Branchiostoma belcheri

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Branchiostoma belcheri
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Leptocardii
Order: Amphioxiformes
Family: Branchiostomatidae
Genus: Branchiostoma
Species:
B. belcheri
Binomial name
Branchiostoma belcheri
(Gray, 1847)

Amphioxus or lancelets (Branchiostoma) are members of the Chordata phylum of which all members have a notochord at some point while they are alive. [1] B. belcheri have a notochord, dorsal nerve cord, pharynx, buccal cavity, cirri, tail, dorsal fin, nerve cord, segmented muscle, and ocelli. They are distinguishable by a slightly round dorsal fin, eighty slender preanal fin-chambers, narrow caudal fin, and obtuse angles between fins. [2] They obtain food by filter feeding. [3] They were first reported in 1897 near the Amakusa Islands, specifically off Goshonoura Island, south of Amakusa-Kamishima Island. [4] These islands are located on the west coast of Kyushu, the island furthest south of the four main isles of Japan. In addition to the location of the siting, information regarding reproductive period and morphology was also obtained. [5] B. belcheri are gonochoric, reproducing via external fertilization. [6] B. belcheri are an endangered species, threatened by the influx of pollutants of land-based origin into the sea such as cleaning agents, chemical waste, garbage, mining waste, pesticides, petroleum products, and sewage. [7] [8] [9]

Contents

Anatomy and morphology

Branchiostoma belcheri exhibit all basic characteristics of the Chordata phylum including the notochord, dorsal nerve cord, pharynx, buccal cavity, cirri, tail, dorsal fin, nerve cord, segmented muscle, and ocelli. Starting with the internal structure, said organisms have true coeloms where the critical organs develop. [1] [3] The notochord runs down the animal's back with a smooth layer of connective tissue on the exterior, lining an interior core of cells that are loosely packed and contain fluid-filled spaces. [1] The outer sheath resists changes in fluid pressure in the notochord, allowing for flexibility while simultaneously withstanding contraction. In this way the notochord can be characterized as a hydrostatic organ. [1] During muscle contraction, the notochord also plays an important role in withstanding shortening of the body. [10] The dorsal nerve cord is located behind the notochord. The pharynx is another structure made from cartilage for feeding, found near the throat. It is composed of pharyngeal slits that are located between pharyngeal bars which are necessary for feeding. [1]

As for the external structure, the tail extends from the body, following the end of the digestive tract. This is a distinguishing characteristic from other worm types as the anus is located all the way at the bottom, on the back side of animal. The dorsal fin is located on the back side of the animal and is made rigid by cartilage. The nerve cord is found in a similar region and originates as a hollow tube. Within the nerve cord are small clusters of light-sensitive cells called ocelli. Finally, muscle allows for movement and maintenance of bodily position, which can be found in discrete segments along the sides of the body. [1]

Aside from the internal and external structures of B. belcheri, it is worth noting their movement. While typically sedentary, B. belcheri has swimming capabilities, moving in the upward direction. This is done via contractions that originate from the anterior end and move down longitudinal muscle one myotome at a time. Contractions alternate between the left and right side of the body, prompting side-to-side twisting that pushes the body forward and allows for spiral swimming about its axis. [10] In addition to these structures are those which produce eggs or sperm, also known as the gonads as well as organs homologous to those of vertebrates such as the pronephric kidney or an endostyle (ciliated, grooved organ that aids in filter feeding via production of mucoproteins). [1] [10] [11] It can be seen that B. belcheri exhibit a genome similar to that of vertebrates, however it is simpler as their genomes did not undergo entire duplications as did that of vertebrate lineages. [12] [13]

Branchiostoma belcheri have three major morphological characteristics that distinguish them from other Branchiostoma. Firstly, they are identifiable by their slightly round dorsal fin that has an obtuse end. Secondly, B. belcheri consist of more than eighty slender preanal fin-chambers. Thirdly, B. belcheri have a narrow caudal fin, as well as an obtuse angle between the dorsal and super-caudal fins, likewise between the prenatal and sub-caudal fins. [2]

