Platorchestia paraplatensis, Serejo & Lowry, 2008

Serejo, C. S. & Lowry, J. K., 2008, The Coastal Talitridae (Amphipoda: Talitroidea) of Southern and Western Australia, with Comments on Platorchestia platensis (Krøyer, 1845), Records of the Australian Museum 60 (2), pp. 161-206 : 189-190

Platorchestia paraplatensis n.sp.

Figs 21–24

Type material. HOLOTYPE: male, 19.3 mm AM P69144 (slides, stubs J037–J042, J043 [part P7]) . PARATYPES: male, 15.1 mm (stub J044, habitus) AM P69145 ; 1 female, 12.5 mm, AM P69146 (1 slide, stub J036, SEM micrographs) ; 355 specimens, AM P69147 , juvenile male, 11 mm, AM P69149 (stub J043 [part, G2]) ; west of Blackwell Reach , Swan River, Perth (32°1.91'S 115°45.72'E), flat beach with stones and large patches of Enteromorpha, C. Serejo, J.K. Lowry & D. Jones , 26 Oct. 2003, WA 763 ; 242 specimens, AM P69148 , east of Point Walter , Swan River, Perth, (32°01'S 115°47'E), flat sandy beach with large patches of Enteromorpha, C. Serejo, J.K. Lowry & D. Jones , 26 Oct. 2003, WA 764 .

Etymology. This species named “paraplatensis” indicates its similarity to P. platensis .

Diagnosis. Male antenna 2 and pereopod 7 strongly sexually dimorphic. Male gnathopod 1 cuspidactylate. Coxa 6 posterior lobe with anterodistal corner subquadrate, with process, 1–2 marginal setae, posterior margin perpendicular to ventral margin, outer surface with ridge. Pleopod 2 with 3 median marginal robust setae; pleopod 3 with 3 distal marginal robust setae.

Description

Male, 15.1–19.3 mm. Eye medium, 1⁄5–1/3 head length. Antenna 1 short, rarely longer than article 4 of antenna 2. Antenna 2 shorter than head and first 3 pereonites; peduncular articles expanded; peduncular articles with sparse, small robust setae. Lower lip distolateral setal tuft absent. Mandible left lacinia mobilis 5-dentate. Maxilliped palp, article 2 with mediodistal lobe, article 4 present, reduced.

Gnathopod 1 sexually dimorphic; subchelate; posterior margin of carpus and propodus with rugose lobe; propodus subtriangular; palm transverse; dactylus slightly shorter than palm, without ventral setal row, cuspidactylate. Gnathopod 2 sexually dimorphic; subchelate; basis slightly expanded; merus without medial lobe; carpus triangular, reduced, enclosed by merus and propodus; palm acute, with welldeveloped midpalmar notch, posterodistal corner without protuberance; dactylus fit into a sulcus internally and narrowed distally. Coxae 2–4 as wide as deep. Pereopods 3–7 cuspidactylate; dactylus without row or patch of dorsal short setae. Pereopod 4 significantly shorter than pereopod 3. Pereopod 4 dactylus thickened and pinched posteriorly, different to pereopod 3 dactylus. Pereopod 5 propodus distinctly longer than carpus. Pereopod 6 not sexually dimorphic, shorter than pereopod 7; coxa 6 posterior lobe with anterodistal corner subquadrate, with process, and with 1–2 marginal setae, posterior margin perpendicular to ventral margin, outer surface with ridge; carpus slender. Pereopod 7 sexually dimorphic; basis without lateral sulcus, posterodistal lobe present; distal articles expanded; carpus oblong. Coxal gills convoluted or simple. Pereopods 3–5 gills smaller than gills 2 and 6.

Pleopods 1–3 well developed, biramous; peduncle slender. Pleopod 1 peduncle without marginal setae. Pleopod 2 peduncle with 3 marginal robust setae and one slender seta. Pleopod 3 peduncle with 3 subdistal robust setae. Epimera 1–3 with posterior margin slightly serrated, posteroventral corner of epimera 1–2 produced. Uropod 1 peduncle with 15 robust setae in two rows, distolateral robust seta absent; inner ramus subequal in length to outer ramus and with 4 marginal inner setae and 3 marginal outer setae; outer ramus without robust setae. Uropod 2 peduncle inner margin with 7–10 setae and outer margin with 3–4 setae; inner ramus subequal in length to outer ramus; inner ramus with 3 marginal inner setae and 2 marginal outer setae; outer ramus with 1–2 marginal robust setae. Uropod 3 peduncle with 2–3 robust setae; ramus subequal in length to peduncle, triangular, narrowing distally; with 2 marginal setae, and 4–5 apical setae. Telson longer than broad, apically incised, with marginal and apical robust setae; each lobe with 3–5 robust setae; dorsal midline entire.

