Cephalochordata (Lancelets)

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Cephalochordata

(Lancelets)

Phylum Chordata

Class Cephalochordata

Number of families 1

Thumbnail description
Exclusively marine species with slender, fish-like shape, tapered at both ends; easily recognized by the externally visible v-shaped lines that separate the iterated muscle blocks and by the oral cirri that guard the mouth opening against unwarranted particles

Evolution and systematics

Cephalochordates (also called acraniates or lancelets) possess key features that characterize them as chordates. Those characters include the notochord, dorsal nerve cord, branchial basket, and a post anal tail with fins. It is generally believed that cephalochordates are the closest living relatives to the vertebrates. The two groups share unique traits such as the organization of the main body musculature into separate segments and the organization of the circulatory system. Another feature, which unites cephalochordates and vertebrates, is a caecum that is probably homologous to the vertebrate liver. Thus, the morphological evidence is strong. In fact it is so strong that it was used as evidence against phylogenetic conclusions, which were drawn solely from DNA sequences when a molecular study placed the cephalochordates as relatives of echinoderms, not even within the chordates.

The potential for fossilization of cephalochordates is limited because the animals are soft bodied. Even their main skeletal element, the notochord, consists of specialized muscle tissue. Nevertheless several fossils have been described that could have affinities to cephalochordates.

Pikaia gracilens from the Canadian Burgess Shale (Cambrian, ca. 530 million years old) is a promising candidate for consideration as a cephalochordate fossil. It has the typical tapered body form at both ends, and it has been argued that it possessed a notochord. However, Pikaia also shows characters not found in extant cephalochordates. In some fossils the lines, interpreted as myocommata, are w-shaped rather reminiscent of vertebrates than of the simpler v-shape of cephalochordates. Also, the anterior end of some fossil remnants displays tentacles, a feature not found in living cephalochordates, but known, for example, from hagfishes.

The nature of several other fossils (e.g., Cathaymyrus diadexus [Cambrian], Lagenocystis pyramidalis [Ordovician], Palaeobranchiostoma hamatotergum [Permian]) remains more equivocal. An instructive example is the fossil Yunnannozoon lividum (Cambrian). Originally reconstructed as a cephalochordate, it was re-described as a worm-like hemichordate only to be interpreted as a sea pen (phylum Cnidaria) shortly thereafter.

Evolutionary relationships among cephalochordate species are unclear. However, it is thought that the genus Epigonichthys is more derived compared to Branchiostoma.

The class Cephalochordata (some experts now categorize it as a subphylum) contains one order, Amphioxiformes, and one family, Branchiostomatidae.

Physical characteristics

As the closest living relatives of vertebrates, cephalochordates occupy an eminent phylogenetic position. They are invaluable in reconstructing the evolutionary step from invertebrate to vertebrate along the lineage that leads to mammals and humans. Therefore, cephalochordates are of great interest to numerous researchers and consequently their anatomy is well known.

A median fin is present along the entire dorsal length of the animals, extending at the anterior end into a short rostral fin and at the posterior end a tail fin. The ventral fin runs between anus and atriopore. In front of the atriopore paired fin-like folds, the metapleural folds, extend to the anterior region of the pharynx. The dorsal and ventral fins contain so-called fin chambers enclosing coelomic cavities and fin rays that consist of a substance that could serve as an energy store.

The epidermis consists of a single-celled layer, a primitive feature compared to the multi-cellular epidermis in vertebrates. It contains gland cells that secrete a thin sheet of mucus covering the surface of the animal. Different types of sensory cells are distributed in the epidermis, especially in the rostral region.

The dorsal nerve cord of cephalochordates has a central canal that is enlarged at the anterior end of the cord. This enlarged region is known as the cerebral vesicle and is homologous to parts of the vertebrate brain as indicated by morphological similarities (infundibular cells, Reissner's fiber) and similarities in the expression patterns of developmental genes. A peculiarity of the cephalochordate neural cord is the presence of numerous pigmented ocelli, the so-called pigment-cup ocelli. In addition, several more structures in the neural cord are probably light sensitive, e.g., the Joseph cell receptors and the lamellar body in the cerebral vesicle. A photoreceptor in the anterior cerebral vesicle of larvae shows significant similarities to the paired eyes of vertebrates and could be homologous to them. Outside the central nervous system one finds receptors (e.g., corpuscules de Quatrefage), especially in the epidermis of the buccal cirri, velar tentacles, the atrium, and along the metapleural folds.

