Cephalochordata

A collection of acraniates from Marovo Lagoon in the New Georgia Group of the Solomon Islands is described. Five species are recorded— Asymmetron lucayanum, A. cultellus, Branchiostoma malayana, B. indicum and B. lanceolatum or B. haeckeli: each is discussed briefly in relation to its taxonomic and biogeographic interest.

@article{doi101111j146979981969tb02131x,
    author = "Gibbs, P. E. and Wickstead, J. H.",
    title = "On a collection of Acrania (Phylum Chordata) from the Solomon Islands",
    year = "1969",
    journal = "Journal of Zoology",
    abstract = "A collection of acraniates from Marovo Lagoon in the New Georgia Group of the Solomon Islands is described. Five species are recorded— Asymmetron lucayanum, A. cultellus, Branchiostoma malayana, B. indicum and B. lanceolatum or B. haeckeli: each is discussed briefly in relation to its taxonomic and biogeographic interest.",
    url = "https://doi.org/10.1111/j.1469-7998.1969.tb02131.x",
    doi = "10.1111/j.1469-7998.1969.tb02131.x",
    openalex = "W2027162172"
}

@incollection{doi101016b9780122825026500145,
    author = "Wickstead, J. H.",
    title = "CHORDATA: ACRANIA (CEPHALOCHORDATA)",
    year = "1975",
    booktitle = "Elsevier eBooks",
    url = "https://doi.org/10.1016/b978-0-12-282502-6.50014-5",
    doi = "10.1016/b978-0-12-282502-6.50014-5",
    openalex = "W2897933019",
    references = "doi101002jmor1050540103, doi101016b9780122825019500065, doi101017s0025315400000102, doi101038150474a0, doi1010381901061d0, doi101093aibsbulletin6323c, doi101242jcss27228551, openalexw2973821230, openalexw301193904, openalexw574596740"
}

@incollection{wickstead1975chordata,
    author = "Wickstead, John H.",
    title = "CHORDATA: ACRANIA (CEPHALOCHORDATA)",
    year = "1975",
    booktitle = "Reproduction of Marine Invertebrates",
    url = "https://doi.org/10.1016/b978-0-12-282502-6.50014-5",
    doi = "10.1016/b978-0-12-282502-6.50014-5",
    openalex = "W2897933019",
    pages = "283-319",
    references = "doi101002jmor1050540103, doi101016b9780122825019500065, doi101017s0025315400000102, doi101038150474a0, doi1010381901061d0, doi101093aibsbulletin6323c, doi101111j109636421910tb00516x, doi101242jcss262246243, doi101242jcss27228551, openalexw2973821230, openalexw301193904, openalexw574596740"
}

@incollection{wickstead1975chordata1,
    author = "Wickstead, J. H",
    editor = "Giese, A. C. and Pearse, J. S.",
    title = "Chordata: Acrania (Cephalochordata)",
    year = "1975",
    booktitle = "Reproduction of Marine Invertebrates II, Entoprocts and Lesser Coelomates",
    publisher = "New York, Academic Press, p. 283-319",
    note = "talkorigins\_source = {true}; raw\_reference = {Wickstead, J. H., 1975, Chordata: Acrania (Cephalochordata), in Giese, A. C., and Pearse, J. S., eds., Reproduction of Marine Invertebrates II, Entoprocts and Lesser Coelomates: New York, Academic Press, p. 283-319.}"
}

@incollection{bereiterhahn1984cephalochordata,
    author = "Bereiter-Hahn, Jürgen",
    title = "Cephalochordata",
    year = "1984",
    booktitle = "Biology of the Integument",
    url = "https://doi.org/10.1007/978-3-642-51593-4\_45",
    doi = "10.1007/978-3-642-51593-4\_45",
    pages = "817-825"
}

Abstract The germinal and non‐germinal cells of the ripe lancelet testis are described by transmission electron microscopy. The visceral peritoneum of the testis is composed of myoepithelial cells, and the haemal layer consists of regions of narrow sinuses and conspicuously thicker blood vessels filled with blood plasma and bounded by basal laminae. Within the germinal epithelium and the testicular lumen, the non‐germinal cells, which are not abundant, contain conspicuous lysosomes and mitochondria with tubular cristae, indicating that they may be involved in steroid synthesis. In the ripe testis, the non‐germinal cells do not appear to be organized into a blood‐testis barrier. Ail types of spermatogenic cells may be flagellated and are joined in small groups by intercellular bridges. During differentiation of the spermatids, the Golgi complex is associated with formation of the acrosomal vesicle near the posterior pole of the cell. A remarkable feature is the dual origin of the subacrosomal material: one component originates at the posterior end of the spermatid, and the other at the anterior end. Subsequently, the two components merge into one after the acrosomal vesicle has migrated to its definitive anterior position in the mature spermatozoon.

