Gnorimosphaeroma Menzies, 1954

Wetzer, Regina, Wall, Adam & Bruce, Niel L., 2021, Redescription of Gnorimosphaeroma oregonense (Dana, 1853) (Crustacea, Isopoda, Sphaeromatidae), designation of neotype, and 16 S-rDNA molecular phylogeny of the north-eastern Pacific species, ZooKeys 1037, pp. 23-56 : 23

Gnorimosphaeroma Menzies, 1954

Isopoda : Sphaeromatidea : Sphaeromatoidea : Sphaeromatidae

Gnorimosphaeroma Menzies, 1954: 5; Kussakin 1979: 406; Harrison and Ellis 1991: 939.

Nishimuraia Nunomura, 1988: 1.

Type species.

Spheroma oregonensis Dana, 1853; now Gnorimosphaeroma oregonense (Dana, 1853); by original designation.

Diagnosis.

Body vaulted, dorsal surfaces smooth or polished in appearance, without setae. Eyes lateral, simple, without posterior lobe. Pleon consisting of 4 visible segments (as determined by lateral sutures), sutures (except first) long extending from lateral margin, separated medially by 24-28% pleon width; pleonite 1 entire, posterior margin even, narrower than remainder of pleon, not extending to pleon lateral margins. Pleotelson vaulted, anteriorly as wide as pleon, without dorsal process; posterior margin entire, simple, arcuate. Maxilliped palp articles 2-4 medial margins lobate, article 2 not expanded. Penial processes entirely separate, basally close set, short (not extending beyond pleopod peduncles). Uropod rami lamellar, similar in size, exopod shorter than endopod, inserted near anterolateral angle of peduncle; endopod lateral margin simple, finely serrate or smooth, distally broadly rounded; both rami distally broadly rounded or narrowly rounded.

Description.

Body vaulted, dorsal surfaces smooth or polished in appearance, without setae; coxal and other margins smooth, with ability to conglobate; not or weakly sexually dimorphic. Head with rostral point present, dorsally visible, simple, not separating antennular bases; without paired incisions in front of eyes, lateral margins not laterally extended to body outline (antennules more or less ventral). Eyes lateral, simple. Pereonite 1 lateral margins not anteriorly produced, not laterally enclosing head, pereonites 2-7 with posterior margin not raised, pereonite 1 anteriorly with 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. Coxae distally narrow, those of pereonites 2-7 overlapping the one behind, rounded, with ventral 'lock and key’ processes, with 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, narrower than remainder of pleon, not extending to pleon lateral margins; sutures (except first) running to lateral margin, all separate, sutures long (separated medially by 24-28% pleon width); pleonal sternite absent; dorsal surface without process; posterior margin even, with ‘keys’. Pleonite 5 posterior margin entire (not fused with pleotelson). Pleotelson vaulted, anteriorly as wide as pleon, without dorsal process; posterior margin entire, simple, arcuate; ventrolateral margins forming ridge.

Marsupium formed from four pairs of oostegites, arising from pereonites 1-4; anterior pocket absent, posterior pocket absent, oostegites overlapping at mid-line (except 1).

Antennule peduncle with basal articles medially not in contact, 1 and 2 robust, article 3 slender; article 1 not produced, without anterior lobe; article 2 approximately 0.5 as long as article 1; with articles 2 and 3 colinear, article 3 longer than article 2; article(s) not flattened; flagellum shorter than peduncle, longer than peduncular article 3. Antenna peduncle articles all colinear (or curving regularly), less robust than antennule, peduncular articles all of similar thickness.

Epistome anteriorly narrow, with median weak constriction, anteriorly flush with head, not projecting; elongate. Mandible incisor wide, 4-cuspid; lacinia mobilis present; spine row normal; present, molar process gnathal surface with transverse ridges, rounded. Maxillula lateral lobe robust setae with some or all serrate, mesial lobe with major robust setae, 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, anteromesial (upper) marginal ridge without long curved serrate robust setae.

Mouthparts of female not metamorphosed.

