Munidopsis quadrata Faxon 1983

<i>Munidopsis quadrata</i> Faxon, 1983 (Fig. 8 <i>j–n</i>, Supplementary Fig. S2.) Material examined <i>Lectotype.</i> MEXICO: Nayarit, off Tres Marias Islands, leg. USFC Steamer <i>Albatross</i>, Stn 3425, 18.iv.1891, 21.316660°N, 106.40000°W, 680 fms (1244 m): 1 M 9.8 mm (MCZ IZ CRU-4560). <i>Paralectotype.</i> MEXICO: Nayarit, off Tres Marias Islands, leg. USFC Steamer <i>Albatross</i>, Stn 3424, 18.iv.1891, 21.25000°N, 106.38333°W, 676 fms (1236 m): 1 ov. F 9.9 mm (MCZ IZ CRU-4559). <i>Non-type specimens</i>. USA: San Diego Trough, off California, leg. Robert Hessler, Spencer Luke, R /V <i>Melville</i>, Stn SIO69-486, 13.xii.1969, 32.41670°N, 117.49670°W, 1250 m: 6 M 6.6–10.0 mm (SIO-BIC C638). — USA: San Diego Trough, off California, leg. Harim Cha, Kent Trego and students, R/V <i>New Horizon</i>, otter trawl, 27.x.2007, 32.60330°N, 117.54180°W, 1210 m: 1 ov. F 9.9 mm (SIO-BIC C11814), 1 ov. F 7.8 mm (SIO-BIC C11025). Diagnosis Carapace granulated, dorsally covered in small granules, laterally unarmed, with dorsal deep furrows, cervical grooves indistinct. Gastric and cardiac region with a longitudinal protuberance. Rostrum triangular, broad base and lateral margins convergent, serrated, directed upwards. Frontal margins slightly oblique, slightly concave behind ocular peduncle. Orbit not distinctly excavated, outer orbital angle blunt. Anterolateral angle unarmed or armed with a small spine. Branchial margin unarmed. Abdominal somites with transverse ridge, somites 2–5 armed with a spine. Telson divided into 9 plates. Sternite 3 moderately broad, anterolateral angles produced, sternite 4 subtriangular. Eyes unarmed, movable, epistomial spine absent. Article 1 of peduncle with welldeveloped dorsolateral and distolateral spines. Article 1 of antenna with distomesial spine. Mxp3 merus 0.5–0.6× longer than ischium, subrhomboidal in lateral view. P1 slender, elongate; meri and carpi with small spines on all surfaces, fixed finger without denticulate carina on distolateral margin. P2–4 slender, spinose; dactyli moderately slender, curving, flexor margin with dactylar teeth along all margins decreasing proximally. Epipods absent from all pereopods. Eggs Approximately 35–55 eggs of 1 mm. Colouration Unknown. Distribution Northeast Pacific, from Mexico to Canada. Genetic data <i>COI</i>, <i>16S</i> rRNA and <i>28S</i> rRNA. Remarks <i>Munidopsis quadrata</i> belongs to a group of species with a frontal margin without delimited orbit, telson with 9–10 plates, eye movable, unarmed, with peduncle larger than cornea. The most closely related species is <i>M. carinipes</i> from Panama and Costa Rica. Indeed, there is no support for considering these species as different taxa according to molecular data (Fig. 2, 3 <i>b</i>). However, both taxa are morphologically highly divergent, and the distribution ranges do not overlap. Both species are easily distinguished by the following characters: • The anterolateral angle of the carapace is often armed with a small spine in <i>M. quadrata</i> whereas this angle is always unarmed in <i>M. carinipes</i>. • The dorsal surface of the carapace is porose and smooth in <i>M. carinipes</i> whereas this is densely covered in granules in <i>M. quadrata</i>. • Abdominal somite 2 is armed with a median broad spine in <i>M. quadrata</i> whereas this spine is absent in <i>M. carinipes.</i> • Abdominal somites 3–5 are armed with a median small spine in <i>M. quadrata</i>, whereas these spines are much broader in <i>M. carinipes</i>. • The rostral margins are subparallel in <i>M. carinipes</i> whereas these are convergent in <i>M. quadrata</i>. • The P1 meri and carpi are spinose in <i>M. quadrata</i> whereas these are unarmed in <i>M. carinipes</i>. • The P2–4 meri are dorsally and ventrally carinated and smooth in <i>M. carinipes</i>, whereas these are cylindrical in cross-section, spinose and not carinated in <i>M. quadrata</i>. • The walking legs P2–4 are more slender in <i>M. quadrata</i> (propodus 6–7× as long as high) than in <i>M. carinipes</i> (propodus 4–5× as long as high). All these morphological characters are consistent for all specimens examined from Panama and Costa Rica (<i>M. carinipes</i>), and from Mexico and California (<i>M. quadrata</i>). Therefore we decided to maintain the species status of both taxa, despite the lack of genetic support. Explanatory hypotheses for the molecular and morphological incongruence in this case would be: (1) recent speciation including incomplete lineage sorting for the analysed markers, (2) hybridisation or introgression between both species, or (3) extreme phenotypic variability. The lack of morphotypes of <i>M. carinipes</i> in <i>M. quadrata</i> populations and <i>vice versa</i> led to the prevalence of hypotheses (1) and (2) over hypothesis (3). To test these hypotheses, we would need broader population sampling and nuclear markers or ultimately genomic data.

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