Female (sexually dimorphic characters), 12.5 mm. Antenna 2 peduncular articles narrow. Gnathopod 1 parachelate; posterior margin of merus, carpus and propodus with rugose lobe vestigial or absent; propodus subrectangular; palm acute; dactylus much longer than palm. Gnathopod 2 mitten-shaped; basis produced proximally, 1.7× longer than wide, posterior margin of merus, carpus and propodus with rugose lobe; palm obtuse, smooth; dactylus shorter than palm. Pereopod 7 distal articles slender. Oostegites setae with simple straight tips. Oostegites 2–4 moderately setose (around 24 setae). Oostegite 5 with about 11 setae, posterior margin with fewer setae than anterior margin. Uropod 2 peduncle inner margin with 4–6 setae.

Habitat. Under Enteromorpha sp. on the margin of Swan River, brackish water.

Remarks. The genus Platorchestia includes supra-littoral and terrestrial species, which is reflected by a degree of sexual dimorphic characters in male antenna 2 and pereopod 7 as pointed recently by Miyamoto & Morino (2004). Attempts to elucidate the P. platensis complex started with Jo (1988), who compared P. platensis based on material from Denmark, the Netherlands and Florida with previous identifications of P. platensis from the Western Pacific and other similar species. Serejo (2004) did a list of the 13 species included in the genus and also compared P. platensis , based on the type material from Montevideo, Uruguay, with the Atlantic P. monodi (Mateus et al., 1986) found on the Brazilian coast. Recently Miyamoto & Morino (2004) described the supralittoral Platorchestia pacifica from Taiwan comparing it to P. platensis (from Kent, England) and allies. Miyamoto & Morino (2004) divided Platorchestia into three groups according to the sexual dimorphism in antenna 2 and pereopod 7. Platorchestia paraplatensis n.sp. is part of group 1, which includes species with strong sexual dimorphism in antenna 2 and pereopod 7 including P. joi Stock & Biernbaum, 1994 ; P. munmui Jo, 1988 , P. pacifica ; P. pachypus (Derzhavin, 1937) and P. platensis . These species are generally found on the supralittoral zone of beaches, mouth of rivers, estuaries etc. Three of these species, P. pacifica , P. platensis and P. paraplatensis n.sp., have gnathopod 1 cuspidactylate, and the anterodistal corner of posterior lobe of coxa 6 with process. The outer plate of maxilla 2 with pectinate setae was pointed out by Miyamoto & Morino (2004) as diagnostic for P. pacifica , and can also be seen in P. paraplatensis n.sp. and in the types of P. platensis . Although similar to the Atlantic P. platensis and P. pacifica in the characters above, the Australian species can be distinguished from these species by pleopod 2 bearing 3 marginal median setae versus 3 marginal proximal seta on P. pacifica and 4 marginal median setae on P. platensis ; pleopod 3 with 3 marginal distal setae, versus 9 marginal setae and 6 facial on P. pacifica and 4 marginal distal setae on P. platensis . Also the gnathopod 2 notch is well defined and the dactylus is narrowed distally when compared with the P. platensis lectotype 12.3 mm ( Serejo, 2004, fig. 10), which has male gnathopod 2 palm sinuous, lacking a conspicuous notch with dactylus not narrowing distally. Oostegite 5 has around 11 setae and the posterior margin has fewer setae than anterior margin, compared with P. pacifica , which has 8 setae distributed evenly on both sides.

Figures

Fig. 21. Platorchestia paraplatensis n.sp., paratype male, 15.1 mm, habitus and H, AM P69145, holotype male, 19.3 mm, other parts, AM P69144, Blackwell Reach, Swan River, Perth, Western Australia. Scales for habitus and H: 1 mm; remainder: 0.1 mm.

Fig.22. Platorchestiaparaplatensis n.sp., holotype male, 19.3 mm,AM P69144, Blackwell Reach, Swan River, Perth,WesternAustralia. Scales represent 0.5 mm.

Fig. 23. Platorchestia paraplatensis n.sp., holotype male, 19.3 mm, U1–3, T, EP1–3, AM P69144; paratype female, 12.5 mm, G1–2, AM P69146; juvenile male, 11.0 mm, G2, AM P69149, Blackwell Reach, Swan River, Perth, Western Australia. Scales for U3, T: 0.2 mm; remainder: 0.5 mm.

Fig. 24. Platorchestia paraplatensis n.sp., holotype male, 19.3 mm, AM P69144, PL1–3, BR2–6; paratype female 12.5 mm, O2–5.Scales for PL and BR: 0.2 mm and H: 1 mm.