@article{doi101111j146363951989tb00934x,
    author = "Holland, Nicholas D. and Holland, Linda Z.",
    title = "The Fine Structure of the Testis of a Lancelet (=Amphioxus), Branchiostoma floridae (Phylum Chordata: Subphylum Cephalochordata= Acrania)",
    year = "1989",
    journal = "Acta Zoologica",
    abstract = "Abstract The germinal and non‐germinal cells of the ripe lancelet testis are described by transmission electron microscopy. The visceral peritoneum of the testis is composed of myoepithelial cells, and the haemal layer consists of regions of narrow sinuses and conspicuously thicker blood vessels filled with blood plasma and bounded by basal laminae. Within the germinal epithelium and the testicular lumen, the non‐germinal cells, which are not abundant, contain conspicuous lysosomes and mitochondria with tubular cristae, indicating that they may be involved in steroid synthesis. In the ripe testis, the non‐germinal cells do not appear to be organized into a blood‐testis barrier. Ail types of spermatogenic cells may be flagellated and are joined in small groups by intercellular bridges. During differentiation of the spermatids, the Golgi complex is associated with formation of the acrosomal vesicle near the posterior pole of the cell. A remarkable feature is the dual origin of the subacrosomal material: one component originates at the posterior end of the spermatid, and the other at the anterior end. Subsequently, the two components merge into one after the acrosomal vesicle has migrated to its definitive anterior position in the mature spermatozoon.",
    url = "https://doi.org/10.1111/j.1463-6395.1989.tb00934.x",
    doi = "10.1111/j.1463-6395.1989.tb00934.x",
    openalex = "W2071969053",
    references = "doi101007bf00310102, doi1010160378432078900155, doi101016b9780122825026500121, doi101111j146363951979tb00594x, doi101111j146363951988tb00906x, doi101111j146364091979tb00640x, openalexw1506527793, openalexw605514449, openalexw654005152, wickstead1975chordata"
}

A method for artificial fertilization of lancelet eggs is described, and the egg coats are studied for the first time by transmission electron microscopy. Large, ovarian oocytes and spawned, unfertilized eggs (which are about 140 µm in diameter) are surrounded by a coarsely granular vitelline layer about 1 µm thick and a jelly layer a few micrometers thick. The egg cortex is crowded with a monolayer of cortical granules, each with an average diameter of approximately 3.5 µm. About 20 to 30 s after insemination, a cortical reaction occurs almost simultaneously over the entire egg surface. The cortical granules undergo exocytosis, and part of their content evidently forms a dense layer 30 nm thick against the inside of the vitelline layer: both layers together constitute the fertilization envelope, which begins elevating from the egg surface. By 80 s after insemination, the jelly layer has disappeared, and beneath the fertilization envelope the bulk of the ejected cortical granule material has become organized into a hyaline layer with a finely fibrogranular consistency. By 20 min after insemination, the perivitelline space between the fertilization envelope and the egg surface has attained its maximum width of roughly 150 µm, and both the hyaline layer and the vitelline layer component of the fertilization envelope are much attenuated and remain so until hatching about 9 h after insemination. Egg coats are compared among major deuterostome groups, and the results imply that the ancestral chordate may have been an unspecialized appendicularian.

