Philomedes orbicularis Brady 1907

“Philomedes orbicularis

(Plate I., figs. 1-15)

Shell of the female seen from the side almost circular, except in the region of the sinus at the ventral margin, length about one-fourth greater than the height; anterior extremity obtusely angulated above the rostrum, posterior broadly and evenly rounded, dorsal margin rather boldly arched throughout its whole length, ventral strongly arched from the deep subrostral sinus backwards. Seen from above the outline is broadly ovate, rounded behind and submucronate in front, greatest width situated in the middle and equal to about two-thirds of the length. Surface of the shell smooth and densely clothed with a villous coating of very short hairs; margins of the rostrum and subjacent sinus fringed with stiff setae. Shell of the male somewhat elongated, height not greatly exceeding one-half of the length, rostrum and sinus less developed than in the female; posterior extremity obliquely subtruncate, slightly sinuated in the middle and rounded off ventrally; dorsal margin well arched, ventral forming a continuous flattened curve. In the young condition, the shell of the female in lateral view has the postero ventral angle sharply produced and is mucronate when seen dorsally. The soft parts of the animal have the typical characters of the genus, but the antennal setae form two distinct series, the distal set being about twice as long as the proximal; the secondary branch of the female antenna is two-jointed, the basal joint bearing a few short marginal setae, the second joint four marginal setae, one of which is longer than the rest and plumose, also a single lash-like apical seta. The secondary branch of the male antenna is much like that of P. Brenda, but less robust and its marginal setae are shorter. The principal tooth of the second maxilla is very large and strong, somewhat hatchet-shaped, the two extremities produced into strong cutting lobes, with a third smaller lateral tooth – the appearance of the whole organ, however, varying very much according to the position in which it is seen. The other limbs present no features calling for special remark. Length, 2.5 mm.

P. orbicularis was found in two gatherings (May 23, 1902 and June 15, 1902). It is in all respects very similar to the well-known European species P. brenda and may perhaps be fairly looked upon as a southern variation of that form. There is the same characteristic villous covering and the size is nearly the same, but there is a total absence, in the adult, of any posterior angulations of the shell, which is also considerably more tumid than that of the northern species. The rather well marked constriction of the anterior, and the broadly rounded character of the posterior extremity, when seen dorsally, are also noteworthy features. The form of the shell in P. brenda seems to be the same throughout life; at any rate the smallest specimens I have seen present characters exactly the same as those of the adult. But one or two small specimens which occurred along with P. orbicularis, and which I at first took to be the young of that species, were very distinctly angulated posteriorly, and I now think that they belong probably to the following species P. assimilis, especially as they have not the villous covering of P. orbicularis.” (Brady 1907, p. 4-5)

Philomedes orbicularis Brady

Philomedes orbicularis Brady, 1907:4, pl. 1: figs. 1–3, 6–15.—Müller, 1912:26 [key], 33.—Kornicker, 1971:191, fig. 16.

Philomedes trithrix Kornicker, 1971:176, figs. 6–12.

LECTOTYPE.—Dry specimen, length 2.45 mm, height 2.08 mm, on slide in Brady collection, Hancock Museum, Newcastle-on-Tyne, designated herein. Specimen illustrated by Kornicker (1971:191, fig. 16).

TYPE-LOCALITY.—Winter Quarters, McMurdo Sound.

