Ischyrocerus fractus, King & Holmes, 2004

King, Rachael & Holmes, J. M. C., 2004, A new species of Ischyrocerus (Crustacea: Amphipoda) from Ireland, with a review of Ischyrocerus anguipes and Ischyrocerus minutus from the North-East Atlantic, Journal of Natural History 38 (14), pp. 1757-1772 : 1762-1767

Ischyrocerus fractus sp. nov.

( figures 3–5 )

Type material

Thirty-five specimens, Lough Hyne, Co. Cork (Irish Grid Reference W 097280), underwater light-trap, 4 m, August, 1985. HOLOTYPE ” ( NMI 58.2002 ). ALLOTYPE „ ( NMI 59.2002 ). PARATYPES 10„„, 8””, 11 juveniles ( NMI 60.2002 ); 2„„, 2”” ( NHM 2002.899 – 902 ) .

Other material

Numerous specimens, Lough Hyne, Co. Cork (Irish Grid Reference W 097280), underwater light-trap at night at various depths from 2 to 20 m, 1981–2002, also in weed collected by hand in shallow water (NMI).

Description

Female. Size: 2.2 mm. Head with lateral cephalic lobes broadly produced. Eyes large, rounded. Antenna 1 slender, sparsely setiferous, more than half body length, peduncle article 3 much longer than 1 and sub-equal to 2; flagellum with four articles, the first flagellar article longer than the following three, each of which bears a single aesthetasc; accessory flagellum distinct, with two articles, characteristically standing up from the principal flagellum. Antenna 2 slender, longer than antenna 1, sparsely setiferous; peduncular article 5 distinctly longer than article 4; flagellum with four to five articles. Mouthparts typical of genus.

Coxal plates 1–4 sub-equal; coxa 5 similar in length to those preceding but with small posterior lobe; coxae 6 and 7 short. Gnathopods subchelate, similar in shape and size, moderately setiferous; basis narrow without anterior lobe; carpal lobe broad; propodus oval (slightly wider in gnathopod 1), palm delimited by a pair of robust setae; dactylus long and serrated. Pereopods 3–4 relatively short, sparsely setiferous. Pereopods 5–7 longer than preceding pereopods, progressively longer from 5 to 7, also sparsely setiferous. Basis of pereopod 5 narrow, widening proximally in pereopods 6 and 7.

Epimera rounded; epimera 2–3 with small notch and seta on the posterodistal corner. Uropods 1–2 biramous, peduncle elongate, with few robust setae; rami unequal lengths. Uropod 3 outer ramus shorter than inner ramus, tapering and recurved, with a dorsal row of five denticles, with a small sub-apical seta; inner ramus straight, reaching well beyond the end of the outer ramus and bearing a small sub-apical seta. Telson not cleft, with a pair of dorsolateral clusters of fine and robust setae at mid-length, apex rounded. Ovigerous females with three to six eggs in the brood pouch.

Male. Size: 2 mm. Similar to female, although smaller. Eyes relatively larger than those of the female. Flagellum of antenna 1 with five to six articles, bearing several aesthetascs. The pattern of aesthetascs is typically as follows: the first flagellar article with four to five groups of aesthetascs on the inner ventro-lateral side (proximally four together, then three to four pairs), the following articles with the aesthetascs in pairs, the second to last article with a long and a short aesthetasc, and the last article with one aesthetasc.

Gnathopods and pereopods similar to those of the female, although slightly larger in proportion to body size. Propodus of gnathopod 2 slightly more elongated than in gnathopod 1.

Colour

Body: pale white with scattered brown chromatophores in dorsal bands, similar to the patterns of I. minutus . Eyes: dark brown surrounded by a light brown ring.

Etymology

The specific name fractus is Latin for`broken’ and highlights the fragile nature of these animals when handled.

Distribution Known only from Lough Hyne, Co. Cork, Ireland.