Pereopod 1 ambulatory; dactylus secondary unguis short, robust, simple; setae on superodistal corner of merus only very long. Pereopod 2 similar in proportion to pereopod 3; dactylus with secondary unguis simple, short and stout. Pereopods 3-7 dactylus with secondary unguis simple. Pereopods with inferior margins of ischium to carpus without dense setulose fringe, ischium superior margin without sinuate acute robust seta, pereopods 1-3 or 4 ischium superior margin with few long stiff slender setae. Pereopods 1 (or 1-3), inferior margins of merus, carpus and propodus palm pereopod 1 only with robust setae on propodus inferior margin.

Penial processes entirely separate, basally close set, short (not extending beyond pleopod peduncles), widest near base, apex bluntly rounded.

Pleopod 1 rami not operculate; exopod lamellar; rami exopod with longitudinal axis weakly oblique; endopod of similar proportions to exopod, mesial margin lamellar, distally triangular, endopod proximomedial heel absent; exopod distally rounded or distally subtruncate or truncate, exopod distal margins not serrate. Pleopod 2 endopod ca. as long as exopod; exopod distal margins not deeply serrate; appendix masculina inserted basally, with straight margins, distally abruptly narrowed, longer than and extending beyond endopod (1.14 × as long as endopod), distally narrowly rounded. Pleopod 3 exopod transverse suture present, endopod of similar proportions to exopod. Pleopod 4 rami with PMS; exopod transverse suture present, incomplete, thickened transverse ridges absent, lateral margin not thickened, with short simple marginal setae; endopod thickened transverse ridges absent; mesial margin without deep distal notch; endopod without proximomedial lobe. Pleopod 5 exopod transverse suture present, entire, thickened transverse ridges absent, lateral margin with short simple setae, lateral margin not thickened, with 3 discrete scale patches; scale patches flush or weakly domed; endopod with thickened transverse ridges absent, endopod without proximomedial lobe.

Uropod rami not strongly flattened, not forming part of continuous body outline; exopod shorter in length than endopod, exopod lamellar, inserted near anterolateral angle of peduncle-endopod, lateral margin simple, finely serrate or smooth, distally broadly rounded; endopod lamellar, distally broadly rounded or narrowly rounded. Uropod endopods not in contact posteriorly.

Remarks.

Gnorimosphaeroma is in a general sense quite unremarkable in appearance, with no species showing any sort of dorsal ornamentation of tubercles, processes, or pereonal and pleonal ridges that characterize so many genera of Sphaeromatidae . As such, there is a lack of readily obvious characters by which to identify the genus. Gnorimosphaeroma , on morphological criteria, is most similar to the genera Bilistra Sket & Bruce, 2004, Exosphaeroma Stebbing, 1900, Lekanesphaera Verhoeff, 1943, Neosphaeroma Baker, 1926 and Sphaeroma Bosc, 1802. The latter three genera can be differentiated from Gnorimosphaeroma in the first instance by having the uropodal exopod lateral margin with one or more serrations or notches (among other characters).

Exosphaeroma is a large genus with 40 species at the last count ( Boyko et al. 2008) that, as presently constituted, contains both smooth bodied species as well as some with coarsely pitted or ridged dorsal surfaces (e.g., see Kensley 1978; Espinosa-Pérez and Hendrickx 2001; Bruce 2003), and also species with greatly enlarged uropodal rami (e.g., see Kensley 1978; Bruce 2003; Wall et al. 2015). Some of the smooth-bodied species of Exosphaeroma are superficially similar to Gnorimosphaeroma , but can be distinguished by the pleonal sutures running to the posterior margin (to the free lateral margin in Gnorimosphaeroma ), as well as pleonite 1 having two flat sub-median lobes on the posterior margin (see Bruce 2003: figs 14E, 18F).

Bilistra is similar in gross morphology and also occupies coastal freshwater habitats. Bilistra differs from Gnorimosphaeroma in having a far shorter uropodal exopod (ca. half as long as endopod), shorter pleonal sutures that run to the pleon posterior margin (not lateral margin); the inferior margins of pereopods ischium or merus to propodus have a dense setulose (fur-like) fringe while the superior margins lack long setae altogether. Bilistra is presently restricted to New Zealand, but there is also one species in South Africa, from supralittoral brackish pools and tidal streams that is currently classified as Pseudosphaeroma barnardi Monod, 1931 that is in need of redescription and formal reassignment to Bilistra (NLB, pers. obs.).

