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Amphioxus – Habitat, Habit and External Features

amphioxus (lancelet)

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Amphioxus has long been of considerable interest to zoologists and there has been a great deal of investigation into its structure and development. While there is no basic chordate type, Amphioxus, the lancelet is closet to our idea of the primitive ancestors of the craniates. In addition to its chordate characters it has many primitive and many specialized features. It is perhaps best regarded as a modern chordate which has diverged relatively little from a more generalized common chordate ancestor.

The lancelet is found in shallow coastal waters around the Mediterranean, the North sea and the English Channel. It is also extremely common on the Chinese coast near Amoy; at certain seasons of the year, it is so plentiful that it becomes a favored article of food and is sold in the markets. The related genus Asymmetron has a tropical and southern distribution, while Amphioxides  is a pelagic form. The species described here is Amphioxus lanceolatus (Branchiostoma lanceolata). It inhabits localities where the sea-floor is sandy and spends most of its time almost buried, in a vertical position, with the head end protruding. Being a ciliary feeder, it subsists on microscopic creatures in the plankton; the method entails almost ceaseless feeding. Occasionally, it swims, especially at night, by rapid and sinous lateral movements of the whole body. Sometimes it lies passively on its side on the sand. It is able to burrow with great rapidity, proceeding with the pointed end  foremost and the expose part of the body vibrating rapidly from side to side. The sexes are separate and fertilization is external. There is a free- swimming larval stage which shows pronounced asymmetry; this feature persists in the adult, but is not so easily recognized.

A.lanceolatus has a whitish translucent body, compressed laterally and about 40 to 50 mm in length. It is pointed at both ends. There is a shallow mediam dorsal fin along the whole length; at the posterior end it passes into a slightly deeper triangular caudal fin around the tail. Ventrally, the caudal fin is continuously with a shallow ventral fin which proceeds forward for about a third of the body length to end at a prominent ventral opening, the atripore, the ventral surface is somewhat flattered and at its sides are the two metapleura folds. These pass forward to the anterior end where they merge into the borders of the oral hood. This lies dorsally and laterally over the anterior end its large aperture may be considered to the moth. From its edge, about twenty stiff cirri radiate outwards. The anus lies on the left side near the end of the body just above the ventral fin.

Because of the semi-transparent nature, the major internal structures can be seen. The V-shaped myotomes, with the apex of the V pointing forward, are prominent; there are about sixty. Small fin-ray boxes supporting the dorsal and ventral fins can be seen with a lens. Below these lies the nerve cord, easily discernible by reason of black pigment spots along it. Beneath the nerve cord is the notochord, characterized by its extreme anterior and posterior extension. In immature specimens, the narrow parallel gill bars show that the pharynx extends from a position immediately behind the oral hood almost half-way down the body. But in mature animals, most of the pharynx is obscured by the row of twenty-six gonads on each side. The whole pharynx is covered laterally and ventrally by downgrowths of the body wall enclosing a cavity called the anrium.

The lower surface of the oral hood of amphioxus is wrinkled by a number of finger-like depression with ridges between them. They constitute the ciliated wheel-organ. A deeper pit in the roof of the hood, to the left of the mid-line is called Hatschek’s pit its function is to secrete mucus. At the back of the oral hood is a vertical partition, the velum. This is pierced by a small aperture sometimes known as the mouth. Since, however, it leads directly into the pharynx it is perhaps preferable to refer to it as the enterostome.

Amphioxus are a group of 30 to 35 species of “fish-like” benthic filter feeding chordates in the subphylum Cephalochordata. Also called lancelets, they are the modern representatives of a group of ancient, segmented coelomate deuterostomes.

Amphioxus (genus Branchiostoma, Latin for “sharp at both ends”) are widely regarded as living proxies for the vertebrate ancestor. Lancelets have many structural features similar to vertebrates, including a notochord, segmental muscles and pharyngeal gill slits.

