Amphioxus: animals which wanted to be vertebrates

Amphioxus is the goal of this article, animals that are included in the Cephalochordata group, inside the Chordata phyllum. Cephalochordata is a group of marine animals placed between invertebrates and vertebrates. Here, we are going to explain the importance of this animals in Zoology and its biology. 

INTRODUCTION

Amphioxus, placed in the Cephalochordata subphyllum, is a marine animal in the Chordata group. Chordata includes, in addition to this group, Urochordata (among which there is Pyrosomida), hagfishes and vertebrates (fishes, amphibians, reptiles, birds and mammals). Despite they represent just a 4% of the amount of organisms in the planet (that correspond to 55,000 species), Chordata has had a very important evolutionary success.

The importance in Zoology of amphioxus is that present all the features of Chordata visible, so other chordata has lost them later or has modified them. These are the features:

• Notochord: dorsal bar placed under the nervous system with a skeletal function.
• Epineuria: dorsal position of nerve cord.
• Endostyle: ventral groove in the pharynx that produce mucus to catch food and also produce iodized compounds. This gives thyroid.
• Caudal fin: locomotive appendix.

Basic features of Chordata in a Cephalochordata (Picture obteined from here).

CEPHALOCHORDATA: AMPHIOXUS

Cephalochordata, known as amphioxus, is a group of 25 species of marine animals with a thin body, laterally compressed and transparent, that measures between 5 and 7 cm.

Brachiostoma lanceolatum (Picture: Hans Hillewaert, Creative Commons)

GENERAL ANATOMY

The skin of cephalochordata consists on one layer of prismatic cells with mucus glands that produce mucus, followed by the basal connective lamina and the dermis.

The most characteristic is notochord, which is composed by cells surrounded by a conjunctive case of actin and paramyosin. These cells have neurons that come from the nerve cord, allowing their contraction in diameter.

General anatomy of a cephalochordate. 1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill’s slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum 20. swim bladder 21. lateral line (Imatge: Piotr Michał Jaworski, Creative Commons)

They are swimming animals, with several fins: they have a dorsal fin, with vesicles placed one after another; a caudal fin and an anal fin, that extends from caudal fin till atriopore, opening from where water leaves the body. This anal fin bifurcates in two sheets and give place two folds to slightly stabilize them, which are known as metapleural folds.

They have a series of muscular fascicles called myomeres, which are in a shape of V with the apex in a forward position.

Oral region has an oral hood cirri to distinguish the entering particles, the Wheel organ (produce water movements) and a diaphragm to regulate the water entrance into the body. Pharynx is perforated for 80 fissures wit the endostyle in the basis, that produce mucus and it is pick into a dorsal lamina, where there are a small bars and then goes to oesophagus.

FUNCTIONS

In order to feed, water with particles gets in through the mouth, it is propelled by the oral hood cirri and then cross the gill’s fissures, where food gets stuck thanks to mucus produced by endostyle, and finally goes to intestines. Here, food particles go to an hepatic cecum and phagocytosis process takes place. Then, water goes to the inner cavity of the body (called atrium) and leaves the body through a pore (atriopore). Digestive system is composed by the oral system, the pharynx with endostyle, the oesophagus and a digestive tube without muscles; which is composed at the same time by the intestine, the hepatic cecum (produce enzymes and absorb nutrients) and the anus, placed in the left side of the body. Its movement is due to a cilium ring.

Circulatory system doesn’t have heart and consists on two circuits: the ventral circuit goes from caudal fin to head and the dorsal, the other way around. The circulatory liquid goes to pharynx fissures to become oxygenated and has amebocytes, but it has not respiratory pigments, so breathing takes places by diffusion.

Excretory system is formed by solenocytes, cells that filter the blood from arteries, placed in the nefritic crest, that connects the atrium with a channel, so that allows that excretory products are expelled with the water in the atrium.

Nervous system consists on a simple nerve cord with a vesicle in the anterior part. This cord, in each metamere, emits two dorsal mixed nerves (with sensitive and motor nerves), which are branched off in two branches: a sensitive dorsal branch and a mixed ventral branch. This ventral branch goes to viscera, tegument and muscles. Sensitive system is constituted by a pigment spot (sensitive to light) and chemoreceptors.

About reproduction, each animal has just one sex (dioic animals), but its anatomy is very similar. They present between 25 and 38 gonads and to do the lay, the body wall is broken.

HABITAT

Amphioxus lives buried in sand seafloor of the shallow and coastal waters and in estuaries all over the world.

Common amphioxus (Branchiostoma lanceolatum) (Picture from UniProt)

REFERENCES

• Notes of the Chordata subject of the Degree in Biology of the University of Barcelona
• Brusca & Brusca (2005). Invertebrates. Ed. Mc Graw Hill (2 ed)
• Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Integrated principles of Zoology. Ed. Mc Graw Hill (13 ed)
• Cover picture: Ricardo R. Fernandez

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