C), but the upper lethal temperature dose, defined as the temperature at which 50% of the test animals die (LD-50), is 95°F (35°C).

Salinity

Salinity, or the level of salt content in seawater, can affect invertebrates. Species that have evolved to live in freshwater can rarely live in salt water, and few marine species can tolerate low salinities or freshwater. This becomes quite apparent when one studies species richness (number of species) as one moves down a river into an estuary. Species richness (number of species) is relatively high in freshwater, then decreases considerably as salinity increases to about 5ppt, where most freshwater species cannot exist. Species richness then increases with salinity as more low-salinity-tolerant species are encountered. Species richness is at its greatest at the mouth of the estuary, where fully marine species occur with estuarine species.

Salinity can effect the morphology of organisms. For example, the shape, number, and size of tentacles of Cordylophora caspia polyps is affected by salinity. The scyphozoan medusae of Rhopiena esculenta can survive at levels of salt concentration as low as 8 parts per thousand (ppt), the scyphistomae to 10 ppt and the planulae to 12 ppt. The estromatolites of Phylloriza peronlesueri, however, form in hypersaline (very salty) waters. A rise in the salt content of the Baltic Sea allowed A. aurita and C. capillata to expand into northern waters, and allowed Rhizostoma pulmo and A. aurita to move from the Azov Sea into the Black Sea. The hydroid Laomedea flexuosa increases its production of gonozooids when the seawater concentration is around 30–40 ppt; at higher concentrations, however, the colonies begin to degenerate. The cephalo-chordate Branchiostoma nigeriense becomes opaque above salinities of 13 ppt.

Ocean currents and turbulence

Moving water is essential to lower metazoans because it supplies food and dissolved gases; prevents the accumulation of sediment; and disperses waste products, medusae, and larvae. Aglaophenia picardi resorbs the tissues of its hydrocauli into the hydrorhiza when the surrounding water is relatively stagnant, but regenerates them when the water begins to move more rapidly. The speed and direction of current flow affect

the form and size of some lower metazoans. The size of hydroids is usually inversely related to the speed of water movement; large specimens are found in calm water and smaller specimens in rougher water. Aglaophenia pluma develops unbranched hydrocauli about 0.6 in (1.5 cm) tall in shallow, turbulent water, but produces branched hydrocauli as high as 19.6 in (50 cm) in deeper water with bidirectional currents. Planar (flat) forms such as A. pluma, Plumularia setacea, and Eudendrium rameum are most abundant where the current tends to flow in one direction, while radial or arborescent (treelike) forms such as Lytocarpia myriophyllum, Nemertesia antenna, and E. racemosum flourish in bi- or multidirectional currents. The distribution of species that inhabit coral reefs and display highly specific patterns of tolerance is greatly affected by water movement. Morphological differences in hydroids and anthozoans are also regarded as indicators of distinct patterns in water movement.

Water depth

The majority of organisms are not able to survive in great depths (below 3,281 ft or 1,000 m). In general, the number of invertebrates is highest in shallow water communities and decreases as water depth increases. However, species diversity may be quite high at great depths on the abyssal plain where the environment has been extremely stable for millennia. Though diversity can be high, biomass may be low in these deep benthic habitats, because the lack of light prevents any primary production. Therefore, these habitats are usually limited by food and depend on organic input from sunlit seas above.

The lack of mixing and primary production result in oxygen-minimum layers in the ocean, and many species are either adapted to lower oxygen concentrations or avoid these areas. The scyphozoans Periphylla periphylla and Nausithoe rubra show high levels of the anaerobic enzyme lactate dehydrogenase, probably as an adaptation to moving at depths between 1,312 and 4,921 ft (400–1,500 m), which has minimal levels of oxygen.