Morphology

Amphioxus are benthic filter feeding chordates that comprise the subphylum Cephalochordata. They are seldom more than 8 cm (3 inches) long and resemble small, slender fish without eyes or a definite head. The notochord (or stiffening rod), gill slits and dorsal nerve cord, characteristic features of chordates, are well-developed in amphioxus.

Amphioxuses reproduce sexually and are ovoviviparous, producing eggs that hatch into microscopic ciliated larvae that are carried with ocean currents to sandy sediments where they mature and enter adulthood. The gonads, the reproductive glands of the male and female, form in a row on the wall of the atrial cavity (Fig. 29-6) and are excreted into the water, where the fertilized eggs are carried by ocean currents until they are discharged from the body. The larvae are planktonic and live for a few weeks before they metamorphose into adults that scavenge nutrients in sandy sediments.

The genome sequence of amphioxus is the most comprehensive and accurate to date for a chordate, and provides a unique opportunity to understand the evolution of gene regulatory elements in a chordate lineage. Genome-wide comparisons of human and amphioxus have identified conserved non-coding sequences that function as tissue-specific enhancers. Excluding transcribed sequences and repetitive sequences, 77 putative ancient chordate conserved non-coding sequences were identified by genome-wide dot plot analysis of amphioxus scaffolds aligned to human chromosomes (Methods). These are the oldest non-coding sequences conserved between mammals and other vertebrates. Using a combination of sequence similarity, genomic location and phylogenetic tree reconstruction, we have assigned these to 17 ancient chordate linkage groups (CLGs).

Pairwise comparisons of stage-specific RNA sequencing data from amphioxus, zebrafish, medaka and frog (Xenopus tropicalis) reveal that the same pattern of gene expression is maintained throughout all developmental stages in these four species (Fig. 3a, b and Extended Data Fig. 1). This suggests that these genes have been retained in multiple copies since the second round of whole genome duplication following the divergence of amphioxus and vertebrates. The highest similarity values are observed for genes that are expressed in the gut and kidney. This is consistent with the hypothesis that the hepatic cecum of amphioxus evolved from a midgut diverticulum of the vertebrate ancestor.

Habitat of Amphioxus

The amphioxus, or lancelet, are small marine chordates (invertebrate subphylum Chordata) that are found widely on tropical and subtropical coasts. Seldom more than 3 in (8 cm) long, they resemble small, slender fishes without eyes or a definite head. They are grouped in two genera, Branchiostoma or Amphioxus, and Epigonichthyes or Asymmetron. They are of great theoretical interest because of their very primitive condition of organisation. Their digestive, vascular, excretory and nervous systems have retained many features of the embryonic stages of the chordates.

Amphioxus have a dorsal hollow nerve cord and gill slits. They also have a notochord, which is a stiffening rod or kephale that runs from the brain end to the anterior portion of the body called the rostrum. This is one of the principal characteristics that distinguish amphioxus from other vertebrates.

Despite their marine origins, amphioxus are extremely sensitive to environmental changes and can be significantly affected by human disturbances of the habitat in which they live. In the present case study, a survey of amphioxus was carried out in seagrass habitats in the Luan River Estuary to establish trends and relationships between these organisms and aquatic environmental variables.

Amphioxus densities and biomass showed a declining trend in the studied area between 1999 and 2011. The highest densities (515 ind/m2) were observed at station l, close to the low-water mark, and the lowest (409 ind/m2) at station 10.

To determine factors that affect the density and biomass of amphioxus, we conducted a correlation analysis between the environmental variables of temperature (T), dissolved oxygen (DO), salinity (S), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N) and phosphorus (PO4-P). Statistical analyses indicated that DO and salinity were important variables for predicting amphioxus population dynamics, while DO and pH showed only a weak relationship with this species.

During this study, amphioxus B. belcheri tsingtauense were kept alive in aquaria to allow for the recording of morphological and meristic indices, which are more accurate than those recorded on dead individuals preserved in formaldehyde or alcohol. This allowed the collection of a large number of specimens to make statistically significant comparisons.