Remarks

Ischyrocerus fractus differs from the other Ischyrocerus species primarily by the form of the unspecialized gnathopods, similar in both sexes, and in the minute apical seta on the outer ramus of uropod 3. In all other Ischyrocerus species, the male gnathopod 2 propodus is greatly enlarged (much larger than gnathopod 1, and much larger than that of the female). Though quite small (2–3 mm), numerous similar-sized males were collected in Lough Hyne, in company with similarly sized ovigerous females. All the males possessed well-developed penial processes on the seventh sternite, so were clearly adult. A notable feature of I. fractus in the collections was the frequent absence of the fifth pereopods. Indeed, there was scarcely a specimen with all appendages intact. When pereopod 5 was still attached, it was weak and spindly, indicating possible regeneration. Specimens collected by hand invariably had the antennae and several legs missing and it was possible to obtain reasonable specimens only because of the delicate capturing technique of using a light-trap ( Holmes and O’Connor, 1988). The small size and fragility of the species may account for it having been overlooked previously.

The presence of ventro-lateral clustered groups of aesthetascs on the male antenna 1, which are reminiscent of the callynophore present in many amphipod families ( Lowry, 1986), is noted. The callynophore has been hypothesized to function in prey/carrion detection and also in pheromone detection and is often present in swimming amphipods ( Lowry, 1986). The life history of Ischyrocerus fractus is unknown. However, specimens were mainly found in light-traps and so it is possible that the grouped aesthetascs form some sort of rudimentary callynophore or a precursor to it.

Ecology

Little is known about the ecology of this species, except that it is marine and attracted to light. It was found in shallow water, 2–5 m depth, usually amongst the red alga Gelidium and the sponges Hymeniacidon and Halichondria . Its distribution within Lough Hyne is restricted: along the south shore, Whirlpool Cliff, and off the west end of Castle Island. No specimens were taken in the North Basin or in Barloge Creek. All these sites have been described in detail in numerous papers on the geography and ecology of Lough Hyne (see Kitching and Thain, 1983; Wilson, 1984). No other species of Ischyrocerus was found in the lough, despite intensive searching.

Ischyrocerus minutus Liljeborg

( figures 6–7 )

Ischyrocerus minutus Liljeborg, 1851a: 335 ; 1855b: 128; Sars, 1894: 589, pl. 210.

Ischyrocerus anguipes: Stebbing, 1906: 657 ; Schellenberg (in part), 1942: 207, figure 169c; Stephensen, 1942: 394; Bousfield (in part), 1973: 192, pl. LVIII (IM GN2); Lincoln (in part), 1979: 556, figure 267d.

? Podocerus isopus Walker, 1889: 209 , pl. 11, figures 11–13.

Material examined

Norway: Norveg. Occid. ( ZMO F13581) ; lok? ( ZMO F13582) ; Hitteren ( ZMO F13583) ; Karmø ( ZMO F13584) . Scotland: Shetland Islands, Aith Wick ( NHM 1974.490.11) (labelled as I. anguipes ); Mormond Hills ( NHM 1956.10.10.308) . Ireland: County Cork ( NHM 1925.9.8:1560–1564); Sherkin Island , County Cork

( NMI 74.1987); Dalkey Island, County Dublin ( NMI 58.1986).

Figures

FIG. 3. Ischyrocerus fractus sp. nov. (Lough Hyne, Ireland). Male: (A) habitus. Female: (B) habitus. Scales: 1.0 mm.

FIG. 4. Ischyrocerus fractus sp. nov. (Lough Hyne, Ireland). Male: (A) antenna 1; (I) gnathopod 2; (J) gnathopod 1. Female: (B) antenna 1; (C) antenna 2; (D) maxilliped; (E) mandible; (F) maxilla 1; (G) labium; (H) maxilla 2; (K) gnathopod 2; (L) gnathopod 1. Scales: 0.1 mm, a (A–C); b (D–H); c (I–L).

FIG. 6. Ischyrocerus minutus Liljeborg (Dalkey Island, Ireland). Male: (A) habitus. Female: (B) habitus. Scales: 0.5 mm (note scale difference from figure 1).

FIG. 7. Ischyrocerus minutus [Liljeborg (Dalkey Island, Ireland—except (E)]. Male: (A) gnathopod 1; (B) gnathopod 2; (E) uropod 3 (Norway); (F) uropod 3. Female: (C) gnathopod 1; (D) gnathopod 2; (G) uropod 3. Scales: 0.1 mm. a (A, B); b (C, D); c (E, F); d (G).