The notochord extends from the anterior to the posterior end. It is a skeletal element that is the antagonist of the lateral muscle cells that attach to it. Curiously, it is itself an active tissue. It consists of specialized muscle cells and is innervated from the nerve cord. Vacuoles within and between the cells act as a hydroskeleton and help in adjusting the rigidity of the notochord. The notochord is the main skeletal element. Aside from it, in the pharynx, between the gill slits, the branchial bars contain skeletal, cartilaginous rods. In addition, similar rods support the anterior cirri.

A pattern of repeated V-shaped lines is visible on the side of a cephalochordate. It is similar to the pattern seen in a fish fillet, though simpler, and of course, minute. This pattern is caused by the connective tissue that separates individual muscle segments. The innervation of the muscle cells is quite different from the vertebrates. There are no ventral spinal nerves that innervate the cells as in vertebrates. Instead the muscle cells themselves form long thin extensions that approach the nerve cord and are innervated directly there.

It has long been believed that cephalochordates possessed excretory cells very similar to polychaetes. Electron microscopy showed that the similarity is superficial. The excretory cells of cephalochordates are unique in the animal kingdom though in evolutionary terms they are probably derived from the so-called podocytes that are found in other deuterostomes. Excretory organs (nephridia; sing. nephridium) are found serially along the dorsal part of the pharynx. They are associated with blood vessels and function in a similar way as the vertebrate kidney. In addition to these nephridia a nephridium is situated just in front of the mouth (Hatschek's nephridium).

The general arrangement of blood vessels is similar to vertebrates, but the vessels are not lined by cells as in vertebrates. The region where a heart would be expected is contractile. In notable similarity with vertebrates a portal vein connects a capillary system around the posterior intestine with a capillary system around the hepatic caecum.

A pharynx with gill slits is common to all chordates: tunicates, cephalochordates, and the more primitive members of the vertebrates. In cephalochordates a vestibule lies in front of the pharynx that is guarded by the cirri. The vestibule contains ciliated tracts (wheel organ), an excretory organ (Hatschek's nephridium), and a shallow groove (Hatschek's pit) that is probably homologous to the adenohypophysis in vertebrates. The pharynx proper begins with the mouth opening that is surrounded by velar tentacles. The pharynx is perforated by up to 200 gill slits. Along its ventral midline runs a groove, the endostyle that produces mucus and thyroxine, indicating homology to the thyroid in vertebrates. The gill slits do not open into the open water, but the whole pharynx is covered by the outer body wall. In this way a space around the pharynx is created, which is called the atrium. The atrium opens in the ventral midline through the atriopore anterior to the anus.

The pharynx leads into the short esophagus and from there the gut extends straight posteriorly and opens through the anus. Ventrally, at about the junction of the pharynx and esophagus a blind ending caecum projects anterior. It is called the hepatic caecum and is probably homologous to the vertebrate liver.

Depending on species, adult animals range in size 0.4–3.2 in (1–8 cm) in total length. The color of all species is whitish to a creamy yellow, sometimes with a tint of pink. The mucus covering the epidermis can reflect iridescently.

For identification and taxonomic purposes the so-called myotome (muscle block) formula is used. Three numbers signify the number of myotomes anterior to the atriopore, between atriopore and anus, and posterior to the anus. In

addition, the number of fin chambers in the dorsal fin serves as a diagnostic character. Epigonichthys differs from Branchiostoma in that it possesses only one series of gonads on the right side of the body, whereas the gonads are paired in Branchiostoma.

Distribution

Cephalochordates are known from continental shelves of the tropic and temperate regions worldwide.

Habitat

Cephalochordates are semi-sessile filter feeders. They can swim vigorously, forward and backward, yet they live most of their life buried halfway in the substrate. Cephalochordates are found in shallow coastal areas, where they inhabit sandy bottoms. They prefer stable, well-reutilated, and smooth-textured grains. Larvae are planktonic and may drift over long distances before colonizing a suitable habitat.

Behavior

Cephalochordates can be found in high densities of over 9,000 animals per square meter. Sexes in cephalochordates are separate, and males and females are equally abundant within a population. Recruitment of larval stages influences the age structure in a population, which in turn depends on the currents.

Depending on the coarseness of the habitat, cephalochordates assume different feeding positions. In coarse sediment they bury their body entirely within the substrate. The preferred position is with their anterior end exposed to the open water. In fine sediment they lie on the substrate.

Feeding ecology and diet

Cephalochordates feed on plankton with plant cells, diatoms being the most important. The entire branchial basket is an adaptation for feeding with the endostyle continuously producing a mucus-net that is transported over the gill slits by cilia and capturing particles out of the water. The food particles together with the mucus net are rolled into a mass and passed on into the esophagus and the posterior intestine where they are digested.

Reproductive biology

Eggs are about 0.00394 in (100 µm) in diameter and contain numerous yolk droplets. Spermatozoa are simple and typical for marine invertebrate species with external fertilization.