Environmental contaminants

Pollution may result from contamination by sewage, hydrocarbons, polyvinyl biphenals (i.e., PCBs), pesticides (e.g., DDT), and heavy metals such as cadmium, copper, lead, mercury, and zinc. Experiments have revealed that exposure to pollutants can lead to sublethal effects in hydroids, including changes in the curvature or branching of the hydrorhiza; loss of hydranths; stimulation of gonozooid production; or changes in the rate of growth. Low concentrations of metal ions such as copper and mercury may inhibit growth regulation in hydroids while increasing the growth rate in Laomedea flexuosa and Clavopsella michaeli. In Elefsis Bay, a polluted area of Greece, populations of Aurelia aurita have multiplied to rates of more than 1,500 medusae per 10 m³. Certain species of Rhizostoma have survived in parts of Madras Harbor that have been polluted by diesel oil; however, the presence of crude petroleum in the waters of Alaska has caused a reduction or cessation in the strobilation in the polyps of A. aurita, and the production of ephyra and polyps with both morphological and behavioral abnormalities. Pelagia noctiluca, a scyphozoan from the Mediterranean Sea, acquires high concentrations of cadmium, lead, mercury and zinc. Individuals of the species Chrysaora quinquecirrha have been found to have highly concentrated levels of the herbicide pendimethalin in their tentacles; they show no change in behavior at concentrations of the pesticide that are lethal to fishes such as perch. The dumping of raw sewage in may tropical areas of the world destroys coral reefs by increasing turbidity that prevents light penetration, increasing sediment loads that smother corals, and increasing nutrient loads that encourage algae growth that can out-compete corals.

Species interactions

Competition occurs when organisms require the same limited resources, such as food, living space, or mates; or when two groups of organisms try to occupy an ecological niche in the same location at the same time. Competition may either be interspecific (between different species) or intraspecific (within the same species). Hydroids, which have a stoloniferous growth pattern, demonstrate two different growth strategies. The first, a guerrilla strategy, is characterized by extensive hydrorhizal growth with little branching and sparsely spaced hydrocauli or polyps; this pattern is exhibited by L. flexuosa. Guerilla behavior is an opportunist-style strategy for reducing interspecific competition for space and the possibility of overgrowth by other organisms. The second strategy, phalanx; results in highly branched hydrorhizae with dense hydranths carried on large hydrocauli. The phalanx strategy is exemplified by Podocoryne carnea and Hydractiniaechinata, which grow on the shells of hermit crabs. Intraspecific variation in the allocation of resources that lead to hydrorhizal growth can be observed in the colonies of H. echinata. This species usually shows either little outward hydrorhizal growth combined with a high rate of reproduction, or extensive hydrorhizal growth combined with a lower rate of reproduction and correspondingly greater competitive ability. Contact with another colony leads the hydrorhiza to produce an abnormally large number of stolons (shoots or runners) armed with nematocysts, which sting and kill the tissue of other hydroids. Guerrilla growth strategies have adaptive value in situations where there is relatively little space available, as on shells occupied by other hydroids, while the phalanx strategy is more advantageous for expanding the colony to shells that are inhabited by hydroids.

Competition for space is of prime importance in the coral reef ecosystem. Most of the aggressive species are small and have slow growth rates, while the less aggressive coral species have faster growth rates and are able to outpace their competitors through rapid growth. The ability to maintain either rapid growth or aggressively dominative practices, but never both, explains why no single species of coral is able to dominate a coral reef. One possible outcome of competition is the extinction of the less successful competitor.

A niche can be subdivided into two or more small niches with minimal overlap, allowing competing organisms to share a resource. Examples of resource partitioning may be found on coral reefs. Small ecological niches can be occupied by similar species if the anatomy, feeding behavior, and territory of each species are only slightly different from those of another. The hydrozoans Hydractinia (retained gonophores), Stylactis (medusoids), and Podocoryne (medusae) have similar morphologies but different reproductive strategies, which allows them to occupy similar niches on the shells of hermit crabs. The competitive ability of colonies may also depend on their size. Podocoryne carnea hydroids show a greater selective advantage in aggressiveness in relation to H. echinata in interspecific competition. In instances of intraspecific competition among different colonies of P. carnea, however, the colony most likely to lose out is the one with the slowest rate of growth.

Feeding mechanisms and behavior

Suspension feeding

Lower metazoans demonstrate a remarkable variety of feeding mechanisms. Most sponges are suspension feeders that subsist on such fine particles as bacterioplankton and dissolved organic matter. Sponges acquire food and oxygen from water that flows through them; this flow is actively generated by sponges beating their flagella (microscopic whiplike structures). This process also acts as a means of waste removal for sponges. The movement of water through sponges is aided by ambient currents passing over raised excurrent (providing outward passage) openings, which creates an area of low pressure above these openings. Sponges are also capable of regulating the amount of flow through their bodies by narrowing or partly closing off various openings. The volume of water passing through a sponge can be enormous—as much as 20,000 times its volume over a 24-hour period.

