Genus Oxycephalus Milne-Edwards, 1830

(Figs 31–34)

Oxycephalus Milne-Edwards, 1830: 385 (key), 396.— Guérin-Méneville 1836: 9.— Milne-Edwards 1838: 307.— Lucas 1840: 240.— Milne-Edwards 1840: 71 (key), 99.— Nicolet 1849: 249.— Dana 1852: 316.— Dana 1853: 1009, 1443.— Bate 1862: 342.— Claus 1871: 155.— Streets 1877: 136.— Claus 1879: 43 (incl. key).— Streets 1879: 278.— Claus 1880: 588.— Claus 1884: 455.— Carus 1885: 427.— Gerstaecker 1886: 487.— Bovallius 1887: 35.— Claus 1887: 68 (incl. key).— Stebbing 1888: 1576.— Bovallius 1890: 46 (key), 54.— Spandl 1924a: 32.— Spandl 1927: 178 (key), 179.— Pirlot 1929: 162.— Barnard 1940: 542 (key).— Hurley 1955: 182 (incl. key).— Fage 1960: 29.— Pillai 1966: 173.— Yoo 1971: 63 (key).— Bowman & Gruner 1973: 49 (incl. key).— Zeidler 1978: 30 (key), 32.— Vinogradov et al. 1982: 404 (key), 404.— Zeidler 1992: 120.— Nair 1995: 6 (key), 9.— Shih & Chen 1995: 190 (key), 190–191.— Vinogradov 1999: 1195 (key), 1196.— Zeidler 1999: 392 –393.

Orio Cocco, 1832: 206.— Cocco 1833: 113 (part).— Prestandrea 1833: 11.— Costa & Costa 1840: 5 (part). — Costa in Hope 1851: 21.

Ornithoramphus Natale, 1850a: 12.—Costa in Hope 1851: 21.— Carus 1885: 428.

Erpetoramphus Natale, 1850b: 11.— Costa in Hope 1851: 22.

Natalius Costa, 1864: 87.— Carus 1885: 427.

Type species. Oxycephalus piscatoris Milne-Edwards, 1830 by monotypy. Type material could not be found at the NHM, MNHN or ANSP (which has some types of Milne-Edwards) and is presumed lost. Despite the loss of type material, and the lack of illustrations at the time, Oxycephalus is a readily distinguished genus and is well established in the literature. The type locality is not given by Milne-Edwards (1830), but later (1840) he gives the Indian Ocean as the locality for his species.

Type species of synonyms. The type species of Orio is O. ornithoramphus Cocco, 1832 by monotypy. Type material could not be located in any Italian, or major European museum, and is considered lost. The description and figures of Cocco (1832) appear to resemble a male Oxycephalus, but are insufficient for a specific determination. The type locality is the Mediterranean Sea, Gulf of Naples, near Messina.

The type species of Ornithoramphus is O. coccoi Natale, 1850 by monotypy. Type material could not be located in any Italian or major European museum, and is considered lost. The figure by Natale (1850a) clearly resembles a male Oxycephalus but it, and the description, are insufficient for a specific determination. The type locality is the Mediterranean Sea, Gulf of Naples, near Messina.

The type species of Erpetoramphus is E. costae Natale, 1850 by monotypy. Type material could not be located in any Italian, or major European museum, and is considered lost. The description and figure by Natale (1850b) are insufficient for a specific determination, but the species appears to belong to Oxycephalus, despite the fact that the first gnathopoda are illustrated as sub-chelate. The type locality is the Mediterranean Sea, Gulf of Naples, near Messina.

The type species of Natalius is N. candidissimus Costa, 1864 by monotypy. Type material could not be located in any Italian, or major European museum, and is considered lost. The descriptions of the genus and species by Costa (1864) seem to correspond to Oxycephalus, but the species description does not contain sufficient detail for a specific determination. The type locality is the Mediterranean Sea, Gulf of Naples.