Eggs and sperm are released into the water. How spawning is synchronized is not known. In subtropical populations the reproductive period stretches over several months and individuals spawn repeatedly. In temperate areas the reproductive period is shorter.

Total, radial cleavage is followed by the formation of a blastula, then gastrulation and neurulation ensue, and a peculiarly asymmetric larva is formed. The large mouth is situated on the left side. A single row of gill slits forms and the larvae start feeding immediately. More gill slits are added and later an additional row of gill slits forms on the right side. When about 12–15 pairs of gill slits are formed the larvae sink to the bottom and metamorphose into juveniles. This metamorphosis mainly involves the formation of the atrium by an outgrowth of the epidermis that surrounds the pharynx. Simultaneously the gill slits are divided and new gill slits are added. From now on the animals only grow in size, adding additional gill slits and segments while developing the gonads to full maturity. The larval period in the plankton lasts from several weeks up to a few months. In the following year the animals become sexually mature. Offshore plankton samples yielded relatively large cephalochordates that resembled larval stages but had developed rudimentary gonads. These were described under the genus name Amphioxides. However, it is more likely that these forms are merely larval forms of typically benthic species that did not encounter a suitable substrate rather than representing a genus with a different life history.

Conservation status

No cephalochordate species is regarded as threatened.

Significance to humans

It is believed that the "crocodile fish" that occurs in southern Chinese mythology is the cephalochordate species Branchiostoma belcheri. The mythical hero Han Yü killed many crocodiles, some of which escaped mortally wounded to the Xiamen region before they died. The maggots that emerged from the crocodile carcasses gradually changed into cephalochordates. This myth originated in the Tan Dynasty (a.d. 616–905).

In southern China local fishermen, deploying traditional techniques, fish for Branchiostoma belcheri. The catch is used for human consumption. Branchiostoma lanceolatum occurs as an appetizer on a gala dinner menu in honor of the influential German zoologist Ernst Haeckel. The greatest importance of cephalochordates to humans, however, is cultural: they occupy a prominent place as a linchpin for the theory of evolution.

Species accounts

List of Species

Smalltail lancelet
Florida lancelet
European lancelet
Epigonichthys cultellus
Bahama lancelet

Smalltail lancelet

Branchiostoma belcheri

order

Amphioxiformes

family

Branchiostomatidae

taxonomy

Branchiostoma belcheri Gray, 1847, Borneo. Possibly three subspecies: B. belcheri belcheri, B. belcheri japonicus, B. belcheri tsingtauense.

other common names

(Listed names were originally coined for the European species Branchiostoma lanceolatum. Because of the morphological uniformity of cephalochordates they are usually used for all species.) English: Amphioxus, lancelet; French: Amphioxus, lancelet; German: Amphioxus, Lanzettfischchen; Spanish: Anfioxo, pez lanceta, lanceta.

physical characteristics

Sixty-five (61–69) myotomes: 38 (34–43) preatriopore, 17 (14–21) atriopore to anus, 10 (8–12) postanal. Two hundred ninety (222–360) dorsal fin chambers. Reaches up to 2.8 in (70 mm) in total length.

distribution

A widespread species found from southern Japan, along the Chinese coast, the Philippines, the northern Australian coast, to Madagascar, and southeastern Africa.

habitat

Sandy bottoms from 3.28–82.02 ft (1–25 m) in depth.

behavior

Seems to avoid mud.

feeding ecology and diet

Consumes microscopic plankton, mainly diatoms.

reproductive biology

Reproductive season in Amoy (China) lasts from May to July. Second reproductive period seems to occur in December.

conservation status

Not threatened. For cultural reasons, Branchiostoma belcheri is protected in the "XIAMEN Rare Marine Creatures Conservation Area."

significance to humans

In some villages at the southern coast of China the majority of fishermen were fishing for Branchiostoma belcheri. Although the importance of this specialized and traditional fishery has declined, it is still viable in the area around Xiamen. The animals

are intended for human consumption. Some animals also are fished for academic purposes and support active research programs around the world.

Florida lancelet

Branchiostoma floridae

order

Amphioxiformes

family

Branchiostomatidae

taxonomy

Branchiostoma floridae Hubbs, 1922, Tampa Bay, Florida.

other common names

physical characteristics

Sixty (56–64) myotomes: 36 (33–40) preatriopore, 16 (14–17) atriopore to anus, 8 (6–11) postanal. Two hundred eighty-six (206–349) dorsal fin chambers. Reaches up to 2.3 in (59 mm) in total length.

distribution

Gulf of Mexico from southwestern Florida westward to Texas. Maybe also along Central American coasts from Mexico to Venezuela.

habitat

Sandy bottoms from 1.64–98.42 ft (0.5–30 m) in depth.

behavior

Probably same as for family.

feeding ecology and diet

Probably same as for family.

reproductive biology

Reproductive period occurs in Tampa Bay, Florida, from late spring to late summer. Individuals can spawn, ripen, and spawn again in the same breeding season.

conservation status

Not threatened.

significance to humans

Branchiostoma floridae occurs in very shallow water where it is accessible without boats. A population in Tampa Bay, Florida, has recently become a major source especially for molecular studies of embryonic development.