Sponges are size-selective particle feeders. Their aquiferous systems create a series of "sieves" of varying mesh size. The largest diameter of incurrent openings is usually around 0.002 in (50 µm), which keeps larger particles from entering the aquiferous system. A few species have larger incurrent pores, reaching diameters of 0.006–0.0069 in (150–175 µm). Some sponges trap roughly 90% of all bacteria in the water they filter. Other sponges also take significant amounts of dissolved organic matter into their aquiferous systems. In some demosponges, 80% of the organic matter that is filtered through their aquiferous system is too small to be seen by light microscopy. The other 20% is composed primarily of bacteria and dinoflagellates. Other sponges harbor symbionts such as green algae, dinoflagellates (zooxanthellae), or cyanobacteria, which also provide them with nutrients.

Predation

Many invertebrates are predators that feed on protozoans, other invertebrates, and fishes. The discovery of several Mediterranean species of sponges that capture and digest entire animals came as a surprise to marine biologists. These species of the family Cladorhizidae have no choanocytes or aquiferous systems, but anatomic and biological analysis revealed the presence of spiky filaments with raised hook-shaped spicules. These carnivorous sponges capture and hold small crustaceans with their spicules, which act like Velcro® tape when they come in contact with the crustaceans' exoskeletons. Once captive, the crustaceans cannot free themselves. They struggle for several hours, which indicates that the spicules do not produce any paralyzing or toxic secretions. Cells then migrate around the helpless prey, and digestion takes place outside the cell walls.

Most cnidarians are carnivorous, using cnidocytes on their tentacles to capture prey. Polyps, the sessile stage of cnidarians, are generally believed to be passive predators, feeding on animals that blunder into their tentacles. Some cnidarian medusae possess sensory structures resembling primitive eyes; they are active predators. Many corals and anemones feed by suspending thin strands or sheets of mucus over the surface of their colony. The sticky mucus collects fine particles of nourishment from the water; cilia present on the organisms drive the food-laden mucus into the mouths of coral or anemones. Many species have developed complementary adaptations such as ectodermal ciliary currents on their tentacles, oral discs or columns, or the ability to position themselves strategically within the water's flow pattern.

In sea anemones, the presence of nearby food evokes behavior that has two phases: a prefeeding, and a feeding response that leads to the ingestion of prey. The prefeeding response consists of the expansion of the oral disc, the movement of its tentacles, and both the extension and swaying of the column. This prefeeding behavior increases the chances of catching nearby food. The feeding response, which takes place after the prey has made contact with the anemone's tentacles or oral disc, includes the discharge of nematocysts and ingesting movements. Sea anemones are able to detect prey from the prey's emission of small dissolved molecules of

amino acids, tripeptides, and vitamins. The cnidae, which ensure the capture of the prey, are spirocysts and nematocysts. The numerous spirocysts on the tentacles of sea anemones appear to have an adhesive function. The contact of solid food with the tentacles leads to a massive discharge of spirocysts, which hold the prey while the nematocysts inject their toxin. Ingestion is directed by the chemical and mechanical stimuli produced by the immobilized prey. After ingestion, the prey is enclosed by filaments whose cnidoglandular tracts contain nematocysts but no spirocysts. The penetrating filaments of these nematocysts inject more toxin. The prey is then subject to the action of secretory cells that ensure its extracellular digestion.

The prey of sedentary cnidarians is composed of small motile animals such as zooplanktonic larvae, isopods, amphipods, and polychaetes. Sea anemones found closer to shore may complete their diet with larger sessile prey dislodged by wave action or foraging predators. The size of the prey is generally small considering the diameter of the sea anemone, and many species may be considered microphagous. Of the nine common species of Caribbean reef sea anemones, seven are planktivores. Condylactis gigantea and Stoichactis helianthus, which eat macroscopic prey such as gastropods and echinoids, probably depend on heavy wave action in the reef to supply them with prey.

