(Figs 13–14)
Diagnosis. Female: Pereonites all laterally straight; pereonites 3 and 4 longest. Mandible molars blunt. Maxilliped endites with two pairs of short setae. Cheliped propodus inner surface with two setae. Pereopod-1 merus with one ventrodistal seta. Pereopods 4–6 unguis with double row of spinules. Pleopods absent. Subadult male: Pleopods without setae. Adult male: Pleopods with simple setae.
Material examined. Holotype: ♀, 1.75 mm, (USNM 1411522), northeastern Gulf of Mexico, App. Sed. 7, coll. F. Qu, 11 June 2014.
Paratypes: two subadult ♂♂, one ♀, body lengths indeterminate, (USNM 1411523), App. Sed. 6, coll. F. Qu, 11 June 2014; one ♀, body length indeterminate, (USNM 1411524), App. Sed. 8, coll. F. Qu, 11 June 2014; one ♀, body length indeterminate, (USNM 1411525), App. Sed. 5, coll. F. Qu, 11 June 2014; one ♀, body length indeterminate, (USNM 1411526), App. Sed. 10, coll. F. Qu, 10 June 2014; one ♀, 1.4 mm, (USNM 1411527), App. Sed. 1, coll. F. Qu, 12 June 2014; one ♂, 1.1 mm, (USNM 1411528), App. Sed. 3, coll. F. Qu, 11 June 2014; two manca-II, 0.7 mm, three ♀♀, body lengths indeterminate, (USNM 1411529), App. Sed. 4, coll. F. Qu, 11 June 2014.
Description. Adult female. Body (Fig. 13 A). Fairly slender, about 6.9 times as long as broad; length 1.4–1.75 mm (n = 2). Cephalothorax longer than broad, shorter than pereonites 1 and 2 combined, anterior half expanding out, posterior half straight. Pereonites all broader than long, lateral margins straight, pereonites 3 and 4 longest, pereonite-6 shortest. Pleonites short and subequal, epimera without setae. Pleotelson posterior half tapering to rounded apex, with pair of setae, longer than combined length of three pleonites.
Antennule (Fig. 13 B) shorter than cephalothorax; article-1 longer than rest of articles combined, with two or three setae on outer margin; article-2 longer than broad, with one distolateral seta; article-3 less than one half length of article-2, with one distolateral seta; article-4 shorter than article-2, with five simple setae and one PSS. Antenna (Fig. 13 C) with six articles; article-1 (not illustrated) and article-2 naked; article-3 half as long as article-1, with one distal seta; article-4 2.4 times as long as broad, with one distal simple and two PSS; article-5 with distal seta; article-6 small, with two long and one short seta.
Mouthparts. Labrum (Fig. 13 D) conical, laterally setose. Right mandible (Fig. 13 E) incisor with three or four denticles; molar acuminate and unarmed. Left mandible not observed. Maxillule (Fig. 13 F) endite with at least eight terminal spiniform setae; palp with two setae (only the bases recovered).
Maxilliped (Fig. 13 G) basis cordiform, naked; endites with two pairs of small setae; palp article-1 naked, article-2 with three setae on inner margin, article-3 with two long and one short seta on inner margin, article-4 with five setae on inner margin—some of these setae are pectinate. Epignath not recovered.
Cheliped (Figs. 13 H, I, J). Sclerite confluent with rear of cephalothorax. Basis with posterior lobe as long as anterior mass, not reaching pereonite 1, naked; merus triangular, with one mid-ventral seta; carpus about 1.6 times as long as broad, with one ventral seta, shallow carpal shield; propodus inner surface (Fig. 13 J) with two setae near dactylus insertion; fixed finger with one ventral seta, cutting edge with distal bifid tooth; dactylus naked.
Pereopods 1–6 propodus with distal row of spinules decreasing in length laterally (illustrated for pereopod 4 in Fig. 14 E).
Pereopod-1 (Fig. 14 A). Basis 3.8 times as long as broad, naked; ischium with one ventrodistal seta; merus longer than carpus, about 1.9 times as long as broad, with one ventrodistal seta; carpus with three distal setae; propodus longer than merus, with one subdistal seta and one spiniform seta on ventral margin; dactylus and unguis subequal to propodus length, dactylus and unguis naked.
Pereopod-2 (Fig. 14 B). Similar to pereopod-1 except: propodus without spiniform seta; dactylus and unguis together shorter than propodus.
Pereopod-3 (Fig. 14 C). Similar to pereopod-2 except: dactylus and unguis subequal to propodus.
Pereopod-4 (Figs. 14 D, E). Basis 2.2 times as long as broad, naked; ischium with two ventrodistal setae; merus shorter than carpus, with two distal spiniform setae; carpus 1.8 times as long as broad, with three distal spiniform setae; propodus subequal to carpus, with three distal spiniform setae; dactylus and unguis combined length longer than propodus; unguis with double row of spinules.
Pereopod-5 (Fig. 14 F) nearly identical to pereopod-4 except only one seta was observed on the ischium.
Pereopod-6 (Fig. 14 G). Similar to pereopods 4 and 5, but propodus with four spiniform setae.
Pleopods absent.
Uropod (Fig. 14 H). Stout, longer than half length of pleotelson; basal article exopod short, about half length of endopod article-1, with one long and one short seta; endopod two-articled, article-1 naked, article-2 slightly longer than article-1, with five simple setae.
Adult male similar to female but with pleopods with simple setae (endopod and exopod each with less than ten setae); length 1.1 mm.
Subadult male similar to male but pleopods without setae (Fig. 14 I).
Etymology. Named in honour of the tanaidacean expert Dr. Kim Larsen, for his contribution to advancing knowledge of tanaidacean biodiversity in the Gulf of Mexico.
Distribution. Northeastern Gulf of Mexico (offshore Alabama), at the depth range 2212–2289 m.
Remarks. The new species is placed in the genus Pseudomacrinella by the cordiform shape of the maxilliped basis, general shape of the habitus and chelipeds (even to the small bifid distal tooth on the fixed finger), and the stout uropods with a short, one-articled exopod. Pseudomacrinella larseni n. sp. can be distinguished from P. macrocephala by the shape of the pereonites (P. macrocephala has pereonites 1–5 almost equal in length), blunt mandible molars, cheliped propodus with two inner setae, and by the unguis of pereopods 4–6 having a double row of spinules. It is also possible that the bases of pereopods 4–6 in P. larseni are less elongate than in P. macrocephala but the drawing of its pereopod-6 (pereopod VII in Kudinova-Pasternak 1990: 105, fig.8) is compromised.
This species was not abundant but was present at eight of the twelve sampled locations.