Ischyrocerus, Conlan, 2021

Conlan, Kathleen E., 2021, New genera for species of Jassa Leach (Crustacea: Amphipoda) and their relationship to a revised Ischyrocerini, Zootaxa 4921 (1), pp. 1-72 : 59-61

Genus Ischyrocerus n. comb.

Supplementary Table S4

Type species. Ischyrocerus anguipes Krøyer, 1838

Diagnosis (with differences from Neoischyrocerus in bold). Body length at maturity 2–18 mm, though most species are> 5 mm long at adulthood. Primarily known from the Northern Hemisphere in cold temperate to polar waters from 32°N (La Jolla, California; J.L. Barnard 1969) to 81°N in the Arctic Ocean ( Stephensen 1944), collected from algae, hydroids, crabs or substrate unknown, 1 to ~ 2000 m. Widely known in Europe but not in the Southern Hemisphere, although one species was found at 42°S off of Chile (J.L. Barnard 1964).

Pereon: dorsally smooth (most species), ridged or carinate (some species).

Antenna 1: accessory flagellum 2 articles (second minute), flush with the flagellum or rarely projecting forward; antenna 2 peduncle article 4, 115–170% wider than antenna 1 peduncle article 2 (or rarely equal width), setae and setal pattern similar to antenna 1 or setae shorter, occasionally plumose.

Gnathopod 2, adult male: 100–220% the length of gnathopod 1, basis usually not especially elongate, propodus elongate and slender or short and broad (rarely similar to gnathopod 1); coxa 1 70–140% the depth of coxa 2; basis concave or straight; ischium anteriorly rounded or straight; propodus variably shaped, slender or broad (posterior length 120–290% of central width), palm only on the distal portion of the propodus or nearly the full length of the propodus, without a bulge or tooth defining it proximally at the junction of the carpus, palm variously toothed; dactyl 33–75% the length of the propodus.

Gnathopod 2, juvenile male: shorter than the adult male gnathopod 2, palm bearing 1–2 strong spines about midway along the length of the propodus, dactyl ending at the spines.

Gnathopod 2, female: only slightly larger than gnathopod 1, palm of the propodus convex or concave.

Pereopods 3 and 4: propodus, posterior margin bearing setae but not spines; c oxa 4, posterior margin straight to shallowly concave.

Pereopod 5, male: similar to but shorter than pereopods 6 and 7 or basis posteriorly concave; merus not differing.

Uropod 1: peduncle with short ventrodistal spinous process underlying the rami, length ~25–50% of the outer ramus length.

Uropod 3: peduncle bearing 0–2 rows of spines dorsally, ending in a corona of spines at the distal margin (rarely a single seta or single spine); rami with 0–1 spines (outer), 0–4 spines (inner), outer ramus subequal to a third shorter than the inner and bearing 0–9 cusps, occasionally with 1–2 apical spines that are straight or dorsally recurved.

Component species. Ischyrocerus anguipes Krøyer, 1838 (N. Europe and Arctic Ocean); I. latipes Krøyer, 1842 (Arctic Ocean); I. minutus Liljeborg, 1851 (N. Europe) ; I. megacheir ( Boeck, 1871) (40°N– 80°N, Atlantic to Arctic Ocean); I. brevicornis ( Sars, 1879) (E. Greenland, Arctic Ocean); I. tuberculatus ( Hoek, 1882) (Barents Sea, 71°N– 77°N); I. tenuicornis ( Sars, 1885) (N. Europe) ; I. nanoides ( Hansen, 1887) (Arctic, Baffin Bay and W Greenland, 61°N – 81°N); I. megalops Sars, 1894 (N. Europe) ; I. commensalis Chevreux, 1900 (E. Atlantic Canada and Saguenay Fjord); I. brusilovi Gurjanova, 1933 (Russian waters); I. enigmaticus Gurjanova, 1934 (Kara Sea, 78°58ʹN); I. cristatus Gurjanova, 1938 (Sea of Japan); I. elongatus Gurjanova, 1938 (Sea of Japan); I. rhodomelae Gurjanova, 1938 (Sea of Japan); I. serratus Gurjanova, 1938 (Sea of Japan); I. hanseni Stephensen, 1944 (64°N, between Iceland and Greenland); I. albanovi Gurjanova, 1946 (Arctic Ocean); I. laptevi Gurjanova, 1946 (Arctic Ocean); I. chamissoi Gurjanova, 1951 (Russian waters); I. dezhnevi Gurjanova, 1951 (Russian waters); I. krascheninnikovi Gurjanova, 1951 (Russian waters); I. stephenseni Gurjanova, 1951 (Russian waters); I. pelagops J.L. Barnard, 1962 (California) ; I. hortator J.L. Barnard, 1964 (off Chile); I. malacus J.L. Barnard, 1964 (California) ; I. gurjanovae Kudrjaschov, 1975 (Kurile Islands); I. tzvetkovae Kudrjaschov, 1975 (Kurile Islands); I. fractus King & Holmes, 2004 ( Ireland) .