Behavior

Branchiostoma belcheri are gonochoric, reproducing via external fertilization. [6] B. belcheri eggs initially develop into planktotrophic larva, later maturing to benthic adults. [14] They reproduce during the spawning season, which takes place every year for approximately three to six month, and occurs during the spring and summer. Males and females release gametes in the water column. [6] The gametes undergo development such that they exhibit ten distinct periods: zygote, cleavage, blastula, gastrula, neurula, talibud, larva, metamorphosis, juvenile, and adult. Those starting from zygote and ending with metamorphosis are identified as planktonic periods, while the juvenile and adult periods are benthic and live buried into the substrate. [15]

Evolution begins with the embryonic period, followed by a larval period, ceasing with the process of metamorphosis, which lasts anywhere from a few weeks to several months post-fertilization. Post-metamorphic juveniles closely resemble adults, differing only in the fact that juveniles require growth prior to gonad formation. This growth period varies in duration depending on the species, ranging from a few weeks in those characterized as "tropical" to several years for those identified as "temperate". Once gonads evolve, the organisms become sexually mature adults. [6]

Feeding

Branchiostoma belcheri reside in the ocean, specially digging its tail into the sand. They use their mouths to suck in large amounts of water, forcing it out via the slits and holding onto small food particles in the process. The space inside the mouth is known as the buccal cavity, while the outside of the mouth is surrounded by small tentacles called cirri. The mouth, buccal cavity, and cirri are all part of the digestive tract, as is the hepatic cecum which, in tandem with the intestine, functions to secrete digestive juices as well as aid in nutrient absorption. [1] B. belcheri depend on the process of filter feeding to obtain nutrients. Thus, they feed on microorganisms existing in the water taken up by ciliary action during respiration and brought into the pharyngeal cavity. The cilia take in the water, and also act to isolate the food particles. [10]

Distribution and habitat

Present distribution of B. belcheri is in the southern Ariake Sea. Early reports mark them near the Amakusa Islands. [16] Later, they were found on the opposite side of the Amakusa-Kamishima Island in the Ariake Sea. Another occurrence was reported in later accounts, placing the species in the northern Ariake Sea. [17] Once again, the species were observed in another distinct area; on the southern coast of Saga Prefecture. [17] Later observations concluded that amphioxus in waters other than the Ariake Sea and the Amakusa belonged to B. belcheri var. tsingtauense, while those in those regions were an intermediate form between this type and B. belcheri. [18] Amphioxus distribution shows large population numbers in the central portion of Ariake sea, as well as four locations surrounding the Amakusa Islands. [17] They mostly live in sand shores as well as dilute sea waters (15.4% to 33.1% salinities). [10]

As of 1998, Japan Fisheries Resource Conservation Association labeled B. belcheri endangered, listing the species in "Endangered Animals of Japanese Marine and Fresh Water Organisms". [7] Populations have been threatened by the influx of pollutants of land-based origin into the sea such as cleaning agents, chemical waste, garbage, mining waste, pesticides, petroleum products, and sewage. [8] [9] Additionally, aquacultural expansion, illegal fishing, sand mining and sediment transportation, and increasing maritime traffic all contribute to the elimination of amphioxus, as well as destruction of their habitat. [8]

Taxonomy

The occurrence of the amphioxus in the Ariake Sea was reported upon discovery of a small animal, similar to that of a fish, that was referred to as "itachi-uo" by the locals. [17] Further investigation involved obtaining samples from Saga Prefecture. These were originally named "Aratsu" or "Gnadotsu", but were later added to B. belcheri. [17]

Branchiostoma belcheri is related to other species of Branchiostoma as it is marine, distributed in shallow waters, positioned with its anterior end in the sand, mostly sedentary, has a small, slender body, is segmented and transparent, has a trunk, tail, median fins, myotomes, notochord, complete digestive tract, and dorsal nerve cord, is enterocoelous, respirates across body surface, uses protonephridia with solenocytes for excretion, performs external fertilization, has separate sexes and exhibits indirect development. [19]

