Emertonia hessleri Mathiske, Thistle & Veit-Kohler 2021, sp. nov.

Emertonia hessleri Mathiske, Thistle & Veit-Köhler sp. nov.

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Type material. The examined specimens are registered and deposited in the Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, Germany. All specimens of the type material were collected with the D.S. V. Alvin in the San Diego Trough off California (only one station) and during the PUCK expedition off Chile. Station numbers for PUCK indicate “Expedition, station number, depth, core number, sediment horizon”. Additionally, for each specimen from the San Diego Trough an identification number assigned by the collector (David Thistle) is given.

Female holotype, SMF 37246 (adult female on 6 slides): D.S. V. Alvin, T3613 A, 1,050 m.

Male allotype, SMF 37247 (adult male on 1 slide): RV Sonne 156 A— PUCK, St. 7103, 891 m, R1, 0–1 cm.

Female paratype 1, SMF 37248 (adult female on 3 slides): D.S. V. Alvin, T3649 A, 1,050 m.

Female paratype 2, SMF 37249 (adult female on 1 slide): D.S. V. Alvin, T3628, 1,050 m.

Male paratype 3, SMF 37250 (adult male on 4 slides): D.S. V. Alvin, T3649 B, 1,050 m.

Female paratype 4, SMF 37251 (adult female on 5 slides): D.S. V. Alvin, T3609, 1,050 m.

Etymology. This species is dedicated to Dr Robert R. Hessler who is a noted deep-sea ecologist.

Description of female. Habitus (Fig. 17). Total body length measured from anterior tip of rostrum to posterior margin of anal somite (holotype) 0.23 mm, including caudal rami 0.25 mm. Body cylindrical (Fig. 17A, B), slightly depressed dorsoventrally, with prosome slightly wider than urosome. Most pores distributed dorsally and laterally on cephalothorax and some on the free body somites. Sensilla visible on cephalothorax and pedigerous somites. First urosomite and genital double-somite dorsolaterally and third and fourth urosomites along entire posterior margin ornamented with spinules (Fig. 17B). Fourth urosomite with thin, well developed pseudoperculum (Fig. 17A). Anal somite short with minute spinules ventrally and ventrolaterally along the posterior margin (Fig. 17B).

Caudal rami (Fig. 17C). Cylindrical, approximately three times longer than wide, with five setae: seta I absent; seta II slender, dorsally displaced; seta III short, slender, spine-like, situated dorsolaterally, near posterior margin; seta IV longer, with clearly visible fracture plane and with flexible end, V long and slender, with clearly visible fracture plane and several pinnules, situated terminally; seta VI absent; seta VII slender, on dorsal surface. Posterior margin of caudal rami with ventral spinule row.

Antennule (Fig. 18). Eight-segmented. No further details visible.

Antenna (Fig. 19A) Basis unarmed. Endopod two-segmented; enp1 with one abexopodal spinule and one short naked inner seta; enp2 with two spinules along inner margin and subapically with four naked dorsal setae; apical margin with six slender naked setae. Exopod one-segmented with one naked flexible seta and a spinule laterally and two naked setae apically.

Mandibular palp (Fig. 19B) biramous. Basis with two spinules. Endopod two-segmented; proximal segment with two naked inner setae, one short and one long; distal segment apically furnished with five naked and slender confluent, basally fused setae. Exopod one-third length of endopod, one-segmented, with one dorsally displaced and two apical setae.

Maxillule (Fig. 19C). Praecoxal arthrite with four naked setae. Coxal endite with four slender naked setae. Endite of basis with four slender naked setae. Endopod one-segmented, with two slender naked setae. Exopod with three slender naked setae, one dorsally displaced.

Maxilla (Fig. 19D). Praecoxa and coxa fused forming syncoxa with three endites. No separation with basis visible. Proximal endite with two slender, naked setae. Middle endite with two slender, naked setae. Distal endite with two slender, naked setae, one of which short. Allobasis with two naked setae. Endopod two-segmented; proximal segment with three naked setae, one of which slightly displaced; distal segment with two naked setae.

Maxilliped (Fig. 19I). Syncoxa with one naked seta. Basis unarmed. Endopod one-segmented with one small spine adjacent to one long naked seta.