Diagnosis. Body shape elongate and narrow. Head oval. Rostrum distinctly elongate. Eyes occupying most of head surface, except for neck and rostrum; grouped in one field on each side of head. Antennae 1 of males with 2- articulate peduncle; flagellum with large, elongate, crescent-shaped callynophore, with small antero-distal lobe, with aesthetascs arranged in two-field brush medially; with three smaller articles inserted terminally, below anterodorsal corner. Antennae 1 of females with 2-articulate peduncle; callynophore narrowly rectangular, with two smaller articles inserted terminally. Antennae 2 absent in females. Antennae 2 of males 5-articulate; strongly zigzagged, with most articles folded back on each other; extending anteriorly under head and posteriorly between the gnathopoda to pereonite 1; basal article elongate, sub-equal in length to following article; terminal article very short, not folded, pointing posteriorly. Mandibular palp 3-articulate in males, with extremely long first article. Mandibular incisor relatively broad, with several teeth, with small distal lobe medially; in male orientated more or less parallel to palp. Maxillae 1 & 2 absent. Maxilliped with inner lobes completely fused; medial margin of outer lobes with membranous fringe. Coxae all partly fused with pereonites. Gnathopods 1 & 2 chelate; carpal process knife-shaped, armed with prominent teeth. Pereopods 3 & 4 sub-equal in length to pereopods 5 & 6. Pereopod 5 with very broad basis, more than 5 x as wide as following articles, but not operculate, non-locking but may overlap with P6; articles 3–7 inserted terminally to basis. Pereopod 6 with very broad basis, more than 5 x as wide as following articles, but not operculate, does not overlap, or lock, with opposing pereopod; articles 3–7 inserted terminally to basis. Pereopod 7 reduced in size, with basis not particularly enlarged; all articles present; dactylus normal. Uropod 1 with articulated exopoda and endopoda. Uropods 2 & 3; endopod fused with peduncle. Rami of all uropoda lanceolate, usually with serrated margins. Telson fused with double urosomite. Oostegites on pereonites 2–5. Gills on pereonites 2–6; some with folds on pereonite 5 or 6 or both.

Species. Oxycephalus piscator Milne-Edwards, 1830; O. latirostris Claus, 1879 and O. clausi Bovallius, 1887.

Sexual dimorphism. The sexes are very similar in appearance, and apart from the morphology of the antennae and mouthparts, are distinguished as follows. Females have a relatively smaller head and rostrum, and the head is slightly more bulbous around the eyes. In mature males the head is characteristically indented on the dorsal surface, above the eyes, with a slight keel anteriorly. The carpus of gnathopod 2 is relatively more slender in females. The basis of pereopod 7 is distinctly more narrowed distally in females, and sometimes the propodus can be relatively longer in mature females.

Remarks. This genus is distinguished from all others in the family by a combination of characters, mainly the morphology of the head, gnathopoda, double urosomite, and uropoda.

Oxycephalus does not closely resemble any other genus of Oxycephalidae. The shape of the cephalon vaguely resembles Cranocephalus, Leptocotis and Streetsia. Oxycephalus also resembles Calamorhynchus, Cranocephalus and Rhabdosoma in that the coxae are fused with the pereonites, and Rhabdosoma in having uropods 2 & 3 with endopods fused with the peduncle. The chelate gnathopoda of Oxycephalus are very distinctive. Other oxycephalids that have both gnathopoda chelate, although different from Oxycephalus in morphology, are Calamorhynchus, Glossocephalus, Leptocotis, and Rhabdosoma. Thus, Oxycephalus has characters in common with all other genera of Oxycephalidae, except perhaps for Tullbergella, although in that genus the endopod of uropod 3 is fused with the peduncle as in Oxycephalus.

Species of Oxycephalus are a common component of plankton collections. Fage (1960) provides the most comprehensive biogeographical information available for this genus. Species of the genus seem to be preferentially associated with ctenophores, but have also been recorded with medusae and even heteropods. Oxycephalus piscator has been recorded with medusae (Carus 1885) and the ctenophores Leucothea multicornis (Chun 1889) and Mnemiopsis mccradyi (Harbison et al. 1978). Oxycephalus latirostris has been recorded from the ctenophores Cestum veneris and Eurhamphaea vexilligera (Harbison et al. 1978). Oxycephalus clausi has been recorded with a variety of gelatinous plankton, including medusae, colonial radiolarians (Harbison et al. 1978), the heteropod Pterotrachea hippocampus (Harbison et al. 1977), the salps Pegea socia and Salpa cylindrica (Madin & Harbison 1977), but most frequently on ctenophores as follows; Ocyropsis maculata (Harbison et al. 1977, 1978), Cestum veneris, Eurhamphaea vexilligera, Mnemiopsis mccradyi, Ocyropsis crystallina, O. maculata, Beroe sp. (Harbison et al. 1978), Ocyropsis maculata maculata, O. m. immaculata and O. crystata guttata (Zeidler 1999). Gasca & Haddock (2004) also recorded it from Ocyropsis crystallina crystallina and Gasca et al. (2014) provide further records with Cestum veneris.

Oxycephalus is a tropical genus although it sometimes occurs in temperate regions. It occurs worldwide and seems to prefer near-surface waters.