Comb jellies are entirely predatory in their habitats. The long tentacles of ctenophores have muscular cores with an epidermal cover that contains colloblasts, or adhesive cells. The tentacles trail passively through the water or are twirled about by various circular movements of the body. Upon contact with the prey, the colloblasts burst and discharge a strong sticky material. In ctenophores, which bear very short tentacles (orders Lobata and Cestida), small zooplankton are trapped in mucus on the body surface and then carried to the mouth by ciliary currents (along ciliated auricular grooves in ctenophore lobates and ciliated oral grooves in cestids). Under conditions of starvation in an aquarium, lobate adults often swim vertically through the water only to descend with their lobes extended; in this position the width of the lobes may be as much as 116% of the animal's length. The lobe width decreases to about 79% of the body length after food is placed in the aquarium and the animal begins to feed. Once the digestive tract is full, these adults still continue to feed by entangling their prey in mucus, which produces a bolus or clump near the mouth. Quite frequently they will either spit out this ball of food or completely empty their digestive tract and continue to feed. This behavior pattern can continue for several hours until the concentration of food is reduced to the point at which all the prey that have been captured can be ingested.

Most pelagic ctenophores and cnidarians feed primarily on copepods. There are a few intriguing examples of pelagic cnidarians that feed exclusively on one type of prey. The siphonophore Hippopodius hippopus feeds only on ostracods; the hydromedusae Bougainvillia principis feeds mainly on barnacle nauplii; and Proboscidactyla flavicirrata eats only veligers (mollusk larvae). Some cnidarians and ctenophores feed specifically on gelatinous prey or fishes. Ctenophores of the genus Beroe offer well-known examples of selective feeding on gelatinous prey: B. cucumis feeds exclusively on the ctenophore Bolinopsis vitrea, and B. gracilis feeds only on Pleurobrachia pileus. When beroids prey on animals larger than themselves, they appear to attach themselves to the prey and suck its tissues into their mouths. Beroids lack tentacles; however, they do possess some 3,000 macrocilia that are hexagonally arranged and form a ciliary band around the inside of the mouth that beats inward, and forcing tissue from the prey into the beroid's pharynx. Gelatinous species that include high proportions of soft-bodied prey in their diets often eat fish eggs and larvae when they are available.

The diets of gelatinous predators generally show some selectivity, and are dependent on factors including the prey's size; the width and spacing of the predator's tentacles; the predator's swimming behavior and speed; water flow; and the prey's ability to escape. In general, species that catch large prey have few and widely spaced tentacles, while those that feed on small prey have numerous closely spaced tentacles. Most gelatinous predators move while feeding, which allows them to make use of water currents that will bring prey toward their tentacles. Cnidarians that are ambush predators are able to catch large, fast-moving prey; cruising predators prefer small, slow-moving prey. For example, siphonophores, which are ambush predators, tend to select large and relatively swift prey, while Aurelia aurita, which is a cruising predator, selects slow-moving organisms. Swimming offers the advantage of allowing predators to scan larger volumes of water; however, it also has the disadvantage of alerting the prey to the predator's presence. Some predators deal with this disadvantage by remaining stationary while they are "fishing."

Swimming is not the only way for prey to escape. Bivalve veligers will close themselves off when they are disturbed by the medusae of Chrysaora quinquecirrha; 99% of captured veligers are ingested alive.

Ecological significance of predation

The feeding rate of predators can be expressed in terms of clearance rate (volume of water × predator ?1 × time ?1), or in terms of numbers or biomass of prey captured (predator ?1 × time ?1) and are often combined with estimates of predator and prey densities in order to estimate the effects of predation on prey populations. One characteristic of gelatinous predators is that clearance rates tend to be constant even at extremely high prey densities. In species with few tentacles (e.g., Pleurobrachia spp.), however, ingestion is limited by prey handling. Saturation feeding at typical prey densities rarely occurs in situ. Gelatinous predators feeding on small prey seldom fill themselves at natural prey densities. They may not reduce populations, but they do keep them in check. Predators that consume ichthyoplankton often appear in areas of intense spawning activity and are major causes of fish egg and larva mortality. In some areas large schyphomedusae consume high numbers of commercially important fish larvae, and they also compete with fishes for food. Swarms of jellyfish may be so dense that they clog and damage fishing nets. Many hydromedusae species, including Porpita spp., Velella spp., and Physalia spp., also occur in huge concentrations, particularly in tropical seas.