@article{doi1023071541578,
    author = "Holland, Nicholas D. and Holland, Linda Z.",
    title = "Fine Structural Study of the Cortical Reaction and Formation of the Egg Coats in a Lancelet (= Amphioxus), Branchiostoma floridae (Phylum Chordata: Subphylum Cephalochordata = Acrania)",
    year = "1989",
    journal = "Biological Bulletin",
    abstract = "A method for artificial fertilization of lancelet eggs is described, and the egg coats are studied for the first time by transmission electron microscopy. Large, ovarian oocytes and spawned, unfertilized eggs (which are about 140 µm in diameter) are surrounded by a coarsely granular vitelline layer about 1 µm thick and a jelly layer a few micrometers thick. The egg cortex is crowded with a monolayer of cortical granules, each with an average diameter of approximately 3.5 µm. About 20 to 30 s after insemination, a cortical reaction occurs almost simultaneously over the entire egg surface. The cortical granules undergo exocytosis, and part of their content evidently forms a dense layer 30 nm thick against the inside of the vitelline layer: both layers together constitute the fertilization envelope, which begins elevating from the egg surface. By 80 s after insemination, the jelly layer has disappeared, and beneath the fertilization envelope the bulk of the ejected cortical granule material has become organized into a hyaline layer with a finely fibrogranular consistency. By 20 min after insemination, the perivitelline space between the fertilization envelope and the egg surface has attained its maximum width of roughly 150 µm, and both the hyaline layer and the vitelline layer component of the fertilization envelope are much attenuated and remain so until hatching about 9 h after insemination. Egg coats are compared among major deuterostome groups, and the results imply that the ancestral chordate may have been an unspecialized appendicularian.",
    url = "https://doi.org/10.2307/1541578",
    doi = "10.2307/1541578",
    openalex = "W2095689929",
    references = "doi101002jmor1050540103, doi10100797814615681485, doi101083jcb1072731, doi101111j146363951981tb00618x, doi101111j146363951988tb00906x, doi101242jcss27228551, doi1023071443999, doi105962bhltitle55924, openalexw564391913, openalexw654005152"
}

The germinal and non‐germinal cells of the ripe lancelet testis are described by transmission electron microscopy. The visceral peritoneum of the testis is composed of myoepithelial cells, and the haemal layer consists of regions of narrow sinuses and conspicuously thicker blood vessels filled with blood plasma and bounded by basal laminae. Within the germinal epithelium and the testicular lumen, the non‐germinal cells, which are not abundant, contain conspicuous lysosomes and mitochondria with tubular cristae, indicating that they may be involved in steroid synthesis. In the ripe testis, the non‐germinal cells do not appear to be organized into a blood‐testis barrier. Ail types of spermatogenic cells may be flagellated and are joined in small groups by intercellular bridges. During differentiation of the spermatids, the Golgi complex is associated with formation of the acrosomal vesicle near the posterior pole of the cell. A remarkable feature is the dual origin of the subacrosomal material: one component originates at the posterior end of the spermatid, and the other at the anterior end. Subsequently, the two components merge into one after the acrosomal vesicle has migrated to its definitive anterior position in the mature spermatozoon.

@article{holland1989the,
    author = "Holland, Nicholas D. and Holland, Linda Z.",
    title = "The Fine Structure of the Testis of a Lancelet (=Amphioxus), Branchiostoma floridae (Phylum Chordata: Subphylum Cephalochordata= Acrania)",
    year = "1989",
    journal = "Acta Zoologica",
    abstract = "The germinal and non‐germinal cells of the ripe lancelet testis are described by transmission electron microscopy. The visceral peritoneum of the testis is composed of myoepithelial cells, and the haemal layer consists of regions of narrow sinuses and conspicuously thicker blood vessels filled with blood plasma and bounded by basal laminae. Within the germinal epithelium and the testicular lumen, the non‐germinal cells, which are not abundant, contain conspicuous lysosomes and mitochondria with tubular cristae, indicating that they may be involved in steroid synthesis. In the ripe testis, the non‐germinal cells do not appear to be organized into a blood‐testis barrier. Ail types of spermatogenic cells may be flagellated and are joined in small groups by intercellular bridges. During differentiation of the spermatids, the Golgi complex is associated with formation of the acrosomal vesicle near the posterior pole of the cell. A remarkable feature is the dual origin of the subacrosomal material: one component originates at the posterior end of the spermatid, and the other at the anterior end. Subsequently, the two components merge into one after the acrosomal vesicle has migrated to its definitive anterior position in the mature spermatozoon.",
    url = "https://doi.org/10.1111/j.1463-6395.1989.tb00934.x",
    doi = "10.1111/j.1463-6395.1989.tb00934.x",
    number = "4",
    openalex = "W2071969053",
    pages = "211-219",
    volume = "70",
    references = "doi101007bf00310102, doi1010160378432078900155, doi101016b9780122825026500121, doi101111j146363951979tb00594x, doi101111j146363951988tb00906x, doi101111j146364091979tb00640x, openalexw1506527793, openalexw605514449, openalexw654005152, wickstead1975chordata"
}

Abstract Three ontogenetically related coeloms of a lancelet are described by transmission electron microscopy. The fin box coeloms are lined dorsally and laterally by smooth myomesothelial cells of uncertain function. In contrast, there are no myofilaments in the mesothelial cells of the ventral parts of the fin boxes. Similarly, myofilaments are absent from the mesothelia lining all parts of the sclerocoels and the lateral parts of the myocoels (the medial side of the myocoel is a myomesothelium comprising the striated muscles of the body wall). Lancelet coeloms differ from those of other deuterostomes in containing several kinds of formed extracellular materials. All three kinds of coeloms contain distinctive spherules with ramifying processes; dense strands are limited to the myocoels and sclerocoels; and a finely granular secretion is found only at the coelomic surface of the mesothelium lining the sclerocoels. These extracellular materials, which appear to originate from exocytosis of secretory granules from the mesothelial cells, may function biomechanically and for energy storage. The discussion includes a consideration of the so‐called fin rays of lancelets and concludes that none of these structures is homologous with the fin rays of fish.