MATERIAL.—USNM 125839 gravid ♀; USNM 125841, gravid ♀ + 1 adult ♀ + 20 juveniles; USNM 125980, adult ♀; USNM 125981, 2 juveniles; USNM 125984, 219 specimens; USNM 126090, gravid ♀; USNM 126091, gravid ♀; USNM 126092, adult ♀; USNM 126093, juvenile ♀, length only 2.31 mm; USNM 126096, 5 adult ♂ ♂ + 37 females and juveniles; USNM 126095, adult ♀; USNM 127472, gravid ♀; USNM 127473, adult ♀; USNM 127474, 10 adult ♀ ♀ (no eggs visible through shell) + 32 juveniles; USNM 127477, adult ♂; USNM 127479, 1 adult ♂ + 7 adult ♀ ♀ + 1 juvenile; USNM 127482, 1 juvenile; USNM 127484, adult ♀; USNM 127485, 3 gravid ♀ ♀ + 3 adult ♀ ♀ + 8 juveniles; USNM 127487, 2 adult ♂ ♂; USNM 127489, 3 gravid ♀ ♀ + 1 adult ♀; USNM 127495, 3 juveniles; USNM 127496, adult ♀; USNM 136084, gravid ♀; USNM 136085, 726 specimens (gravid ♀ ♀, adult ♀ ♀ without eggs and juveniles, no adult ♂ ♂); USNM 138021, 11 specimens (not dissected); USNM 138158, 1 gravid ♀; USNM 138159, 1 gravid ♀; USNM 138160, 1 gravid ♀; USNM 138163, 1 gravid ♀ (not dissected); USNM 138650, 1 adult ♀ without eggs.

USNM 125839, 125841 from Eastwind station 004 A; USNM 125980, 125981 from Deep Freeze IV, USS Staten Island, station S.I. 2; USNM 125984, 126090–126093, 126095, 126096 from Deep Freeze I, USS Edisto, stations 5, 6, 8 combined, vial 1; USNM 127472–127474, 127477 from Hero Cruise 691, station 6; USNM 127479 from Hero Cruise 691, station 7; USNM 127482 from Hero Cruise 691, station 12-B; USNM 127484 from Hero Cruise 691, station 5; USNM 127485 from Hero Cruise 691, station 3; USNM 127487, 127489, 127495, 127496 from Deep Freeze I, USS Edisto, stations 2, 6, 8 combined, vial 3; USNM 126084, 136085 from Hero Cruise 691, station 22; USNM 136177 from Hero Cruise 691, station 23; USNM 138021 from Hero Cruise 702, station 448; USNM 138158–138160 from XXIV Exp. Ant. Chile, Yelcho station 70–39; USNM 138163 from XXIII Exp. Ant. Chile, Hero station 69–47; USNM 138650 from XXIV Exp. Ant. Chile, Yelcho station 70–30.

In addition, the following specimens collected by the Chilean Antarctic Expeditions were identified and returned to the Departmento de Zoología, Instituto Central de Biología Universidad de Concepción, Concepción, Chile: XXIV, Yelcho station 70–39, 4 gravid ♀ ♀ + 11 specimens; XXIII, Hero station 69–40, 5 gravid ♀ ♀, 9 adult ♀ ♀ without eggs, 41 juveniles; XXIII, Hero station 69–47, 1 gravid ♀, 9 juveniles; XXIV, Yelcho station 70–25, 3 gravid ♀ ♀; XXIV, Yelcho station 70–30, 2 gravid ♀ ♀, 2 juveniles; XXIII, Hero station 69–35, 1 gravid ♀, 3 adult ♀ ♀, 1 juvenile; XXIV, Yelcho station 70–32, 2 gravid ♀ ♀, 8 juveniles; XXIII, Hero station 69–46, 1 gravid ♀, 9 adult ♀ ♀ without eggs, 6 juveniles; XXIII, Hero station 69–42, 1 adult ♀ without eggs, 1 juvenile; XXIII, Hero station 69–33, 2 gravid ♀ ♀, 2 adult ♀ ♀ without eggs; XXIII, Hero station 69–48, 4 juveniles; XXIII Hero station 69–45, 2 adult ♀ ♀ without eggs; XXIII, Hero station 69–28, 1 adult ♀ without eggs; XXIV, Yelcho station 70–28, 1 gravid ♀, 2 juveniles; XXIII, Hero station 69–26, 1 adult ♀ without eggs; XXIV, Yelcho station 70–42, 1 adult ♀ without eggs; XXIII, Hero station 69–37, 1 gravid ♀, 1 juvenile; XXIII, Hero station 69–30, 1 gravid ♀, 2 juveniles; XXIV, Yelcho station 70–44, 1 juvenile; XXIV, Yelcho station 70–46, 1 adult ♀ without eggs.

DISTRIBUTION.—This species was collected only in the Continental subregion of Antarctica at depths of 15 to 405 m (Figure 135).

BENTHIC