Branchiostoma belcheri differ in number of genetic mutations. Mutations in at least one of the sequenced samples are present in twelve percent of genomic sites of the B. belcheri total genome. Approximately thirteen million variations were identified in each B. belcheri genome. Studies show that the ratio of number of transition to transversions for a pair of sequences was very low, suggesting a majority of mutations are recent with high mutation per generation. This data, along with high polymorphism rates, confirms the large-scale genetic diversity exhibited by B. belcheri. In this same study, over six-hundred variations were identified in the B. belcheri mitochondrial genome. Additionally, mutations in genes associated with phagocytic intracellular digestion likely influence the ability of organisms to phagocytize and digest food particles. [20]

Related Research Articles

<span class="mw-page-title-main">Chordate</span> Phylum of animals having a dorsal nerve cord

A chordate is a deuterostomic animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation.

<span class="mw-page-title-main">Vertebrate</span> Subphylum of chordates with backbones

Vertebrates are deuterostomal animals with bony or cartilaginous axial endoskeleton — known as the vertebral column, spine or backbone — around and along the spinal cord, including all fish, amphibians, reptiles, birds and mammals. The vertebrates consist of all the taxa within the subphylum Vertebrata and represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described.

<span class="mw-page-title-main">Hemichordate</span> Phylum of marine deuterostome animals

Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The class Graptolithina, formerly considered extinct, is now placed within the pterobranchs, represented by a single living genus Rhabdopleura.

<span class="mw-page-title-main">Hagfish</span> Family of eel-shaped, slime-producing animal

Hagfish, of the class Myxini and order Myxiniformes, are eel-shaped jawless fish. They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae. Hagfish are marine predators and scavengers. Hagfish defend themselves against predators by releasing copious amounts of slime from glands in their skin.

<i>Haikouichthys</i> Extinct genus of jawless fishes

Haikouichthys is an extinct genus of craniate that lived 518 million years ago, during the Cambrian explosion of multicellular life. Haikouichthys had a defined skull and other characteristics that have led paleontologists to label it a true craniate, and even to be popularly characterized as one of the earliest fishes. Cladistic analysis indicates that the animal is probably a basal chordate or a basal craniate; but it does not possess sufficient features to be included uncontroversially even in either stem group. It was formally described in 1999.

<span class="mw-page-title-main">Tunicate</span> Marine animals, subphylum of chordates

A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata. It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals. They are the only chordates that have lost their myomeric segmentation, with the possible exception of the 'seriation of the gill slits'. However, doliolids still display segmentation of the muscle bands.

<span class="mw-page-title-main">Notochord</span> Flexible rod-shaped structure in all chordates

In zoology and developmental anatomy, the notochord is an elastic rod-like structure found in many deuterostomal animals. Any species that has a notochord at any stage of its life cycle is, by definition, a chordate.

<i>Pikaia</i> Extinct genus of primitive chordates

Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "one of the most famous early chordate fossils," or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.

<span class="mw-page-title-main">Cephalochordate</span> Subphylum of lancelets

A cephalochordate is an animal in the chordate subphylum Cephalochordata. Cephalochordates are commonly called lancelets, and possess 5 synapomorphies, or primary characteristics, that all chordates have at some point during their larval or adulthood stages. These 5 synapomorphies are a notochord, dorsal hollow nerve cord, endostyle, pharyngeal slits, and a post-anal tail. The fine structure of the cephalochordate notochord is best known for the Bahamas lancelet, Asymmetron lucayanum. Cephalochordates are represented in modern oceans by the Amphioxiformes and are commonly found in warm temperate and tropical seas worldwide. With the presence of a notochord, adult amphioxus are able to swim and tolerate the tides of coastal environments, but they are most likely to be found within the sediment of these communities.