@article{doi101111j146363951990tb01081x,
    author = "Holland, Nicholas D. and Holland, Linda Z.",
    title = "Fine Structure of the Mesothelia and Extracellular Materials in the Coelomic Fluid of the Fin Boxes, Myocoels and Sclerocoels of a Lancelet, Branchiostoma floridae (Cephalochordata = Acrania)",
    year = "1990",
    journal = "Acta Zoologica",
    abstract = "Abstract Three ontogenetically related coeloms of a lancelet are described by transmission electron microscopy. The fin box coeloms are lined dorsally and laterally by smooth myomesothelial cells of uncertain function. In contrast, there are no myofilaments in the mesothelial cells of the ventral parts of the fin boxes. Similarly, myofilaments are absent from the mesothelia lining all parts of the sclerocoels and the lateral parts of the myocoels (the medial side of the myocoel is a myomesothelium comprising the striated muscles of the body wall). Lancelet coeloms differ from those of other deuterostomes in containing several kinds of formed extracellular materials. All three kinds of coeloms contain distinctive spherules with ramifying processes; dense strands are limited to the myocoels and sclerocoels; and a finely granular secretion is found only at the coelomic surface of the mesothelium lining the sclerocoels. These extracellular materials, which appear to originate from exocytosis of secretory granules from the mesothelial cells, may function biomechanically and for energy storage. The discussion includes a consideration of the so‐called fin rays of lancelets and concludes that none of these structures is homologous with the fin rays of fish.",
    url = "https://doi.org/10.1111/j.1463-6395.1990.tb01081.x",
    doi = "10.1111/j.1463-6395.1990.tb01081.x",
    openalex = "W1994508914",
    references = "doi101002jmor1051390403, doi101007bf00334754, doi101007bf00351139, doi101083jcb104159, doi101083jcb123646, doi101111j146363951989tb00934x, doi101111j174966321963tb16652x, doi101126science7089565, doi101159000183024, doi105962bhltitle155406, doi105962bhltitle55924, holland1989the"
}

Abstract Adults of the European lancelet were collected at Banyuls‐sur‐Mer (Mediterranean France) in mid‐spring, shortly before the onset of the breeding season. The dorsal and ventral fin rays were studied by light microscopic histochemistry and by transmission electron microscopy (TEM). Each fin ray is a mass of extracellular material that accumulates beneath the mesothelium of a fin box coelom. The fin ray material is rich in lipids, proteins, and neutral mucopolysaccharides. TEM reveals no lipid droplets in this material, which consists entirely of a packed mass of 15–20 nm granules of medium electron density. It is likely that these granules consist of glycoproteins or glycolipoproteins. Our results are consistent with the proposal of Azariah (1965, Journal of the Marine Biological Association of India 7: 459–661) that lancelet fin rays are nutritional reserves supporting gametogenesis during the breeding season.

@article{doi101111j146363951991tb01197x,
    author = "Holland, Nicholas D. and Holland, Linda Z.",
    title = "The Histochemistry and Fine Structure of the Nutritional Reserves in the Fin Rays of a Lancelet, Branchiostoma lanceolatum (Cephalochordata = Acrania)",
    year = "1991",
    journal = "Acta Zoologica",
    abstract = "Abstract Adults of the European lancelet were collected at Banyuls‐sur‐Mer (Mediterranean France) in mid‐spring, shortly before the onset of the breeding season. The dorsal and ventral fin rays were studied by light microscopic histochemistry and by transmission electron microscopy (TEM). Each fin ray is a mass of extracellular material that accumulates beneath the mesothelium of a fin box coelom. The fin ray material is rich in lipids, proteins, and neutral mucopolysaccharides. TEM reveals no lipid droplets in this material, which consists entirely of a packed mass of 15–20 nm granules of medium electron density. It is likely that these granules consist of glycoproteins or glycolipoproteins. Our results are consistent with the proposal of Azariah (1965, Journal of the Marine Biological Association of India 7: 459–661) that lancelet fin rays are nutritional reserves supporting gametogenesis during the breeding season.",
    url = "https://doi.org/10.1111/j.1463-6395.1991.tb01197.x",
    doi = "10.1111/j.1463-6395.1991.tb01197.x",
    openalex = "W1964567395",
    references = "doi101111j146363951990tb01081x"
}

