Amphithyrus Claus 1879

Genus Amphithyrus Claus, 1879

(Figs 9–10)

Amphithyrus Claus, 1879: 4 (key), 15.— Carus 1885: 425.— Gerstaecker 1886: 483.— Claus 1887: 41.— Stebbing 1888: 1485.— Spandl 1924a: 39.— Spandl 1927: 227 (key), 246.— Pirlot 1929: 158.— Hurley 1955: 187 (key), 188.— Bowman & Gruner 1973: 55 (incl. key).— Zeidler 1978: 39 (key), 40.— Vinogradov et al. 1982: 439 (key), 457–458.— Shih & Chen 1995: 225 (key), 231–232.— Vinogradov 1999: 1200 (incl. key).

Type species. Amphithyrus bispinosus Claus, 1879 by page priority. Type material could not be found at the MFN or ZMH and is considered lost. However, the description and figures provided by Claus (1879, 1887), for this and other species of Amphithyrus, readily characterise this genus. No specific type locality is given by Claus (1879), just the “Atlantic”.

Diagnosis. Body shape more or less spherical. Head round. Eyes occupying most of head surface; grouped in one field on each side of head. Antennae 1 of males with 1-articulate peduncle; flagellum with large, crescentshaped callynophore, with aesthetascs arranged in two-field brush medially; with three smaller articles inserted on antero-dorsal 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 zig-zagged, with most articles folded back on each other; extending anteriorly under head and posteriorly between the gnathopoda and pereopoda to pereonite 5; basal article distinctly inflated, about half or less the length of following article; last two articles sub-equal in length to preceding one; terminal article pointing anteriorly. Mandibular incisor relatively broad, with several teeth, with small distal lobe medially; in male orientated more or less parallel to palp. Maxillae 1 consisting of plate-like lobes, with four rounded teeth distally on medial margin. Maxillae 2 consisting of relatively broad, curved lobes, with prominent, rounded medial bulge and pointed tip. Maxilliped with inner lobes completely fused; medial margin of outer lobes with membranous fringe. Gnathopods 1 & 2 complexly chelate; carpal process knife-shaped, conspicuously smooth; propodus with distal margin distinctly excavate, resulting in sub-chelate process with more-or-less pointed postero-distal corner. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis very broad, more than 5 x as wide as merus, not operculate, with simple proximal knob and groove to articulate with P6; articles 3–7 inserted terminally to basis. Pereopod 6; basis very broad, quadrate, more than 5 x as wide as merus, operculate, with fissure, can overlap with opposing pereopod but is without a locking mechanism or telsonic groove; articles 3–7 inserted sub-terminally on basis; merus with antero-distal corner extended, distinctly overlapping carpus medially. Pereopod 7 reduced in size with large basis, with only 1–3 terminal articles. Uropods 1 & 2 with articulated exopoda and endopoda, more-orless lanceolate, usually with serrated margins. Uropod 3; endopod fused with peduncle.

Species. Amphithyrus bispinosus Claus, 1879; A. similis Claus, 1879; A. sculpturatus Claus, 1879; A. glaber Spandl, 1924 and A. muratus Volkov, 1982.

Sexual dimorphism. The sexes are very similar in gross morphology, differing mainly in the morphology of the mandibles and the antennae.

Remarks. This genus has been removed from the family Platyscelidae because it has a number of characters that combined are inconsistent with that family as follows. The first antennae of males have a callynophore with only a one-field brush of aesthetascs; the second antennae of males have a very short, thickened basal article, about one-third as long as the following article, which has a distal bulge dorsally, as found in Paralycaea and Amphithyropsis gen. nov.; the second antennae are absent in females (but also Tetrathyrus); the first maxillae are of a different structure, similar to those found in Paralycaea and Amphithyropsis gen. nov.; pereopod 5 is relatively long and the articles distal to the basis are inserted terminally; the basis of pereopod 6 does not have a telsonic groove, or a ridge/groove locking mechanism on the distal margin, but merely overlaps with the opposing basis; and the telson is not fused with the double urosomite.

Amphithyrus is most like Amphithyropsis gen. nov. in the morphology of pereopod 5, and in that the basis of pereopod 6 has a relatively large, semi-circular fissure.

Virtually nothing is known about the biology of species, but they have been found in association with siphonophores. Amphithyrus bispinosus and A. glaber have been found with Agalma elegans (Harbison et al. 1977), and A. similis has been found with Chelophyes appendiculata (Harbison et al. 1977, Laval 1980).

Amphithyrus is widely distributed in tropical and warm-temperate regions of the world’s oceans, and some species such as A. bispinosus can be relatively abundant.