p1: Hi and welcome to amphioxus biology 101.
p1: In this video we are going to look at the distribution of amphioxus
p1: and we will describe the different species, anatomy, and life cycle of these animals.
p1: At the end of this chapter, you should be able to identify the different
p1: morphological structures of an adult amphioxus,
p1: explain basic features of amphioxus biology,
p1: and you should also be able to describe the life cycle of amphioxus.
p1: As we said in the 1st chapter, amphioxus are small animals
p1: that measure only a few centimetres long
p1: and live burrowed in the sand.
p1: They are filter feeders and can be found in almost all the temperate
p1: and tropical sea waters around the world
p1: as long as the sediment is convenient.
p1: Amphioxus usually live in shallow water at low depths
p1: between 1-50 meters. However, some species were described at more than 200 meters
p1: in a very specific ecosystem, which is a whale fall.
p1: They are 3 genuses of cephalochordates, the Branchiostoma in which more than 30 species
p1: have been described and to which belong the European species Branchiostoma lanceolatum.
p1: There is also the Epigonichthys genus, which is closely related to Branchiostoma,
p1: and finally the Asymmetron genus.
p1: In Epigonichthys and Asymmetron species
p1: there is only one row of gonads on the right side of the adults,
p1: which is different from what we can observe in Branchiostoma.
p1: Moreover, in Asymmetron and Epigonichthys, there are very few species described.
p1: Why are few species described? Because until now
p1: species description was mainly based on morphological and anatomical features
p1: of the animals, and cephalochordates very much look like each other.
p1: Today we have some molecular data that show that there are many more amphioxus
p1: species than previously thought.
p1: If we have a look at the morphology of the adult amphioxus, we can observe that
p1: they have a typical chordate body plan.
p1: They have a dorsal hollow nerve tube
p1: which is enlarged in entire region so this territory is called the cerebral vesicle
p1: and under this hollow nerve tube they have a notochord.
p1: The notochord is like a rigid stick and it is a synapomorphy of chordates.
p1: Chordates include cephalochordates, vertebrates - to which we belong
p1: - and tunicates.
p1: In tunicates (urochordates), there is a notochord but only in the tail region
p1: at the larval stage, and this tail might disappear after metamorphosis.
p1: In vertebrates, the notochord forms during early development,
p1: and it is a very important signaling center.
p1: However, this notochord almost completely
p1: disappears during late development in vertebrates.
p1: In amphioxus, the notochord is observed all along the life of the animal
p1: and runs all along the body, extending beyond the cerebral vesicle in the
p1: entire region, this is why amphioxus
p1: are called cephalochordates.
p1: In the ventral part of the adult amphioxus, we can observe the digestive tube.
p1: In the anterior part, there is a mouth surrounded by cirri that impede
p1: the entry of big particles.
p1: Amphioxus are filter feeders, so they will ingest the sea water
p1: and the small particles that are in the sea water will stick to a mucus
p1: that is produced in the ventral part of the pharynx by a structure called
p1: the endostyle, which is homologous to the vertebrate thyroid gland.
p1: This will form a food string that will move towards the intestine thanks to
p1: the beating of the cillia, of the digestive tube cells.
p1: The digestive tube is associated to a digestive gland, which is called the
p1: hepatic cecum, and which is thought to be homologous to the vertebrate liver.
p1: After digestion the excrement will be expulsed through the anus of the animal.
p1: The ingested water passes through perforations of the pharynx that are
p1: called pharangyal slits, or gill slits,
p1: despite the fact that we know amphioxus don't have gills.
p1: This water enters a cavity, which is called the atrium, which covers the whole pharynx.
p1: Finally, the water will be expelled through the atriopore thanks to the contraction
p1: of the atrium wall muscles.
p1: Amphioxus also prosesses, like vertebrates, striated muscles, segmented gonads
p1: and a post-anal tail. However, contrary to vertebrates, they don't have a
p1: mineralised skeleton, derivatives of migrating neural crest cells
p1: and well-developed heads with paired sensory organs.
p1: This is why amphioxus is often considered as vertebrate-like but simpler,
p1: which is the reason why zoologists of the 19th century
p1: were very interested in these animals supposed to be the ancestor of vertebrates.
p1: However, as you might know (I hope you know), extant animals cannot be
p1: ancestors of extant species.
p1: Cephalochordates are gonochoric animals, meaning some individuals will only produce
p1: oocytes, whereas the others will only produce sperm.
p1: The gonads will produce mature gametes only during several months of the year
p1: and this season, called the spawning season or the breeding season, is different
p1: from one species to another.
p1: During this period, adult amphioxus might spawn once
p1: or several times after the gonads refill.
p1: All the cephalochordate species we did so far spawn at sunset.
p1: They do so by getting out of the sand,
p1: swimming towards the surface, the seawater surface and releasing
p1: their gametes through the atriopore in the water column.
p1: After fertilization, embryonic development starts and several stages follow
p1: one another until the embryo becomes a larva.
p1: Amphioxus embryos and lava live in the plankton,
p1: and so amphioxus have a so-called typical bentho-pelagic life cycle
p1: like it is also the case for many other marine non-vertebrate species.