The Australian cephalochordate fauna consists of two genera and eight species. There are two endemic temperate-water species, Epigonichthys australis and E. bassanus, and six tropical species. Two of the latter, Branchiostoma minucauda and B. moretonensis, are endemic. The remaining four species, B. belcheri, E. cultellus, E. maldivensis and E. lucayanum, are found throughout the Indo-West Pacific Region. This study is based on the morphological study of 10 characters in 200 specimens. New synonymies are given for E. cultellus and the B. belcheri species-complex. The generic names Amphipleurichthys, Bathyamphioxus, Merscalpellus, Notasymmetron and Zeamphioxus are placed in the synonomy of Epigonichthys and the use of the junior synonym, Asymmetron, in place of Epigonichthys, is not supported. Information is presented on the distribution and habitat preferences of each species in Australia together with a key to the species likely to be found in Australia.

@article{doi101071it9941443,
    author = "Richardson, Benjamin J. and McKenzie, AM",
    title = "Taxonomy and distribution of Australian Cephalochordates (Chordata: Cephalochordata)",
    year = "1994",
    journal = "Invertebrate taxonomy",
    abstract = "The Australian cephalochordate fauna consists of two genera and eight species. There are two endemic temperate-water species, Epigonichthys australis and E. bassanus, and six tropical species. Two of the latter, Branchiostoma minucauda and B. moretonensis, are endemic. The remaining four species, B. belcheri, E. cultellus, E. maldivensis and E. lucayanum, are found throughout the Indo-West Pacific Region. This study is based on the morphological study of 10 characters in 200 specimens. New synonymies are given for E. cultellus and the B. belcheri species-complex. The generic names Amphipleurichthys, Bathyamphioxus, Merscalpellus, Notasymmetron and Zeamphioxus are placed in the synonomy of Epigonichthys and the use of the junior synonym, Asymmetron, in place of Epigonichthys, is not supported. Information is presented on the distribution and habitat preferences of each species in Australia together with a key to the species likely to be found in Australia.",
    url = "https://doi.org/10.1071/it9941443",
    doi = "10.1071/it9941443",
    openalex = "W2085485371"
}

Primordial germ cells (PGCs) are described from the gonad of c. 2 cm juvenile Branchiostoma virginiae; early oocytes (c. 10 microm) and enlarging, previtellogenic oocytes (c. 35 microm) are described from the ovary of c. 5 cm adults. The germinal epithelium of the juvenile gonad and adult ovary is composed of both germinal and somatic cells. In the juvenile, somatic cells retain contact with the basal lamina of the germinal epithelium though their perikarya may be displaced towards the lumen; the germinal epithelium is, therefore, a simple but pseudostratified epithelium. In the adult ovary, somatic cells may lose contact with the basal lamina and the epithelium appears to become stratified. PGCs and oocytes are identified as germ cells by the presence of nuage. PGCs and oocytes are polarised epithelial cells. They rest on a basal lamina, extend apically towards a lumen, form adhering junctions with neighbouring cells, and exhibit apical-basal polarity. PGCs and early oocytes have an apical flagellum with an associated basal body, accessory centriole, and one or more striated rootlet fibres. The flagellum is surrounded by a collar of microvilli. Once oocytes begin to enlarge and bulge basally into the connective tissue layer, the flagellum is lost, but the basal bodies and ciliary rootlets are present at the apex of 35 microm oocytes. Similarities of the oogenic pattern in cephalochordates and echinoderms indicate that the establishment of egg polarity in deuterostomes is influenced by the polarity of the germinal epithelium.

