The origin of vertebrates

In this blog we have talked many times about the evolution of different groups of vertebrates: amphibians from lobe-finned fishes, birds from a group of dinosaurs, cetaceans from a group of terrestrial mammals or even about human evolution. But, when and how did vertebrates originate? The answer to this question is below.

INTRODUCTION

People, unconsciously, use incorrectly the term “vertebrate” because this concept is often used to refer what is, in fact, Craniata. From a biological point of view, vertebrates, which are characterized by the presence of a vertebral column surrounding the neural tube, include all tetrapods (amphibians, reptiles, birds and mammals) and all fish except hagfish. When we include all fish, also the hagfish class, then the correct concept is Craniata, characterized by the presence of skull.

Phylogeny of main groups of Chordata  (Picture: from Yo Evoluciono).

Craniata, with Cephalochordata (which includes Amphioxus) and Tunicata or Urochordata (which includes ascidians, salps, pyrosomes and larvaceans), constitute the Chordata.

Craniata, Cephalochordata and Urocordata constitute the Chordata (Picture: Noelways).

THE ORIGIN OF CHORDATA

Before focusing on the origin of the Craniata, let me mention the most accepted theory to explain the origin of chordates, which resulted in Craniata and vertebrates.

This is the Garstang’s theory. It proposes that the origin of chordates, which took place about 570 million years ago during the Precambrian, must be from some deuterostomes (animals in which the mouth is not formed from the blastopore of the early stages of development and that includes echinoderms, hemichordates and chordates). The larvae of these animals have some similarities with a chordate body plan: bilateral symmetry, unidirectional digestive tract and an adoral and circumoral ciliated band. Thus, it is believed that a deuterostom larva, perhaps the auricularia larvae of echinoderms, would have resulted in the evolutionary line of chordates.

Garstang’s theory (Picture: University of Saskatchewan).

Chordates are a phylum of animals, with 55,000 different species. Although all members included in the group of chordates have very different aspects, they can be grouped by the fact that at some point in their development have 5 unique characteristics (synapomorphies):

• Notochord: it is a longitudinal rod positioned in the back of the body, which has the function of preventing the shortening of the body.
• Epineuria: the condition of the central nervous system present in the dorsal part of the body, above the notochord.
• Pharyngeal gill slits: they are openings communicating outside with the pharyngeal cavity.
• Endostyle: is a ventral groove with glandular walls located in the pharynx, the function of which is to secrete mucus to catch food, captures iodine and produce thyroid hormones.
• Caudal fin: a tail that extends beyond the anus.

5 unique features let the classification of Chordates (Picture: askIITians).

WHEN AND HOW DID THE VERTEBRATES APPEAR?

The increasing complexity of chordates gave rise to Craniata, which originated 544 million years ago in the Cambrian seawater.

Simplifying, one group of Prochordates (Urochordates and Cephalochordata) resulted in a procraniata. This was a benthic marine animal (fixed to the bottom) that fed by filtering suspended particles from the water. Thus, the increased mobility of the animal produced skeletal and muscle modifications, which would lead to Craniata. Here, there are some adaptations that led the evolution of Craniata:

• Changes in the muscles and skeleton: the rise in the skeleton would have allowed the increase of the segmented musculature (myomeres), which amended the form: V-shaped (amphioxus) to W-shaped, allowing better control movement. In addition, replacement of notochord by vertebral column explains the origin of vertebrates.
• Physiology: internal systems were modified to meet higher metabolic needs. Therefore, there was the gradual disappearance of cilia of the pharynx for food (ciliary pump) and appeared muscles (muscle pump). With the emergence of the gills, pharynx had a respiratory function. Muscles and digestive glands in the digestive tract also appeared. In addition, the heart and kidneys were originated.
• Nervous System: to locate and capture prey, chordates developed a more complex nervous system. In the frontal part of the neural tube, the brain was originated and was protected by the skull. Furthermore, it appeared a set of sense organs for receiving light, sound, taste, odors, electric current …

ROUGH OUTLINES IN THE DIVERSIFICATION OF CRANIATA

The origin of the Craniata occurred during the Cambrian with the appearance of agnates, that is, animals that had the characteristics of Craniata but had no jaws or any other grasping device. Currently, this group only includes lampreys and hagfish.

Lampreys and hagfish are the only agnates that have survived to nowadays (Picture: RTVE).

The next step was the emergence of the jaws during the Silurian. All jawed animals are included in the group gnathostomes. They include jawed fishes and tetrapods.

THE FIRST CRANIATA: THE OSTRACODERMS

The first vertebrates were a group of agnates called ostracoderms. They had many characteristics of Craniata. They were filter feeders with a powerful pharyngeal pump water into their mouth. The first ostracoderms had the body covered with bony plates. Thus, it is in this group where the bone first appears.

