Thalassomysis tattersalli Nouvel, 1942

Figs 9–10

Thalassomysis tattersalli Nouvel, 1942: 6–8, figs 12–17 (preliminary diagnosis).

Thalassomysis Tattersalli – Nouvel 1943: 59–61, figs 85–102 (detailed description).

Thalassomysis tattersalli – Murano & Krygier 1985: 692–693, fig. 4 (range extension, supplementary description). — San Vicente 2017: table 1 (distribution range). — Mees & Meland 2024: AphiaID 931983 (accepted).

Diagnosis

Covers adult female and non-adult males (the latter described here for the first time). Carapace normal, anterior margin with semicircular projection covering part of eyestalks. Eyes separately set in sublateral position, dorsally with papilla. Eye structure and size of papilla varying with body size (see Notes below and ‘Discussion’). Antennular trunk with three segments separated by transverse articulations. Antennal peduncle 4-segmented with oblique border between small third and larger, dorsally overlapping fourth segment. Antennal scale entire, setose all around, extending half its length beyond antennal peduncle and ⅖ beyond antennular trunk. Scale six times as long as wide, setose lateral margins parallel, tip rounded, no tooth. Labrum rostrally rounded, caudally strongly asymmetrical due to mesially rugose, distally rounded caudal projection from lateral third of caudal margin. Labium with transverse distal margin. Thoracic endopods 1–2 normal; endopods 3–8 and all exopods unknown. Female with four pairs of oostegites. Pleopods 1–5 of subadult male setose, no spines; pleopod 4 biramous, remaining pleopods uniramous. Female pleopods reduced to uniramous setose plates increasing in length caudally. Pleomere 6 with pair of carinae running along ventro-lateral edge and narrowing caudally. Both rami of uropods unsegmented, setose all around, no spines. Telson triangular, slender, 3.2–3.7 times as long as maximum width at basis, and 15–22 times as long as maximum width at convex terminus, lateral margins not serrated, with 11–17 spines on distal ⅔, spines distally somewhat discontinuously increasing in length and width; narrow terminal margin with four spines; total of 27–37 spines, no setae.

Material examined

SOUTHERN OCEAN • 1 ♂ imm. (BL = 13.4 mm, on slides); northern Weddell Abyssal Plain, ANDEEP-II station 135-4; 65°00.06ʹ S, 43°01.19ʹ W to 64°59.97ʹ S, 43°00.91ʹ W; depth 4677.6– 4678.2 m; 11 Mar. 2002; EBS supranet • 1 ♀ imm. (BL = 10.3 mm); eastern Weddell Abyssal Plain, S of Maud Rise and E of Sanae Canyon, ANDEEP-III station 059-5; 67°29.74ʹ S, 00°01.93ʹ W to 67°29.61ʹ S, 00°02.19ʹ W; depth 4655– 4655 m; 14 Feb. 2005; EBS supranet • fragment of one subad. ♂ (posterior ⅔ of body, estimated BL ≈ 14 mm, on slides); Weddell Abyssal Plain, ANDEEP-III station 088-8; 68°03.66ʹ S, 20°27.90ʹ W to 68°03.61ʹ S, 20°27.52ʹ W; depth 4929–4931 m; 27 Feb. 2005; EBS supranet.

First description of (non-adult) males

Eyes proximally connected by a large transverse symphysis (Fig. 10B). Cornea opaque, eyestalk with pattern of transparent and opaque parts. Ommatidia without rhabdom, nonetheless forming a compact laterally positioned cornea (Fig. 10D) delimited from eyestalk. Eyestalk (Fig. 10B, D–E) with large ganglion mass. Eye dorsally with ocular papilla measuring ⅕ of antero-posterior eye extension, in loco facing upwards (Fig. 10A). Small pore at tip of ocular papilla (Fig. 10C). Organ of Bellonci (Fig. 10E) close to basis of papilla; this organ with numerous sensory cells with a neck and (out of focus in Fig. 10E) large vacuole. Antennular trunk without antennular bursa. Vestigial appendix masculina in dissected immature male with BL 13.4 mm. Four barbed setae disto-laterally on disto-median lobe of antennular trunk, no teeth. Eyes bolster-shaped (Fig. 10A), dorsoventrally compressed by a factor of 1.9. Foregut with unidentifiable masticated material, abundant crustacean remains fragmented to small pieces and a minor number of small mineral particles. Ingested setal fragments thicker than found elsewhere in this mysid, so potential ingestion of own exuvia excluded in this case. Thoracic sternite 1 with rounded mid-anterior projection. Thoracomeres 2–8 without mid-sternal processes.