@article{doi101017s0967199400003816,
    author = "Frick, J E and Ruppert, E E",
    title = "Primordial germ cells and oocytes of Branchiostoma virginiae (Cephalochordata, Acrania) are flagellated epithelial cells: relationship between epithelial and primary egg polarity.",
    year = "1997",
    journal = "Zygote (Cambridge, England)",
    abstract = "Primordial germ cells (PGCs) are described from the gonad of c. 2 cm juvenile Branchiostoma virginiae; early oocytes (c. 10 microm) and enlarging, previtellogenic oocytes (c. 35 microm) are described from the ovary of c. 5 cm adults. The germinal epithelium of the juvenile gonad and adult ovary is composed of both germinal and somatic cells. In the juvenile, somatic cells retain contact with the basal lamina of the germinal epithelium though their perikarya may be displaced towards the lumen; the germinal epithelium is, therefore, a simple but pseudostratified epithelium. In the adult ovary, somatic cells may lose contact with the basal lamina and the epithelium appears to become stratified. PGCs and oocytes are identified as germ cells by the presence of nuage. PGCs and oocytes are polarised epithelial cells. They rest on a basal lamina, extend apically towards a lumen, form adhering junctions with neighbouring cells, and exhibit apical-basal polarity. PGCs and early oocytes have an apical flagellum with an associated basal body, accessory centriole, and one or more striated rootlet fibres. The flagellum is surrounded by a collar of microvilli. Once oocytes begin to enlarge and bulge basally into the connective tissue layer, the flagellum is lost, but the basal bodies and ciliary rootlets are present at the apex of 35 microm oocytes. Similarities of the oogenic pattern in cephalochordates and echinoderms indicate that the establishment of egg polarity in deuterostomes is influenced by the polarity of the germinal epithelium.",
    url = "https://pubmed.ncbi.nlm.nih.gov/9276511/",
    doi = "10.1017/s0967199400003816",
    openalex = "W1997728508",
    pmid = "9276511",
    references = "doi101016b9780122825057x50015, doi101016s0074769608600704, doi101038375654a0, doi1015159781400858712, doi105860choice334500, doi105860choice501469, doi105962bhltitle21641, openalexw654005152, openalexw659399033"
}

The formation of an acrosomal process at acrosomal exocytosis in spermatozoa of the amphioxus was described in the present report for the first time. A non-reacted acrosome was located in front of the nucleus, where a cup-shaped acrosomal vesicle covered a conical accumulation of subacrosomal material. When naturally spawned spermatozoa were treated with a calcium ionophore, ionomycin, the acrosomal vesicle opened at the apex and an acrosomal process was projected. The process exhibited a filamentous structure. The reaction followed the mode typically seen in marine invertebrates. These observations suggest that the features and function of the acrosome of amphioxus, whose position is on the border between invertebrates and vertebrates, reflect their ecological adaptation and phylogenic position.

@article{doi102108zsj211079,
    author = "Morisawa, Sachiko and Mizuta, Takanobu and Kubokawa, Kaoru and Tanaka, Hiroyuki and Morisawa, Masaaki",
    title = "Acrosome Reaction in Spermatozoa from the Amphioxus Acrosome Reaction in Branchiostoma belcheri (Cephalochordata, Chordata)",
    year = "2004",
    journal = "ZOOLOGICAL SCIENCE",
    abstract = "The formation of an acrosomal process at acrosomal exocytosis in spermatozoa of the amphioxus was described in the present report for the first time. A non-reacted acrosome was located in front of the nucleus, where a cup-shaped acrosomal vesicle covered a conical accumulation of subacrosomal material. When naturally spawned spermatozoa were treated with a calcium ionophore, ionomycin, the acrosomal vesicle opened at the apex and an acrosomal process was projected. The process exhibited a filamentous structure. The reaction followed the mode typically seen in marine invertebrates. These observations suggest that the features and function of the acrosome of amphioxus, whose position is on the border between invertebrates and vertebrates, reflect their ecological adaptation and phylogenic position.",
    url = "https://doi.org/10.2108/zsj.21.1079",
    doi = "10.2108/zsj.21.1079",
    openalex = "W2142984585",
    references = "doi101002jez1402320116, doi101002jmor1050540103, doi101007bf00219787, doi101016016953479290115r, doi101016s0022532070901632, doi101083jcb193477, doi101083jcb252249, doi101111j146363951989tb00934x, doi1023071538405, doi1023071541578, doi1023071542182, holland1989the"
}

Asymmetron inferum n. sp. is established for the holotype collected during the Hyper-Dolphin/Natsushima cruise in 2003 of the Japan Marine Science & Technology Center (JAMSTEC) off Cape Nomamisaki, southwestern end of Kagoshima Prefecture, at a depth of 229 m. A. inferum is very similar to its congener, A. lucayanum Andrews, 1893 (formerly called Epigonichthys lucayanus) but easily distinguishable from it by the larger number of total myotomes (83 in the former vs. 55-72 in the latter). The genus Asymmetron has been treated as a junior synonym of Epigonichthys, but is recovered as a valid genus distinct morphologically from another valid genus Epigonichthys (sensu stricto).

