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Subfamily Leptomysinae Hansen, 1910

Carcedo, Cecilia, Fiori, Sandra, Hoffmeyer, Mónica (2013): Pseudobranchiomysis arenae, a new genus and species of Leptomysinae (Crustacea: Mysida) in Argentinian sandy beaches. Zootaxa 3647 (3): 479-487, DOI: http://dx.doi.org/10.11646/zootaxa.3647.3.5

Genus Pseudobranchiomysis n. gen. Type species. Pseudobranchiomysis arenae n. sp. Diagnosis. Body robust with pigmented spots throughout. Carapace covering more than half of the cephalothorax in dorsal view, with anterior margin produced into a triangular rostrum with a rounded apex. Eyes well developed, with reniform cornea. Antennular peduncle of male with processus masculinus well developed and very hirsute. Antennal scale large, lanceolate, setose on both margins, with a pointed apex and absence of distal suture. Mandibles with well-defined molar process. Maxilla with distal segment of endopod expanded, longer than broad. Labrum without frontal process. Endopod of thoracopods with 5 - to 7 -segmented carpopropodus. Male pleopods biramous, with a well-developed pseudobranchial lobe; endopod of 1 st pair very small and unjointed, 11 - segmented exopod; 2 nd to 5 th pairs with both rami long and subequal, endopod 11 -segmented and exopod 11 - to 14 -segmented; exopod of 4 th pair with modified setae on antepenultimate, penultimate and ultimate segments. Female pleopods reduced, small and narrow. Telson with posterior cleft armed with many fine spines on convex margins and two long plumose setae at centre. Uropods densely furnished with setae. Inner uropod with row of seven spiniform setae on inner margin, extending from statocyst edge to nearly three-quarters of uropod length. Etymology. The generic name refers to the well-developed pseudobranchial lobes present in male pleopods. Remarks. According to Mauchline’s (1980) identification key of Mysidae, the mysids reported here belong to the subfamily Leptomysinae, as demonstrated by the following diagnostic characters: antennal scale entire, setose all around, without terminal spine; labrum normal and symmetrical; endopods of 3 rd to 8 th thoracic limbs with carpus and propodus fused and subdivided, no oblique articulation; pleopods of male biramous; statocyst present; exopod of uropod undivided and outer margin of uropod exopod without spines. Our specimens were compared to all recognized genera included in the World Register of Marine Species (WoRMS) website (Mees 2012) for Leptomysinae, and were found to bear a closest resemblance to the genus Pseudomysis Sars, 1879. Similar characters to this species where seen in the shape and armature of the telson, particularly in the distal cleft with each apical lobe convex and armed with fine spines, lateral margins with regularly arranged spiniform setae and two long and plumose setae at the base of the cleft. However, there are some small differences: whereas in Pseudomysis each apical lobe has single, long and stout spiniform setae at the apex, in our specimens each apical lobe has two spiniform setae at the apex, the external setae longer than the internal setae. Also, Pseudobranchiomysis, has more spinose setae on the lateral margin of the telson and more spines on the cleft margin than in Pseudomysis. Our specimens also differ from Pseudomysis in the form of the anterior margin of the carapace; the absence of a distal articulation on the antennal scale; the presence of large pseudobranchial lobes in male pleopods; the lower number of endopod and exopod segments in the male pleopods, and the different arrangement of spiniform setae in the uropodal endopods. We assign our specimens to a proposed new genus Pseudobranchiomysis, characterized by a combination of the three main features: antennal scale setose all around with a pointed apex and distal suture absent; male pleopods with large pseudobranchial lobes and telson with posterior cleft armed with many fine spines on the convex margins. Pseudobranchiomysis arenae n. sp. (Figs. 1–6) Type specimens. Holotype: adult male, 18 mm, (DZI-MLP 26.847). Allotype: adult female, 25 mm, (DZI-MLP 26.848). Paratypes: two adult males and two adult females (DZI-MLP 26.848). Other specimens examined. 10 females, 22.44–29.02 mm (LB-CR 8 A); 10 males, 14.55–19.48 mm (LB-CR 8 B), 10 juveniles 7.20 mm - 12.62 mm (LB-CR 8 C). Type locality. Monte Hermoso, Buenos Aires Prov., Argentina (38 ° 59´S, 61 °06´W), 25 November 2010, 1 m, sledge with net, collected by Cecilia Carcedo. Description. Body robust. Carapace (Figs. 1 A, B) covering more than half the cephalothorax in dorsal view, with anterior margin produced into triangular rostrum. Eyes (Fig. 2 A) large, depressed dorsoventrally; cornea well pigmented, reniform in dorsal view; peduncles short and stout. Antennular peduncle of male (Figs. 2 A, B) more robust than that of female; first segment 1.2 times as long as broad, with two strong plumose setae and three minor at distal end of outer margin; second segment short, inner margin longer than outer, with a