Some species have such a significant ecological effect that they are considered "keystone species." If these species disappear or appear in an environment, the entire habitat can shift dramatically. For example, active predation by sea stars can significantly affect prey populations. The crown-ofthorns sea stars (Acanthaster spp.) feed on coral polyps in tropical reef habitats worldwide. Occasionally, population explosions of A. planci occur that can have devastating effects on coral reefs. Thousands of square miles (kilometers) of bare coral skeletons can result. Species composition and diversity of other inhabitants are affected secondarily as algae and other organisms colonize the reef. Organisms that depend on live coral for survival either leave the area or die.

Aggressive and defensive behavior

Gastropods, pycnogonids, sea stars, sea urchins, fishes, and sea turtles are predators that feed on invertebrates. Moreover, jellyfishes and comb jellies can be predators of other cnidarians and ctenophores. Sea turtles, especially Dermochelys coriacea, feed on scyphomedusae such as A. aurita. In addition, birds may add scyphozoans to their diets. Some benthic animals like the nudibranchs may feed on the scyphistomae of Cyanea capillata and A. aurita. A single nudibranch can consume as many as 200 polyps per day; however, not all nudibranchs are able to eat scyphozoans. As in the pelagic environment, scyphozoans in their benthic stages may eat one another; for example, the scyphistomae of A. aurita eat the planulae larvae of C. capillata as well as the larvae of their own species. In addition to natural predation, scyphozoans may be affected by fisheries.

In general, these species defend themselves against predators by the production of physical structures or the emission of various chemicals. Sponges are the most diverse source of marine natural products; some of these compounds offer potential pharmacological benefits. Compounds isolated from sponges vary widely in structural complexity; they include sterols, terpenoids, amino acid derivatives, saponins, and macrolides. The toxins produced by temperate and tropical sponges have been shown to deter predation by fishes, asteroids, and gastropods. The organic component of sponges consists primarily of NaOH-soluble and insoluble protein, NaOH being the chemical formula for sodium hydroxide, or lye. About 56% of Antarctic sponges are toxic. Leucetta leptorhapsis and Mycale acerata are highly toxic; however, the asteroid Perknaster fucus antarcticus is a specialist and is able to feed on M. acerata without succumbing to its toxins. This finding suggests that some asteroids have evolved physiological mechanisms that neutralize or sequester (compartmentalize) sponge toxins. In many cases these secondary metabolites are not necessarily toxic, but may make the sponge distasteful to predators and may be more effective in deterring predation. These compounds not only help the sponges avoid predators, but also prevent infection by microorganisms. In addition, they allow the sponges to compete for space with other sessile invertebrates such as ectoprocts, ascidians, corals and even other sponges. Clinoid sponges are among the most common and destructive endolithic (living embedded in rock surfaces) borers on coral reefs worldwide. Cliona, Anthosigmella, and Spheciospongia of the order Hadromerida; and Siphonodictyon, of the order Haplosclerida are siliceous sponges known to bore into hard substrates. Such sponges are able to excavate galleries in calcareous material by removing small fragments of the mineral by specialized archaeocyte cells. The cells secrete chemicals that dissolve the calcareous substrate. When infested corals are split open, clinoid sponges appear as brown, yellow, or orange patches lining the corroded interiors of the coral skeleton.

The sponge Cinachyra antarctica has distinctively long spicule tufts that emerge from the spiral conules on their surface. This species, found throughout Antarctica at depths of 59–2,496 ft (18–761 m), uses its spicules to protect itself from predators. When spicules are removed, C. antarctica is made vulnerable to predators.