Swimming legs (Fig. 20; Table 4). With rami characteristic for Emertonia species belonging to the andeep - group; intercoxal sclerites without ornamentaion.

P1 (Fig. 20A). Coxa with several spinules of different sizes along outer margin. Basis with spinules along inner margin, one unipinnate inner and one bipinnate outer seta. Endopod slightly longer than exopod; both rami two-segmented and armed with spinules along outer margin. Enp1 unarmed; with three pinnules along inner apical margin. Enp2 with two equally long unipinnate setae, the inner one slightly stronger and accompanied at base by two long spinules. Exp1 with one unipinnate outer spine. Exp2 with two unipinnate outer spines and terminally with one unipinnate outer and one naked inner seta.

P2–P4 (Fig. 20B–D). Coxae without ornamentation. Basis of P2 with spinule row near naked outer seta. Basis P3 unarmed and basis P4 with naked outer seta. Exopods three-segmented and slightly longer than endopods; all segments with spinules along outer margin in P2 and P3. P4 exp1 sparsely armed and exp2 and exp3 unarmed along outer margin. Spines of P4 exp2 and exp3 accompanied at base by spinules. P2 exp1 and exp2 with strong serrated spine. P3 exp1 with strong naked and exp2 with strong serrated spine. P2–P3 exp2 with spinules along inner margin. P4 exp1 and exp2 with two spinules on small ledges along inner margin. P2–P3 exp3 with four elements. P2 exp3 with two strongly serrated outer spines and two apical setae, the outermost of which long, naked and spine-like and the inner long and flexible with two long pinnules. P3 exp3 as in P2, but inner apical seta more plumose and outer apical spine with bifid tip. P4 exp3 with two serrated spines (a short outer and a very long terminal). P2–P3 endopods one-segmented; with spinules along outer margin, two short rows of fine spinules and an additional terminal spinule row on posterior surface. P2 enp with three long spinules along inner margin, the proximalmost one positioned on a ledge. P2–P3 endopods apically with long rigid seta, being uniplumose in P2 and bipinnate with spine-like pinnules in P3. P4 endopod two-segmented, armed with spinules along outer margin; enp1 with one long inner spinule and a spinule row along apical margin; enp2 with two groups of three spinules arising from small ledges near apical margin and with one short, naked, spine-like distal seta exhibiting two deep serrations at tip.

P5 (Fig. 21A). Baseoendopods fused medially, completely separated from small exopods; with long, naked outer basal seta on either side and short spinule rows on surface near outer margins; anterior surface with one pore near spinule row and several integumental pits. Endopodal lobes moderately developed, separated by medial cleft; each with halfmoon-shaped spinule row and two stout apical setae, the outer being unipinnate with two spinules and the inner bipinnate. Exopod with one small spinule and three naked spine-like setae, the inner one being shortest, the outer longest and the middle one more flexible, and directed outwardly (contrary to the other setae).

P6 and genital field (Fig. 21B). Sixth pair of legs represented by small fused outgrowths.

Description of male. Habitus (Fig. 22A, B). As in female but slightly smaller and second and third urosomites not fused. Total body length measured from anterior tip of rostrum to posterior margin of anal somite (allotype) 0.18 mm, including caudal rami 0.19 mm. Spermatophore located in right side of the body. Ornamentation of body somites as in female.

Caudal rami (Fig. 22C). For description see female.

Antennule (Fig. 23). Seven-segmented (not all segments visible in Fig. 23B). Segment I with spinule row along outer margin, segment III relatively small, segment IV rounded and bulbous, with two spinules along anterior margin and one stout spinule on apical margin (Fig. 23A). Armature formula:

I (0)

II (8) eight slender naked setae of different sizes

III (1) one naked seta

IV (2 + aes) two slender naked setae and one aesthetasc

V and VI no armature discernible

VII (3) three slender naked setae, one of which minute

Swimming legs as in female, sexual dimorphisms only in P5 and P6 (Fig. 21C, D).

P5 (Fig. 21C). Baseoendopods fused medially; with very long, naked outer basal seta on either side and short spinule rows on anterior surface surface near outer margins. Exopod discrete at base; with one small spinule (not visible on one side) and three naked spine-like setae, the middle seta of which being more flexible, longest and outwardly directed.

P6 (Fig. 21D) represented by small, medially fused plates, each bearing two stout setae.