European lancelet

Branchiostoma lanceolatum

order

Amphioxiformes

family

Branchiostomatidae

taxonomy

Branchiostoma lanceolatum Pallas, 1774, Cornwall, England.

other common names

English: Amphioxus, lancelet; French: Amphioxus, lancelet; German: Amphioxus, Lanzettfischchen; Spanish: Anfioxo, pez lanceta, lanceta.

physical characteristics

Sixty-one (59–65) myotomes: 36 (34–38) preatriopore, 14 (11–17) atriopore to anus, 11 (10–14) postanal. Two hundred twelve (183–288) dorsal fin chambers. Reaches up to 2.4 in (60 mm) in total length.

distribution

Most widespread species in the genus. Reported from the North Sea to the Mediterranean and the Black Sea. Also found in the Suez Canal, off the East Africa coast, Oman, India, and Sri Lanka. Specimens from the Solomon Islands also may belong to this species.

habitat

Sandy bottoms from 3.28–196.85 ft (1–60 m) in depth.

behavior

Probably same as for family.

feeding ecology and diet

Semi-sessile filter feeders. Feed on plankton organisms, especially diatoms, but foraminiferans, radiolarians, and cladocerans were also found in content analyses of the gut.

reproductive biology

Reproductive period starts in April in the Mediterranean, and in June/July in the North Sea. Spawning seems to occur in the evening; spermatozoa are shed before the eggs.

conservation status

Not threatened.

significance to humans

Several European marine biological stations supply this species to educational and research institutions.

No common name

Epigonichthys cultellus

order

Amphioxiformes

family

Branchiostomatidae

taxonomy

Epigonichthys cultellus Peters, 1877, Moreton Bay, Australia.

other common names

physical characteristics

Gonads on right side of body only. Fifty-one (48–52) myotomes: 31 (29–36) preatriopore, 15 (13–18) atriopore to anus, 9 (9–13) postanal. Two hundred twenty (180–254) dorsal fin chambers. Reaches up to 1.6 in (40 mm) in total length.

distribution

Solomon Islands, Queensland, Indonesia, Philippines, western Australia, Thailand, Sri Lanka, and Tanzania.

habitat

Sand and mud from about 13.12–65.6 ft (4–20 m).

behavior

Probably same as for family.

feeding ecology and diet

Probably same as for family.

reproductive biology

Probably same as for family.

conservation status

Not threatened.

significance to humans

None known.

Bahama lancelet

Epigonichthys lucayanus

order

Amphioxiformes

family

Branchiostomatidae

taxonomy

Epigonichthys lucayanus Andrews, 1893, Alice Town, North Bimini, Bahamas.

other common names

physical characteristics

Gonads on right side of body only. Sixty (55–62) myotomes: 37 (35–52) preatriopore, 16 (13–18) atriopore to anus, 8 (6–9) postanal. Three hundred seven (167–484) dorsal fin chambers. Reaches up to 0.83 in (21 mm) in total length.

distribution

Tropical and subtropical regions of Atlantic, Pacific, and Indian Oceans.

habitat

Coral sands to mud flats, from 0–65.61 ft (0–20 m) in depth (up to 3,001.97 ft [915 m] for larvae).

behavior

Probably same as for family.

feeding ecology and diet

Semi-sessile filter feeder. Feeds on plankton organisms, especially diatoms and coccolithophorids.

reproductive biology

Probably same as for family.

conservation status

Not considered threatened.

significance to humans

None known.

Resources

Books

Gans, C., N. Kemp, and S. Poss, eds. The Lancelets (Cephalochordata): A New Look at Some Old Beasts. The Results of a Workshop. Volume 42, Israel Journal of Zoology. Jerusalem: Laser Pages, 1996.

Gee, Henry. Before the Backbone. London, Weinheim, New York, Tokyo, Melbourne, Madras: Chapman & Hall, 1996.

Harrison, Frederick W., and Edward E. Ruppert, eds. Microscopic Anatomy of Invertebrates. Vol. 15. Cephalochordata (Acrania). New York, Chichester, Weinheim, Brisbane, Singapore, Toronto: Wiley-Liss, 1997.