Eyes and antennula not available in fragment of subadult male with estimated BL ≈ 14 mm. This specimen with pyriform penes (Fig. 9B); several setae associated with terminal ejaculatory opening; spermatozoa visible in efferent ducts. Pleopods 1–3, 5 (Fig. 9E–F) of both available non-adult males represent uniramous setose plates increasing in length caudally; pleopod 4 longest, biramous with unsegmented, plate-like, setose endopod. Exopod 4 (Fig. 9D) 2–3-segmented only in subadult male. The still rudimentary aspect of this exopod suggests a much larger (multi-segmented?) exopod 4 in the so far unknown adult male. Exopod of uropods extending 0.2–0.3 times its length beyond endopod and 0.1–0.2 times beyond telson. Endopod not forming an external suture. Statocyst chamber well delimited by internal cuticle only. Statoliths distinct, without mineral (probably accidentally demineralized in present material). No setae, pores or scales detected on telson. Anal lobe distinct, weakly cuticularized.

Notes on immature female

Non-dissected immature female with BL 10.3 mm (Fig. 9A), showing pyriform eyes, dorsoventrally compressed by a factor of 1.4. Ocular papilla measuring ⅓ of antero-posterior eye extension. Cornea compact, well delimited from eyestalk, with ommatidia reaching surface. Cornea brown, well contrasting from opaque eyestalk even after 18 years of preservation in ethanol. Pigmented cornea with ommatidia reaching surface, together with ganglion mass present in eyestalk, suggesting some visual abilities (see ‘Discussion’). No female lobe detected. Thoracic endopod 2 very long (bent in Fig. 9A), extending beyond antennular trunk when stretched anteriorly. Endopods 3–8 broken, as in other conspecific specimens in ANDEEP material.

‘Ovoid organ’

Nouvel (1942, 1943) reported “un petit organe ovoïd longuement pédonculé” on a seta of the maxillula in T. tattersalli. Murano & Krygier (1985) found no such structure in material of this mysid species and assumed that the structure reported by Nouvel could be a “small parasite or tumor” rather than an organ of a mysid. In fact, such ovoids (Fig. 9C) were also found in the present material and identified as settled spores of ellobiopsids (phylum Myzozoa: Ellobiopsida). The dissected subadult male with a total of twelve such spores on thoracic sternites, no spores elsewhere. The immature male with a single spore fixed to a seta of pleopod 2, four spores to the joint between the mandibles and labrum, and several spores on the antennular trunk. The available immature female not examined (not dissected) in this respect.

Type locality and distribution

This species was first described by Nouvel (1942, 1943) based on a single adult female taken during Cruise 1910 of the ‘Campagnes du Prince Albert 1 er de Monaco’, station 2983 in the Gulf of Biscay, 45°28ʹ N, 5°43ʹ W, vertical haul 4500–0 m, here regarded as type locality by monotypy. Murano & Krygier (1985) published one adult and three non-adult females from three samples in the NE Pacific, about 100–750 km off the Oregon coast, 44– 45° N, 125– 134° W, taken with a beam trawl at depths of 2926–3724 m. The ANDEEP records are the first with (non-adult) males and are the first from the southern hemisphere: these specimens were taken with an epibenthic sledge on the Weddell Abyssal Plain at three stations at depths of 4655–4931 m, 65– 68° S, 00°– 43° W. The great distances between the three sea areas recorded so far point to a panoceanic abyssal distribution.