Defensive and feeding activities are closely associated in most cnidarians; the tentacles of most anemones and jellyfishes serve both purposes. In some cases, however, both functions are performed by separate structures. Sea anemones and corals have developed several specialize structures used to defend against territorial invasion. Three of these structures, namely acrorhagi (special fighting tentacles), catch tentacles, and sweeper tentacles, are modified feeding tentacles. The mesentenic filament is another modified defensive structure. The acrorhagi are located at the margin of the anemone's body column. When these anemones make physical contact with one another, usually with their tentacles, the acrorhagi expand and apply themselves on the target organism. The ectodermal tissue of the acrorhagus lifts away from its underlying mesenclyme (cellular jellylike material) while the acrorhagus discharges its nemactocysts, and the ectoderm then clings to the target organism. This process is called peeling. As a result of continued discharges from the nematocysts, the victim's tissue beneath the acrorhagial peel becomes necrotic and dies. Catch tentacles (in sea anemones) and sweeper tentacles (in scleractinian corals) develop from feeding tentacles that undergo a morphological change when the organism comes into contact with appropriate other species. In response to weeks of contact, the feeding tentacles alter their form, structure, and complement of cnidae. Catch and sweeper tentacles do not adhere to potential food objects; when they are touched by prey, they actually retract. Sweeper tentacles emerge at night; as their name implies, they flail about or undulate. They can reach 5–10 times the length of feeding tentacles. The coral Montastrea cavernosa is a mildly aggressive coral, capable of destroying the tissue of a variety of subordinate coral species with its mesenterial filaments. But its own mesenterial filaments can be destroyed by M. annularis when both are placed together. These species have sweeper tentacles, which are multipurpose structures with the capacity to regulate the distance between colonies, thus functioning as organs of competition.

Although the ability of hydroids to resist predation is often attributed to their nematocysts and associated toxins, the chemical compounds make them much less attractive to a potential hydroid predator. Some species of hydroids secrete chemicals that deter feeding by the pinfish Lagodon rhomboides. After having been treated with potassium chloride, which forces them to discharge their nematocysts, both Halocordyle disticha and Tubularia crocea become palatable; this suggests that these species rely on nematocysts to defend themselves against predators. However, species such as Corydendrium parasiticum, Eudendrium carneum, Hydractinia symbiologicarpus, and Tridentata marginata, remain unpalatable after their nematocysts have been discharged.

Resources

Books

Arai, N. A. A Functional Biology of Scyphozoa. London: Chapman and Hall, 1997.

Bergquist, P. R. Sponges. Berkeley: University of California Press, 1978.

Nybbaken, J. W. Marine Biology: An Ecological Approach. New York: Harper and Row, 1982.

Periodicals

Fautin, D. G. "Reproduction of Cnidaria." Canadian Journal of Zoology 80 (2002): 1735–1754.

Gili, J. M., and R. G. Hughes. "The Ecology of Marine Benthic Hydroids." Oceanography and Marine Biology: An Annual Review 33 (1995): 351–426.

Harbison, G. R., and R. L. Miller. "Not All Ctenophores Are Hermaphrodites. Studies on the Systematics, Distribution, Sexuality and Development of Two Species of Ocyropsis." Marine Biology 90 (1986): 413–424.

Kass-Simon, G., and A. A. Scappaticci, Jr. "The Behavioral and Developmental Physiology of Nematocysts." Canadian Journal of Zoology 80 (2002): 1772–1794.

McClintock, J. B. "Investigation of the Relationship Between Invertebrate Predation and Biochemical Composition, Energy Content, Spicule Armament and Toxicity of Benthic Sponges at McMurdo Sound, Antarctica." Marine Biology 94 (1987): 479–487.

Purcell, J. E. "Pelagic Cnidarians and Ctenophores as Predators: Selective Predation, Feeding Rates and Effects on Prey Populations." Annales de l'Institut Océanographique 73, no. 2 (1997): 125–137.

Reeve, M. R., and M. A. Walter. "Nutrionatal Ecology of Ctenophores — A Review of Recent Research." Advances in Marine Biology 15 (1978): 249–287.

Stachowicz, J. J., and N. Lindquist. "Hydroid Defenses Against Predators: The Importance of Secondary Metabolites Versus Nematocysts." Oecologia 124 (2000): 80–288.

Van-Praet, M. "Nutrition of Sea Anemones." Advances in Marine Biology 22 (1985): 65–99.

Other

"Spiky sponge Cinachyra antarctica." Underwater Field Guide to Ross Island and McMurdo Sound, Antarctica. Scripps Institution of Oceanography Library. <http://scilib.ucsd.edu/sio/nsf/fguide/porifcra16.html>

Alexandra Elaine Rizzo, PhD

Eliane Pintor Arruda, MSc

Dennis A. Thoney, PhD