protuberance at distal corner of inner margin with about ten plumose setae; third segment robust, as long as broad, with a small protuberance on superior margin with three setae. Processus masculinus well developed and very hirsute. Female antennular peduncle (Figs. 3 A, B) with first segment as in male and with a strong plumose seta near distal end of inner margin in dorsal view; second segment as in male; third segment as long as broad, with row of six plumose setae near distal end of outer margin, a small protuberance on superior margin with three setae and seven plumose setae at distal end of inner margin. Antennal scale (Fig. 2 C) lanceolate, with pointed apex, without distal suture; about five times as long as broad, more than twice as long as peduncle; densely furnished with plumose setae on both margins. Antennal peduncle (Fig. 2 C) 3 -segmented, first segment short, second and third segments 1.5 times as long as broad, with eight to ten setae at distal end of inner margin. Mandible (Fig. 2 D) with well-defined molar process; mandibular palp 3 -segmented; first segment inconspicuous, second segment very dilated, 1.5 times as long as third; second and third segments with plumose setae on lateral margins. Maxillule (Fig. 2 F) with outer lobe bearing robust setae on distal margin, inner lobe with plumose setae. Maxilla (Fig. 2 E) with endopod 2 -segmented; distal segment expanded, longer than broad and furnished with numerous strong setae on outer margin; exopod with 22 setae. Labrum (Fig. 2 G) without anterior process. Marsupial pouch in female formed by three pairs of oostegites increasing backward in size, anterior pair small, posterior pair large. First and second thoracopods (Figs. 4 A, B) with endopod short, robust, with five segments and exopod 14 - and 15 -segmented respectively; third to eighth thoracopods robust with terminal claw. Endopod of third thoracopod (Fig. 4 C) with ischium one-third longer than merus and 5 -segmented carpopropodus. Endopod of fourth (Fig. 4 D) to seventh thoracopods with ischium one-third longer than merus and 6 -segmented carpopropodus. Endopod of eighth thoracopod (Fig. 4 E) with ischium one-third longer than merus and 7 -segmented carpopropodus. Exopod of third to eighth thoracopods 15 -segmented. Pleopods of male developed, two branched, with well-developed pseudobranchial lobes. First pleopod (Fig. 5 A) with endopod reduced to unsegmented lobe, exopod 11 -segmented. Second pleopod (Fig. 5 B) with endopod 10 -segmented and exopod 11 -segmented. Third pleopod (Fig. 5 C) with endopod 10 -segmented and exopod 12 - segmented. Fourth pleopod (Fig. 5 E) with endopod 10 -segmented and exopod 14 -segmented; antepenultimate segment of exopod armed with long and strong seta; penultimate and ultimate segments both armed with long spiniform seta (Fig. 5 D). Fifth pleopod (Fig. 5 F) with 12 -segmented exopod; 10 -segmented endopod. Pleopods of female (Fig. 5 G) reduced to unsegmented single lobes, flattened and knife-shaped. Telson (Figs. 6 A, B) about twice as long as broad, equal in length to sixth abdominal somite. Apex of telson cleft, sides of notch convex, furnished with fine close-set pectinate spines and centre defined by two long plumose setae. Cleft about 1 / 7 of length of telson. Lateral margins of telson armed with 23 spinose setae, including two pairs of terminal setae. Uropods (Fig. 6 C) broadly lanceolate in form and densely setose on both margins. Outer uropod unjointed, 1.3 times longer than inner uropod. Inner uropod with large statocyst, 1.2 times longer than telson, inner margin with row of seven spinose setae, extending from statocyst edge to near three-quarters of uropod length; proximal setae closely set, distal ones more separated (Fig. 6 D). Etymology. The specific name is derived from the Latin arenae, sand grains, and refers to the pigmented spots throughout the body segments, giving it the appearance of grains of sand from the beach. Ecological note. This species occurred in large numbers throughout the entire sampling period (two years) in the surf zone (0–1 m depth) of Monte Hermoso and Pehuen Có sandy beaches. Females were observed carrying eggs or larvae in the brood pouch during spring-summer, mainly during November and December of both years. The large number of specimens found in samples indicates that this species is a gregarious form and likely endemic to this particular zone. The population of this mysid shared the surf community mainly with Arthromysis magellanica (Mysida), Artemesia longinaris (Decapoda), Leptoserolis bonaerensis (Isopoda) and Monocorophium insidiosum (Isopoda). Temperature conditions ranged between 11.5 ºC and 23.3 ºC and salinity between 25.3 and 36.6.

Family Mysidae Haworth, 1825, Published as part of Carcedo, Cecilia, Fiori, Sandra & Hoffmeyer, M��nica, 2013, Pseudobranchiomysis arenae, a new genus and species of Leptomysinae (Crustacea: Mysida) in Argentinian sandy beaches, pp. 479-487 in Zootaxa 3647 (3) on page 480, DOI: 10.11646/zootaxa.3647.3.5, http://zenodo.org/record/220451