@article{doi102108zsj211131,
    author = "Nishikawa, Teruaki",
    title = "A New Deep-water Lancelet (Cephalochordata) from off Cape Nomamisaki, SW Japan, with a Proposal of the Revised System Recovering the Genus Asymmetron",
    year = "2004",
    journal = "ZOOLOGICAL SCIENCE",
    abstract = "Asymmetron inferum n. sp. is established for the holotype collected during the Hyper-Dolphin/Natsushima cruise in 2003 of the Japan Marine Science \& Technology Center (JAMSTEC) off Cape Nomamisaki, southwestern end of Kagoshima Prefecture, at a depth of 229 m. A. inferum is very similar to its congener, A. lucayanum Andrews, 1893 (formerly called Epigonichthys lucayanus) but easily distinguishable from it by the larger number of total myotomes (83 in the former vs. 55-72 in the latter). The genus Asymmetron has been treated as a junior synonym of Epigonichthys, but is recovered as a valid genus distinct morphologically from another valid genus Epigonichthys (sensu stricto).",
    url = "https://doi.org/10.2108/zsj.21.1131",
    doi = "10.2108/zsj.21.1131",
    openalex = "W2118652344",
    references = "doi101007s002390040238x, doi101016b9780122825026500145, doi101016b9780122825057x50015, doi101071it9941443, doi101093bioscience1610752a, doi101111j146979981969tb02131x, doi101242jcs371303, doi105134175986, doi105134176018, openalexw3216129607, wickstead1975chordata"
}

@misc{crossref2005cephalochordata,
    title = "Cephalochordata (Chordata)",
    year = "2005",
    booktitle = "Van Nostrand's Scientific Encyclopedia",
    url = "https://doi.org/10.1002/0471743984.vse1596",
    doi = "10.1002/0471743984.vse1596",
    openalex = "W4245623911"
}

BACKGROUND: The lancelet Asymmetron inferum (subphylum Cephalochordata) was recently discovered on the ocean floor off the southwest coast of Japan at a depth of 229 m, in an anaerobic and sulfide-rich environment caused by decomposing bodies of the sperm whale Physeter macrocephalus. This deep sulfide-rich habitat of A. inferum is unique among the lancelets. The distinguishing adaptation of this species to such an extraordinary habitat can be considered in a phylogenetic framework. As the first step of reconstruction of the evolutionary processes in this species, we investigated its phylogenetic position based on 11 whole mitochondrial genome sequences including the newly determined ones of the whale-fall lancelet A. inferum and two coral-reef congeners. RESULTS: Our phylogenetic analyses showed that extant lancelets are clustered into two major clades, the Asymmetron clade and the Epigonichthys + Branchiostoma clade. A. inferum was in the former and placed in the sister group to A. lucayanum complex. The divergence time between A. inferum and A. lucayanum complex was estimated to be 115 Mya using the penalized likelihood (PL) method or 97 Mya using the nonparametric rate smoothing (NPRS) method (the middle Cretaceous). These are far older than the first appearance of large whales (the middle Eocene, 40 Mya). We also discovered that A. inferum mitogenome (mitochondrial genome) has been subjected to large-scale gene rearrangements, one feature of rearrangements being unique among the lancelets and two features shared with A. lucayanum complex. CONCLUSION: Our study supports the monophyly of genus Asymmetron assumed on the basis of the morphological characters. Furthermore, the features of the A. inferum mitogenome expand our knowledge of variation within cephalochordate mitogenomes, adding a new case of transposition and inversion of the trnQ gene. Our divergence time estimation suggests that A. inferum remained a member of the Mesozoic and the early Cenozoic large vertebrate-fall communities before shifting to become a whale-fall specialist.