The ostracoderms include two main groups: Pteraspidomorphii and Cephalaspidomorphii

Pteraspidomorphii measured between 20 cm and 2 meters, with a small mouth surrounded by small plates, with well-developed eyes and unpaired fins. An example is Astraspis, which was between 12 and 35 cm long, it had an armor of  bone plates between 3-5 mm, its eyes were protected by plates, it had pharyngeal  slits and had sensory channels in the plates.

Astraspis (Picture: Deviant Art).

Cephalaspidomorphi were freshwater animals with a pair of pectoral fins, allowing them to control their body: tilt, deflection and rotation. An example is Cephalaspis, a small animal less than 30 cm long and covered with a heavy cuirass and a cephalic one-piece shield.

Cephalaspis (Picture: Rod6807, Creative Commons).

REFERENCES

• Apuntes de Cordados de la Licenciatura en Biología (UB).
• Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. Ed. McGraw-Hill (13 ed.)
• Liem, Bemis, Walker & Grande (2001). Functional Anatomy of the Vertebrates. An Evolutionary Perspective. Brooks Cole Pub (3 ed)

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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Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.

Marine unicorns: Pyrosomida

This week, it has been spread through Social Media a video of a marine animal that is hardly seen. A group of divers, while they was diving in the Philippines, had the opportunity to observe a marine unicorn or Pyrosomida. Or maybe not! Some professionals said that this is a Thysanoteuthis squid egg case. In fact, it’s easy to distinguish a Thysanoteuthis squid egg case from a Pyrosomida: while in the egg case the balls (eggs) are distributed forming a spiral, in the Pyrosomida is an homogeneous mesh of organisms.  In this article we will talk about Pyrosomida. 

INTRODUCTION

Pyrosomida are an order of marine animals included in the Chordata (a group that also includes vertebrate animals). In concrete, they are inside the Thaliacea class in the Urochordata or Tunicata group. There are 2,000 species widely distributed in all oceans, from close to shore to big depths. The reason of their name, Tunicata, is that they present a tough tunic that covers and protects the animal and that contains cellulose. Urochordata or Tunicata can be classified into three classes: Ascidians (or sea squirts), Larvacea and Thaliacea.

Urochordata classes. (A) Ascidian (Picture: Gronk, Creative Commons), (B) Larvacea (Picture: Rocco Mussat Sartor, Università degli Studi di Torino); and (C) Thaliacea (Picture: Mingorance Rodríguez, Creative Commons)

THALIACEA

The Thaliacea are pelagic organisms similar to a lemon or a barrel, with a transparent and gelatinous body; and for this reason is difficult to see them when they are in the sea surface. Each individual is constituted for a belt of circular musculature and an inhalant and exhalent siphon in opposed poles. They can live in two ways: while some of them are solitaries, that is that each individual live independently one from another; other form colonies that can measure some metres.

They get around with body contractions, so they bomb water through the body and propel themselves with a water stream. This also allow them to breath and to feed on the particles of the water.

Most of them are luminescent and produce bright light during the night.

PYROSOMIDA

As we said, Pyrosomida are a group of marine pelagic animals included in the Thaliacea, which have been explained in the previous section.

The individuals live grouped in colonies, that measure between 20 and 30 cm in the Mediterranean, despite in tropical seas they can be 4 meters long and, in extreme cases, more than 10 meters. Each individual is called blastozooid and measures few millimetres, the body present an oral siphon inside the colony and a cloacal outside. Despite each organism maintains its individuality, they live together under the same tunic. Colonies have an inner cavity, which communicates with the exterior through an opening.

They are filtering animals, so water pass through faringial fissures with feeding particles and, thanks to an organ that produces mucus (called endostyle) and Listers tongues, they form a feeding cord.

Concerning to reproduction, each individual release gametes inside the colony and then these are expeled outside. After fecundation, it is formed the oozooid, called ciatozooid, which produce four individuals by budding, and these constitute the tip of the new colony. These four organisms, by budding, produce the entire colony.

Pyrosomida are the most bioluminescent organisms of the zooplanckton because they produce a blue light that can be seen easily some metres under the water.

Young colony of Pyrosomida. These animal measured about 1 cm long. (Picture: Nick Hobgood, Creative Commons). 
Pyrosoma atlanctium (Picture: Show_ryu, Creative Commons)

REFERENCES

• Notes of the subject Chordata of the Degree in Biology of the University of Barcelona
• Hickman, Roberts, Larson, Anson & Eisenhour (2006). Integreted principles of Zoology. Mc Graw Hill (13 ed).
• Storer, TI; Usinger, RL; Stebbins, RC & Nybakken JW (1975). Zoología general. Ed. Omega (6 ed)
• http://www.sensaciones.org/down/aquanet/aquanet-34.pdf

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