Reproduction and Genome of Amphioxus

Lancelets (Amphioxus, singular amphioxus) are little worm-like marine animals that spend much of their lives buried in the sea floor and filter-feed through their jawless, ciliated mouths. They resemble some ancient chordate ancestor and are therefore of special zoological interest. Their combination of primitive features such as a notochord, a dorsal hollow (tubular) nerve cord and gill arches or slits with a segmented body musculature that has been derived from somites and a post-anal tail is unique among invertebrates.

The genome of amphioxus has recently been sequenced using a whole-genome shotgun approach.30 This 520 megabase (Mb) genome assembly shows excellent base-level accuracy and long-range coherence, as assessed by comparisons to finished clone sequences and expressed sequence tags. The assembly is based on 19 pairs of chromosomes.

Extensive conserved synteny has been observed between the amphioxus genome and the genomes of various vertebrates, including humans. These observations are consistent with the hypothesis that amphioxus represents an ancient chordate ancestor that shared a common genome-scale event with vertebrates, which occurred before the split between urochordates and cartilaginous fish and bony vertebrates.

Amphioxus also provides an opportunity to study chordate embryonic development in a model system that has the same life history as the final adult and is amenable to genetic manipulations. Consequently, amphioxus is widely used in laboratories worldwide to study aspects of embryonic development and cell differentiation.

The life cycle of amphioxus is characterized by a transition from the zygote period to metamorphosis and beyond. Embryos and larvae are planktonic, but adults are benthic, although some species of amphioxus have evolved to be capable of a limited amount of drift from their habitat.

Branchiostoma lanceolatum grows to a maximum of about 6 cm in length. Embryos and larvae undergo ten developmental periods, from the zygote to metamorphosis. Each of these periods is accompanied by significant changes in gene expression patterns that are consistent with the hourglass model for evolution of chordates. The zygote and metamorphosis periods are planktonic and the juvenile and adult periods are benthic.

Activities and Habits of Amphioxus

Throughout the ecosystem, the primary role of amphioxus is to filter small particles from the water. As cephalochordates, amphioxus are largely passive feeders that do not move or hunt for their food. They spend most of their time burrowing in the substrate, with only their head exposed. The burrows are most frequent in clean gravel and sand, but also occur in silty sediments. The amphioxus’s mouth openings are furnished with a hood of fringelike structures called cirri that form a coarse filter to screen out larger particles, which are then passed by the gill basket into the body for digestion.

In the Dapu River Estuary, the most frequent habitat for Branchiostoma belcheri tsingtauense is sandy seafloor sediments of 5-15 m depth. During the summer of 2011, a correlation was observed between amphioxus density and sand grain size distribution: the higher the proportion of sand with granularity 0.063-0.5 mm, the higher the amphioxus density.

The genome of amphioxus provides insights into the last common chordate ancestor. Although the ancestor of amphioxus and vertebrates diverged early in the Cambrian period, their slower morphological and molecular evolution makes them good surrogates for the ancestral chordate genome in terms of gene content, exon-intron structure, and chromosome organization. In addition, amphioxus harbors genes duplicated at a small scale during the 2R whole-genome duplication in vertebrates.

Conserved non-coding sequences can also provide clues about ancient chordate cis-regulatory elements. Genome-wide comparisons between mammals and teleost fish have revealed up to 3,100 conserved non-coding sequences, but few are known from higher phylogenetic distances. In amphioxus, we identified 77 putative chordate conserved non-coding sequences (>60% identity over >50 bp) by searching for conservation outside of coding sequences and requiring conservation in at least one other vertebrate species.

These conserved sequences are concentrated in specific regions of the amphioxus genome, and several amphioxus scaffolds exhibit high hits to the same region of human chromosome 17 in the Oxford Grid. Based on this pattern, we can partition the human genome into 135 amphioxus-specific segments that retain relict signals of the ancestral chordate karyotype despite extensive chromosomal rearrangement.