<span class="mw-page-title-main">Lancelet</span> Order of chordates

The lancelets, also known as amphioxi, consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are modern representatives of the subphylum Cephalochordata. Lancelets closely resemble 530-million-year-old Pikaia, fossils of which are known from the Burgess Shale. However, according to phylogenetic analysis, the lancelet group itself probably evolved around the Cretaceous, 97.7 million years ago for Pacific species and 112 million years ago for Atlantic species. Palaeobranchiostoma from the Permian may be part of the fossil record of lancelets; however, due to poor preservation, some doubt about its nature remains.

<span class="mw-page-title-main">Shark anatomy</span>

Shark anatomy differs from that of bony fish in a variety of ways. Variation observed within shark anatomy is a potential result of speciation and habitat variation.

Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by connective fascia (myosepta) and most easily seen in larval fishes or in the olm. Myomere counts are sometimes used for identifying specimens, since their number corresponds to the number of vertebrae in the adults. Location varies, with some species containing these only near the tails, while some have them located near the scapular or pelvic girdles. Depending on the species, myomeres could be arranged in an epaxial or hypaxial manner. Hypaxial refers to ventral muscles and related structures while epaxial refers to more dorsal muscles. The horizontal septum divides these two regions in vertebrates from cyclostomes to gnathostomes. In terrestrial chordates, the myomeres become fused as well as indistinct, due to the disappearance of myosepta.

<span class="mw-page-title-main">Lamprey</span> Order of jawless fish

Lampreys are an ancient lineage of jawless fish of the order Petromyzontiformes. The adult lamprey is characterized by a toothed, funnel-like sucking mouth. The common name "lamprey" is probably derived from Latin lampetra, which may mean "stone licker", though the etymology is uncertain. Lamprey is sometimes seen for the plural form.

In evolutionary developmental biology, inversion refers to the hypothesis that during the course of animal evolution, the structures along the dorsoventral (DV) axis have taken on an orientation opposite that of the ancestral form.

Branchiostoma floridae, the Florida lancelet, is a lancelet of the genus Branchiostoma. The genome of this species has been sequenced, revealing that among the chordates, the morphologically simpler tunicates are actually more closely related to vertebrates than lancelets. An embryo of a Florida amphioxus has a larval pharynx with gill slits that is asymmetrical. The gill slits in the larval pharynx form in the center of the embryo when it is in its earliest stage of development (primordial) meaning the thick layer of endoderm is overlapped by a thin layer; which aids into making the B. floridae asymmetrical from left to right. The lancelet Branchiostoma floridae maintains a high level of Fox transcription factor gene diversity, with 32 distinct Fox genes in its genome, and 21,229 clusters of cDNA clones, making it very useful to the research community.

<i>Branchiostoma lanceolatum</i> Species of lancelet

Branchiostoma lanceolatum, the European lancelet or Mediterranean amphioxus, is a lancelet in the subphylum Cephalochordata. It is a marine invertebrate with a notochord but no backbone and is used as a model organism to study the evolutionary development of vertebrates.

Buccal cirri are feeding structures found in the oral hood of primitive jawless organisms called amphioxus. The word buccal is derived from the term bucca which means "cheek" and cirri is derived from the Latin word cerrus meaning a tendril or a small and flexible appendage.

In biology, solenocytes are elongated, flagellated cells commonly found in lower invertebrates, such as flatworms, chordates and several other animal species. In terms of function, solenocytes play a significant role in the excretory systems of their host organism(s). For example, the lancelets, also referred to as amphioxus, utilize solenocytic protonephridia to perform excretion. In addition to excretion, these cells contribute to ion regulation and osmoregulation. With this in mind, solenocytes form subtypes of protonephridium and are often compared to another specialized excretory cell type, i.e., flame cells. Solenocytes have flagella, while flame cells are generally ciliated.

Branchiostoma bennetti, the mud lancelet, is a lancelet of the genus Branchiostoma endemic to the Gulf of Mexico, where it is found in the northern part.

Linda Zimmerman Holland is a research biologist at Scripps Institution of Oceanography known for her work examining the evolution of vertebrates.

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