Gnorimosphaeroma pereopod setation is inconsistently illustrated, even within species, despite being a potentially significant character. The redescription given here, and figures of Hoestlandt (1975) show long setae on the superior or superodistal margin of the merus and long setae on the inferior margin of the ischium and merus. Such setae were not mentioned or figured in Menzies’ (1954) genus diagnosis or species descriptions. Such setae are also apparently absent from all northwestern species (e.g., Hoestlandt 1975, 1977; Kwon and Kim 1985; Nunomura 1998, 1999a, 2007).

Neotype designation.

It has been long established that all of Dana’s (1852) isopod material, and therefore all the type material for the many species of isopod that he named, was lost with the sinking of the ship USS ' Peacock ' on the bar of the Columbia River in 1841 ( Bruce 1986: 220; 2004: 228; 2009: 211; Poore and LewTon 1993: 234). Gnorimosphaeroma oregonense (Dana, 1853) is one such species.

Species of Gnorimosphaeroma are uniform in appearance, and to date no assessment has been made of intrinsic variability within species. Some species of Gnorimosphaeroma occur sympatrically and there are many exceedingly similar species. At present few species have been described in full detail. Furthermore, records of G. oregonense are somewhat inconsistent in the details presented and the material is not always available for re-examination, so that it is not always possible to confirm the correct identity of previous records and indeed also on occasion, new material. We consider that designating a neotype is necessary to clearly characterize the identity of this species, to allow for the genus to be precisely diagnosed based on the type species and to permit unambiguous identification and separation from other sympatric congeneric species.

Dana (1853) did not indicate a specific type locality, but stated that the species had been obtained from “Puget’s Sound, Oregon; also, Bay of San Francisco, California". One may infer that the first mentioned location is the type locality but that remains an inference, and furthermore one cannot be certain that the material consists of only one species, given that there are four species in the region and also that the morphology of purported species apparently changes from low to high latitudes (present study). The neotype has been chosen from specimens collected as near as practically possible to the original type locality, and is now Stanley Park, 49.294°N, 123.155°W (British Columbia, Canada), ca. 150 km north of Puget Sound.

Included species.

Gnorimosphaeroma albicauda Nunomura, 2005, G. akanense Nunomura, 1998, G. anchialos Jang & Kwon, 1993, G. boninense Nunomura & Satake, 2006, G. chejuense Kim & Kwon, 1988, G. chinense (Tattersall, 1921), G. hachijoense Nunomura, 1999b, G. hoestlandti Kim & Kwon, 1985, G. hokurikuense Nunomura, 1998, G. insulare (Van Name, 1940), G. iriei Nunomura, 1998, G. kurilense Kussakin, 1974, G. naktongense Kwon & Kim, 1987, G. noblei Menzies, 1954, G. oregonense (Dana, 1853), G. ovatum (Gurjanova, 1933), G. paradoxa (Nunomura, 1988), G. pulchellum Nunomura, 1998, G. rayi Hoestlandt, 1969, G. rebunense Nunomura, 1998, G. saijoense Nunomura, 2013, G. shikinense Nunomura, 1999b, G. tondaense Nunomura, 1999b, G. trigonocaudum Nunomura, 2011, G. tsutshimaense Nunomura, 1998.

Notes.

The original diagnosis of the genus was provided by Menzies (1954: 5). A more complete diagnosis of the genus is provided here (see above). Menzies (1954) suggested that Neosphaeroma pentaspina Baker, 1926 could possibly be attributed to Gnorimosphaera were it to be redescribed, while Harrison and Holdich (1984) indicated some shared characters, notably the pleon suture, but the species is presently considered as incertae sedis. Smooth-bodied Sphaeromatidae similar to Gnorimosphaeroma are summarized in the genus remarks above and reoccur in several sphaeromatid clades. In their molecular analysis Wetzer et al. (2018) demonstrated that this a plesiomorphic trait and that Neosphaeroma is basal to or nested within the Cymodoce clade and is not closely related to Gnorimosphaeroma .