Ischyromene Racovitza, 1908

Bruce, Niel L, 2006, A new species of Ischyromene Racovitza, 1908 (Sphaeromatidae: Isopoda: Crustacea) from intertidal marine habitats in New Zealand, Zootaxa 1220, pp. 19-34 : 20-23

Genus Ischyromene Racovitza, 1908

Ischyromene Racovitza, 1908 : LXII; Harrison & Holdich, 1982: 85.— Kensley & Schotte, 1989: 217.— Poore, Lew Ton & Bruce, 2002: 238.

Type species: Ischyromene lacazei Racovitza, 1908 , by monotypy.

Species included ( New Zealand species in bold): I. australis ( Richardson, 1906) , Cape Town, South Africa; I. australoides ( Barnard, 1940) , South Africa; I. barnardi ( Menzies & Glynn, 1968) , Puerto Rico; I. bicarinata Harrison, 1981 , Israel (Mediterranean); I. bicolor ( Barnard, 1914) , South Africa; I. brunnea ( Vanhöffen, 1914) , St. Paul Is., southern Indian Ocean; I. codii ( Nobili, 1906; figured in Nobili 1907), Tuamotu Islands; I. condita ( Hurley & Jansen, 1977) , New Zealand; I. cordiforaminalis ( Chilton, 1883) , New Zealand; I. eatoni ( Miers, 1875) , Kerguelen Is.; I. hirsuta ( Hurley & Jansen, 1971) , New Zealand; I. huttoni ( Thomson, 1879) , New Zealand; I. insulsa ( Hurley & Jansen, 1977) , New Zealand; I. kokotahi sp. nov., New Zealand; I. lacazei Racovitza, 1908 (type species), Atlantic and Mediterranean France; I. magna Barnard, 1954 , South Africa; I. menziesi ( Sivertsen & Holthuis, 1980) , Tristan da Cunha; I. mortenseni ( Hurley & Jansen, 1977) , New Zealand; I. polytyla Harrison & Holdich, 1982 , Queensland, Australia; I. rubida ( Baker, 1926) , New South Wales, Australia; I. sapmeri ( Kensley, 1976) , St. Paul Is., southern Indian Ocean; I. scabricula ( Heller, 1868) , South Africa; I. tuberculata ( Menzies, 1962) , Chile.

Incertae sedis: Currently housed in the genus but regarded as incertae sedis: I. ovalis ( Barnard, 1914) , South Africa; I. macrocephala ( Krauss, 1843) , South Africa.

Diagnosis

Pleon of 4 segments, with 2 long, separate sutures, which run to the posterior margin. Pleotelson posterior margin perforate, with posteriorly closed and dorsally directed foramen, or with ventrally open exit channel. Penial processes short, not extending beyond pleopod peduncle. Pleopod 1 with both rami indurate (i.e. thickened) or the endopod with an indurate mesial margin; pleopods 1–3 endopods noticeably shorter than the exopods. Uropod rami biramous, lamellar.

Description

Body vaulted, dorsal surfaces smooth, with sparse setae, with ability to conglobate; not or weakly sexually dimorphic. Head with rostral point present, simple, not separating antennular bases; anterior margin simple, without incision, lateral margins not laterally extended to body outline. Eyes lateral, simple. Pereonites smooth or nodular or ornamented (to various degrees); 2–7 or 5–7 with posterior margin raised; pereonite 1 lateral margins not anteriorly produced, not laterally enclosing head, anteriorly without keys. Sternite 1 without cuticular mesial extensions. Pereonite 6 simple, without bosses, processes or marginal extensions. Pereonite 7 as wide as pereonite 6, forming part of body outline, dorsally without bosses, processes or marginal extensions or posterior margin with posteriorly produced rim. Coxae ventrally wide, those of pereonites 2–7 overlapping anterior over posterior, rectilinear, coxae without ventral ‘lock and key’ processes, without grooved articulation; those of pereonite 6 not large, not overlapping those of pereonite 7. Pleon consisting of 4 visible segments (as determined by lateral sutures); pleonite 1 entire, posterior margin even, as wide as remainder of pleon, extending to pleon lateral margins; pleonal sternite present; sutures running to posterior margin, all separate, lateral or both sutures long; dorsal surface without process; posterior margin even, with ‘keys’. Pleotelson vaulted, anteriorly as wide as pleon, without dorsal process; posterior margin forming posteriorly directed ventrally open tube or with dorsally directed ventrally and posteriorly enclosed short tube or with subapical round or Y­shaped foramen connected to posterior by narrow slit, with ventral thickened rim; lateral margins simple. Membrana cingula absent.

Antennule peduncle with basal articles medially not in contact, articles 1 and 2 not robust, articles 1–3 of similar stoutness; article 1 not anteriorly produced, without anterior lobe; article 2 approximately 0.5 as long as article 1, without anterodistal lobe; articles 1 and 2 not flattened; with articles 2 and 3 collinear, article 3 as long as or shorter than article 2; flagellum shorter than peduncle, longer than peduncular article 3. Antenna peduncle articles all collinear, articles less robust than antennule, peduncular articles all of sub­similar thickness.

Epistome anteriorly narrow, without median constriction, anteriorly flush with head, not projecting, anteriorly not prominently extended, short, wrapping around labrum.