@article{doi101186147121487127,
    author = "Kon, Takeshi and Nohara, Masahiro and Yamanoue, Yusuke and Fujiwara, Yoshihiro and Nishida, Mutsumi and Nishikawa, Teruaki",
    title = "Phylogenetic position of a whale-fall lancelet (Cephalochordata) inferred from whole mitochondrial genome sequences",
    year = "2007",
    journal = "BMC Evolutionary Biology",
    abstract = "BACKGROUND: The lancelet Asymmetron inferum (subphylum Cephalochordata) was recently discovered on the ocean floor off the southwest coast of Japan at a depth of 229 m, in an anaerobic and sulfide-rich environment caused by decomposing bodies of the sperm whale Physeter macrocephalus. This deep sulfide-rich habitat of A. inferum is unique among the lancelets. The distinguishing adaptation of this species to such an extraordinary habitat can be considered in a phylogenetic framework. As the first step of reconstruction of the evolutionary processes in this species, we investigated its phylogenetic position based on 11 whole mitochondrial genome sequences including the newly determined ones of the whale-fall lancelet A. inferum and two coral-reef congeners. RESULTS: Our phylogenetic analyses showed that extant lancelets are clustered into two major clades, the Asymmetron clade and the Epigonichthys + Branchiostoma clade. A. inferum was in the former and placed in the sister group to A. lucayanum complex. The divergence time between A. inferum and A. lucayanum complex was estimated to be 115 Mya using the penalized likelihood (PL) method or 97 Mya using the nonparametric rate smoothing (NPRS) method (the middle Cretaceous). These are far older than the first appearance of large whales (the middle Eocene, 40 Mya). We also discovered that A. inferum mitogenome (mitochondrial genome) has been subjected to large-scale gene rearrangements, one feature of rearrangements being unique among the lancelets and two features shared with A. lucayanum complex. CONCLUSION: Our study supports the monophyly of genus Asymmetron assumed on the basis of the morphological characters. Furthermore, the features of the A. inferum mitogenome expand our knowledge of variation within cephalochordate mitogenomes, adding a new case of transposition and inversion of the trnQ gene. Our divergence time estimation suggests that A. inferum remained a member of the Mesozoic and the early Cenozoic large vertebrate-fall communities before shifting to become a whale-fall specialist.",
    url = "https://doi.org/10.1186/1471-2148-7-127",
    doi = "10.1186/1471-2148-7-127",
    openalex = "W2083183346",
    references = "doi101007bf00160154, doi101016b9780122825026500145, doi101038290470a0, doi101093bioinformatics149817, doi101093bioinformatics178754, doi101093bioinformaticsbtg180, doi101093nar25244876, doi101093nar2781767, doi101111j155856461985tb00420x, doi102108zsj211131, doi1023072408678, openalexw3217097258, wickstead1975chordata"
}

@incollection{nielsen2011phylum,
    author = "Nielsen, Claus",
    title = "Phylum Cephalochordata",
    year = "2011",
    booktitle = "Animal Evolution",
    url = "https://doi.org/10.1093/acprof:oso/9780199606023.003.0063",
    doi = "10.1093/acprof:oso/9780199606023.003.0063",
    pages = "358-364"
}

@incollection{holland2015cephalochordata,
    author = "Holland, Linda Z.",
    title = "Cephalochordata",
    year = "2015",
    booktitle = "Evolutionary Developmental Biology of Invertebrates 6",
    url = "https://doi.org/10.1007/978-3-7091-1856-6\_3",
    doi = "10.1007/978-3-7091-1856-6\_3",
    pages = "91-133",
    references = "doi102108zsj211079, hatschek1893the"
}

@incollection{pandian2018cephalochordata,
    author = "Pandian, T. J.",
    title = "Cephalochordata",
    year = "2018",
    booktitle = "Reproduction and Development in Echinodermata and Prochordata",
    url = "https://doi.org/10.1201/9780815364733-9",
    doi = "10.1201/9780815364733-9",
    pages = "193-194"
}

@incollection{crossref2023cephalochordata,
    title = "Cephalochordata, n.",
    year = "2023",
    booktitle = "Oxford English Dictionary",
    url = "https://doi.org/10.1093/oed/4175808430",
    doi = "10.1093/oed/4175808430"
}

@incollection{paululat2025acrania,
    author = "Paululat, Achim and Purschke, Günter",
    title = "Acrania (Cephalochordata, Lancelets)",
    year = "2025",
    booktitle = "Metazoa – Morphology and Evolution of Animals",
    url = "https://doi.org/10.1007/978-3-662-69904-1\_10",
    doi = "10.1007/978-3-662-69904-1\_10",
    openalex = "W4412557476",
    pages = "183-193"
}

@misc{crossrefNonecephalochordata,
    title = "Cephalochordata",
    year = "None",
    booktitle = "AccessScience",
    url = "https://doi.org/10.1036/1097-8542.120700",
    doi = "10.1036/1097-8542.120700"
}

@misc{tNonecephalochordata,
    author = "T., Mondal and C., Raghunathan",
    title = "Cephalochordata",
    year = "None",
    booktitle = "Data Products",
    url = "https://doi.org/10.26515/fauna/2/2025/cephalochordata",
    doi = "10.26515/fauna/2/2025/cephalochordata"
}