Remarks. Species were retained in Ischyrocerus if they lacked the key characters noted above for Neoischyrocerus in the appearance of the male gnathopod 2, pereopod dactyls or uropod 3 spination pattern. Ischyrocerus fractus is the only species known in this genus where the male’s gnathopod 2 propodus is very little different from the female’s. It is also the smallest known at adulthood for this genus (2 mm).

Excluded, but uncertain generic status. Ischyrocerus camptonyx Thurston, 1974b from subantarctic Signy Island is not Ischyrocerus . It is possibly an undescribed species of Jassa or synonymous with J. alonsoae , in which case Thurston’s name would take precedence. Jassa thurstoni Conlan, 1990 (called J. falcata form 2 by Thurston) and Pleojassa moorei n. sp. (called J. falcata form 3) are also known from Thurston’s collections there. For I. camptonyx , hallmarks of the genus Jassa , rather than Ischyrocerus are the spines at the tip of the antenna 2, the sinuous palmed gnathopod 2, the strong overlap of the merus over the carpus on pereopods 3 and 4, the typical Jassa -like uropod 3 with long peduncle lacking mid-dorsal spines (but with a corona of spines around the distal margin), a lateral setal brush and the strong hooked spines at the tip of the outer ramus, and the telson with a long seta at each corner rather than a spine. However, Thurston describes the uropod outer ramus “with three stout hooked spines dorsally near apex and a minute comb with five-six teeth laterally”, which does not correspond to his illustration and are not Jassa -like. Possibly, though, his description could be interpreted differently. One of the three spines may be the apically immersed, dorsally recurved spine typical of Jassa , the other two spines are cusps, and the five–six teeth are minute dorsal cusps proximal to the two large ones. If so, then this also speaks of I. camptonyx as being a Jassa , either its own species or synonymous with J. alonsoae . It is not a Pleojassa , even though the male’s second-gnathopod resembles that of P. moorei , because this genus lacks a gill on gnathopod 2 while I. camptonyx possesses one. Thurston considered that the few males available for study were juvenile because they all lacked a thumb as in Jassa . However, some of these specimens were larger than the adult, ovigerous females. The female allotype was 4.5 mm and the male holotype was 5.5 mm. This suggests that the male holotype is actually an adult that will not produce a thumb, in which case it is not Jassa . Therefore, until the range of variation can be assessed in Thurston’s specimens, this species should remain in Ischyrocerus with a question as to its proper generic placement.

Uncertain status of Ischyrocerus anguipes in the Southern Hemisphere. Sars’ (1894) excellent illustrations of I. anguipes may have resulted in some mis-identifications by early workers in the Southern Hemisphere. Alternatively, their identifications were correct and I. anguipes was being introduced by shipping if the specimens came from ports. K.H. Barnard’s (1916) “ I. anguipes ” could have been a species of Neoischyrocerus , however, as the specimens were 3 mm and the male’s second gnathopod propodus was 3.5 x longer than wide with the dactyl nearly the full length of the propodus, which is typical of adult male Neoischyrocerus .

Schellenberg’s (1953) “ I. anguipes ” from L̹deritzbucht, Namibia was a 4.8 mm male which he stated differed from K.H. Barnard’s (1916) “ I. anguipes ” in its gnathopod 2 morphology. “The metacarpus of the 2nd gnathopod is shaped in the same way but stronger and exhibits a spination like on the first gnathopod. The palm is evenly finely corrugated (or: “wavy”) along its almost entire length.” (Der fast gleich geformte, aber stärkere Metacarpus des 2. Gnathopoden zeigt die Bestachelung wie am 1. Gnathopoden. Die Palma ist fast in ihrer ganzen Länge gleichmässig fein gewellt.) (translated from German to English by Jan Beermann, Alfred Wegener Institute, Bremerhaven, Germany). The similarly shaped gnathopods 1 and 2 suggest that this specimen was a female or juvenile, or an adult male of a different species, as adult male I. anguipes have a gnathopod 2 that is more than twice the length of gnathopod 1, with a concave, rather than convex palm. Schellenberg illustrated the uropod 3 outer ramus as 5-cusped, terminating in a basally immersed, dorsally recurved spine similar to Jassa . The corona of spines at the peduncle’s distal margin is typical of Ischyrocerus and Jassa .

“ I. anguipes ” captured off the coast of Ceylon in ~ 150 m depth were briefly described by Walker (1904) but not illustrated. One male and one ovigerous female were 2.5 mm long. The male’s gnathopod 2 was similar to that of I. anguipes , but this could also be Neoischyrocerus which has similarly shaped, though more pendulous second gnathopods with the dactyl in the largest males nearly the full length of the propodus.

Chilton (1921) described “ I anguipes ” from Lyttelton, New Zealand which were up to 6 mm long, saying that they closely resembled Sars’ (1894) illustrations of that species. His description was minimal, however, and he provided no illustrations.