p1: The larva will stay in the plankton and feed on phytoplankton
p1: during several weeks to several months, and then this larva will undergo a process
p1: called metamorphosis and will become a juvenile.
p1: This juvenile will go back to the seafloor, and after several months, mature gonads
p1: will develop and the juvenile will become an adult, able to reproduce.
p1: Embryonic development is very similar between the different cephalochordate
p1: species studied so far.
p1: After the gametes fuse to form the zygote, the fertilization membrane, also
p1: also called chorion, will detach from the zygote
p1: as it is also the case in other marine animals like sea urchins.
p1: Then the 1st division starts, and the embryo forms a morula.
p1: After this stage, blastulation occurs and the embryo acquires a bowl shape,
p1: consisting of a single layer of cells surrounding a cavity, which is called
p1: the blastocoel.
p1: Then part of the blastula flattens,
p1: and this region, which is called the vegetative plate will thereafter form
p1: the mesoendermal layer of the embryo.
p1: This plate will invaginate during gastrulation
p1: and two embryonic layers will be formed:
p1: the ectodermal cell layer and the mesoendodermal cell layer,
p1: and the first opening of the embryo, the blastopore, will also form during
p1: this gastrulation process.
p1: The external cell layer of the gastrula is called the ectoderm
p1: and will give rise to the central nervous system and to the epidermis,
p1: whereas the internal layers, the mesendoderm
p1: will give rise to mesodermal and endodermal structures.
p1: At this stage, all the cells of the ectoderm have cilia, and the embryo starts
p1: to move inside the chorion.
p1: Until this stage, embryonic development looks very much like
p1: what is observed in sea urchins.
p1: At the end of gastrulation the embryo hatches, neurulation starts,
p1: and embryogenesis becomes vertebrate-like.
p1: In the dorsal part of the embryo,
p1: the ectoderm first forms a neural plate
p1: that will roll itself to form a neural tube,
p1: whereas the rest of the ectoderm will detatch
p1: to form the epidermis that will come to cover the neural tube
p1: and to close the blastopore.
p1: In the dorsal axial parts of the mesendoderm, the notochord will form
p1: whereas in the paraxial region, the somites will form by an enterocoelic process.
p1: These somites are homologous to the ones of vertebrates, and will form
p1: the body muscles of amphioxus.
p1: Once the neural tube and the first somites are in place, the embryo elongates
p1: in the posterior region by adding new somites.
p1: At the late neural stage, the pharyngeal region, which corresponds to the
p1: anterior endoderm, anterior digestive tube
p1: will enlarge and the mouth will open on the left side,
p1: whereas the first pharyngeal slit will open on the right side.
p1: At the same time, the endostyle and the club shaped gland, which is a structure specific
p1: to amphioxus embryos, will develop in the pharyngeal region.
p1: So the embryo is now a larva and the anus opens.
p1: This larva feeds on phytoplankton and will keep on adding new somites in the
p1: posterior region and new pharyngeal slits.
p1: Once the number of pharyngeal slits reaches a value which depends on the species,
p1: the larva will undergo metamorphosis.
p1: The amphioxus larva is asymmetric, with the mouth being on the left side and
p1: the pharyngeal slits on the right side.
p1: Moreover, somites are also asymmetric, with the left somites more anterior
p1: ﻿than the right ones.
p1: During metamorphosis, several morphological modifications occur:
p1: first, the slits duplicate and half of them migrate to the other side of the body
p1: so that at the end, we have two rows of pharyngeal slits in the adult.
p1: The mouth will also migrate to a ventral midline position,
p1: so at the end the adult will show a perfectly symmetric pharyngeal region,
p1: contrary to what we can see in the larva.
p1: At the same time, the atrium will form so the atrial wall will grow and fuse in
p1: the ventral region, forming a cavity around the pharynx which is called the atrium.
p1: And the hepatic cecum, which is not visible in the lava, will grow.
p1: Finally during metamorphosis, the epider mal cells of the lava will lose their cilia
p1: and the juvenile will then go back to the sea floor.
p1: In vertebrates, like amphibians and flat fish, metamorphosis is controlled
p1: by the thyroid hormone T3, also called triiodothyronine,
p1: and this hormone acts through the binding to specific receptors that are called
p1: thyroid hormone receptors or TR alpha and beta.
p1: In amphioxus it has been shown that metamorphosis
p1: is not controlled by T3, but by a derivative of T3, which is called Triac.
p1: This molecule also acts through the binding to the unique thyroid hormone
p1: receptor TR of amphioxus.
p1: This means that metamorphosis controlled by thyroid hormone receptors
p1: which would bind to T3, or a derivative of T3,
p1: is an ancestral feature in the chordate clade.
p1: So to conclude, in this chapter we have seen that we can find amphioxus all around
p1: the world in the sandy bottom of tropical and temperate shallow waters,
p1: that they have a classic bentho-pelagic life cycle,
p1: that they are filter feeders with a mor phology that is vertebrate-like but simpler,
p1: and that the external development is echinoderm-like until gastrulation
p1: and becomes similar to vertebrate embryonic development at later stages.