Mandible incisor wide, multicuspid; lacinia mobilis present, tri­cuspid; spine row normal; molar process gnathal surface with transverse ridges, rounded. Maxillule lateral lobe RS with some or all serrate, medial lobe with 4 major RS, these setae being heavily serrate. Maxilla with setae on middle and lateral lobes serrate. Maxilliped palp articles 2–4 medial margins lobate, article 2 not expanded; endite distal margin rounded, with clubbed RS, dorsomedial margin with long curved serrate RS.

Pereopod 1 ambulatory; dactylus secondary unguis short, robust, trifid. Pereopod 2 similar in proportion to pereopod 3; dactylus with secondary unguis trifid, short and stout. Pereopods 3–7 dactylus with secondary unguis trifid. Pereopods with inferior margins of ischium to carpus bearing dense setulose fringe, ischium superior margin with sinuate acute RS; pereopods 1–3 or 4 ischium superior margin without long stiff slender setae. Pereopods 1 (or 1–3), inferior margins of merus, carpus and propodus palm without conspicuous RS.

Penial processes entirely separate, basally in contact, short (not extending beyond pleopod peduncles), straight, apex bluntly rounded.

Pleopod 1 rami operculate; with exopod indurate; rami collinear; endopod of similar proportions to exopod, medial margin indurate, distally triangular, endopod proximomedial heel present; exopod distally rounded, exopod distal margins not serrate. Pleopod 2 endopod markedly longer than exopod; exopod distal margins not deeply serrate; appendix masculina inserted basally or sub­basally, proximal lobe absent, with straight margins or terminally spatulate, about as long as endopod, distally bluntly rounded. Pleopod 3 exopod transverse suture absent, endopod longer that exopod. Pleopod 4 rami without PMS; exopod transverse suture absent, exopod thickened transverse ridges present, exopod lateral margin not thickened, with short simple marginal setae; endopod thickened, transverse ridges present; mesial margin without deep distal notch; without proximomedial lobe. Pleopod 5 exopod transverse suture present, entire, thickened transverse ridges present, lateral margin without short simple setae, not thickened; with 3 discrete scale patches on protruding lobes; endopod with thickened transverse ridges present, endopod with proximomedial lobe.

Uropod rami not strongly flattened, not forming part of continuous body outline; exopod lamellar, exopod similar in length to endopod, proximally inserted (at anterolateral angle), lateral margin simple, smooth, distally broadly rounded; endopod lamellar, distally broadly rounded.

Female. Mouthparts not metamorphosed. Marsupium formed from 3 pairs of oostegites, anterior pocket absent, posterior pocket present, oostegites overlapping at midline. Otherwise generally similar to the male.

Remarks

Ischyromene Racovitza, 1908 , a long­established genus of 23 mostly small species (<5 mm), remains poorly known. Species of the genus, in the past mostly identified as species of Dynamenella Hansen, 1905 , were allocated to Ischyromene by Harrison & Holdich (1982) in their revision of the genus. Ischyromene is readily recognized by the posterior margin of the pleotelson being perforate, short penial processes in males, pleopod 1 with both rami indurate (i.e. thickened) or the endopod with an indurate mesial margin, pleopods 2 and 3 exopods being noticeably shorter than the endopods, and the pleon with two separate sutures which run to the posterior margin. Superficially the genus resembles both Dynamenella and Paradella , genera not known from New Zealand, but the shape and thickened cuticle of pleopod 1 (usually easy to observe) readily distinguishes Ischyromene from those genera.

Ischyromene belongs to a group of primarily Southern Hemisphere genera (the ‘ Ischyromene group’; see Bruce 1995) characterized by the wholly or partly indurate (thickened) first pleopods (among other characters). Related and often sympatric genera include Cymodocella Pfeffer, 1887 and Dynamenopsis Baker, 1908 , these genera being distinguished by a prominent posteriorly directed and ventrally closed tube in the former, and large coxae on pereonite 6 in the latter ( Harrison & Holdich 1982).

Most species of the genus are minimally described and very poorly characterized. Detailed descriptions have been published for only two species — I. lacazei Racovitza, 1908 ( Schüller & Wägele 2005a, b) and I. polytyla Harrison & Holdich, 1982 . The genus has a discontinuous distribution with two species known from the eastern Atlantic and Mediterranean, and one species from Puerto Rico; most species are from temperate and cool­water Southern Hemisphere regions. A character state present in several New Zealand species is the presence of dense and laterally extended setae on the margins of the pereonites and pleon (present species; see also Hurley & Jansen 1977), and of an anterior spine in the pleotelson foramen. Species also differ in being smooth bodied (e.g. I. huttoni ) or ornamented on the pleotelson (most species). The posterior margin of the pleotelson may have a posteriorly directed and ventrally open sinus or may have a dorsally directed and posteriorly closed foramen. At present there is insufficient data to assess the phylogenetic significance of these differing character states within the genus.

Distribution

Predominantly Southern Hemisphere, with most records from south of the Tropic of Capricorn, the exception being I. codii from French Polynesia ( Harrison & Holdich 1982). In the Northern Hemisphere one species is known from the Caribbean and two species from the eastern Atlantic and Mediterranean (not one as stated by Schüller & Wägele 2005a, b); all species are from intertidal or shallow water habitats.