Trapezionida pluto, Macpherson & Machordom, 2025

Trapezionida pluto sp. nov.

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Figs 4 View Fig , 5B View Fig

Etymology

The name refers to one of the Children of the Oceans of the Greek mythology (Pluto). The name is a noun in apposition.

Material examined

Holotype

SOUTHWESTERN INDIAN OCEAN – Saya de Malha Bank • ov. ♀ ( 5.1 mm); SAYA stn DW5407; 11°00′ S, 60°19′ E; 193–198 m depth; 6 Nov. 2022; MNHN-IU-2021-5805. GoogleMaps

Paratypes

SOUTHWESTERN INDIAN OCEAN – Saya de Malha Bank • 1 ♀ ( 4.4 mm); same data as for holotype; MNHN-IU-2022-331 GoogleMaps • 1 ov. ♀ (4.0 mm); SAYA stn DW5423; 11°27′ S, 62°01′ E; 198–210 m depth; 12 Nov. 2022; MNHN-IU-2022-852 GoogleMaps • 1 ♂ ( 4.6 mm); SAYA stn DW5424; 11°28′ S, 62°01′ E; 150–171 m depth; 12 Nov. 2022; MNHN-IU-2021-5756 GoogleMaps .

Description

CARAPACE. Slightly longer than broad. Ridges with dense short plumose setae and scattered thick long iridescent setae. Gastric region with 2 epigastric spines behind rostrum and 4 or 5 spines at each side, longest spines behind supraocular spines. One well-developed parahepatic spine on each side. Cervical groove distinct, with 2 branchial dorsal spines and one postcervical spine on each side. Frontal margins slightly oblique. Lateral margins subparallel and convergent posteriorly. First lateral spine at anterolateral angle, well developed, not reaching level of sinus between rostrum and supraocular spines, one small and one well developed spine between anterolateral spine and anterior branch of cervical groove. Branchial margin with 2 spines, first strongest, followed by 4 smaller spines. Rostrum spiniform, about 0.6 length of remaining carapace, slightly upwards directed, dorsally not carinated. Supraocular spines short, not reaching midlength of rostrum and clearly not reaching end of corneae, subparalell.

PTERYGOSTOMIAN FLAP. Unarmed, anteriorly ending in blunt tip.

THORACIC STERNUM. As long as wide. Surface of thoracic sternites 4–6 smooth, only a few short scales on sternite 4. Sternite 3 four times as wide as long. Sternite 4 anterior margin transverse, nearly contiguous to sternite 3; three times as wide as long, and 2.2 times as wide as sternite 3. Numerous small granules on lateral sides of sternites 6 and 7.

PLEON. Tergite 2 with only one pair of lateral spines on each side of anterior margin ( holotype and one paratype), and with 1–3 additional minute median spines (on two paratypes); tergites 2–3 each with one uninterrupted transverse ridge behind anterior ridge, several small scales between both ridges; tergites 4–5 each with several scales; posteromedian margin of somite 6 straight.

EYES. Ocular peduncles broader than long; corneae dilated, maximum corneal diameter 0.4 distance between bases of anterolateral spines.

ANTENNULE. Article 1 (distal spines excluded) 0.3 carapace length, 2.0 times as long as wide (excluding spines), barely overreaching end of cornea, with two subequal or slightly different-sized distal spines with lateral longer; 2 spines on lateral margin, proximal one short, located at midlength of segment, distal spine much longer.

ANTENNA. Article 1 with strong distomesial spine reaching end of article 2. Article 2 with distomesial and distolateral subequal spines, reaching or exceeding end of article 3, mesial margin with small spine. Articles 3 and 4 unarmed.

MXP 3. Ischium with well-developed spine on flexor distal margin. Merus 0.8 ischium length; flexor margin with two subequal strong spines; extensor margin with small distal spine.

P1. 3.0 (females)–3.5 (male) times carapace length, with scattered long simple iridescent setae and numerous short plumose setae on spines and scales. Merus as long as or slightly longer than carapace length, 2.1–2.5 times as long as carpus, with rows of dorsal and mesial spines; distomesial spine strongest, barely reaching proximal third of carpus. Carpus 0.7–0.9 palm length, 1.4–1.8 times as long as broad, with rows of spines along mesial, lateral and dorsal surfaces, mesial spines stronger. Palm twice as long as broad, with 3 or 4 rows of dorsal spines; one row of spines along both mesial and lateral margins. Finger 1.4–1.5 times palm length; fixed finger with a row of small spines on lateral margin, dactylus with a row of mesial spines, proximalmost spine strongest.

P2–4. Moderately slender, with scattered long simple iridescent setae and numerous short plumose setae along extensor margins of all articles. P2 twice carapace length. Meri shorter posteriorly (P3 merus 0.8 length of P2 merus, P4 merus 0.7 length of P3 merus); P2 merus 0.8 carapace length, five times as long as broad, 1.4–1.6 times as long as P2 propodus; P3 merus 4.5 times as long as broad, 1.3 times as long as P3 propodus; P4 merus 3.5 times as long as broad, 1.1 times as long as P4 propodus. Extensor margins of meri with row of 8–12 proximally diminishing spines on P2–3, 0–3 small spines on P4; flexor margins with 3–5 spines followed proximally by several eminences on P2–4; lateral surfaces unarmed, with sparsely scaly ridges. Carpi with 4 extensor spines on P2–3, only disto-extensor spine on P4; lateral surface with several granules sub-paralleling extensor margin; flexor margin with distal spine. Propodi 4.6–5.0 times as long as broad; extensor margin with two proximal spines on P2–3, unarmed on P4; flexor margin with 9–11 slender movable spines, one fixed well-developed distal spine. Dactyli slender, length 0.7–0.8 that of propodi; flexor margin with 7–9 movable spinules along entire border, and ultimate spinule at base of unguis; P2 dactylus 5.5 times as long as wide.

COLOUR. Base colour of carapace and pleon light orange, darker in gastric area and base of rostrum; tip of rostrum whitish; brownish eyes. P1–4 light orange to whitish; P1 with several transverse orange bands; P2–4 with transverse light orange bands on meri, carpi and propodi, dactyli whitish.

Genetic data

COI (MNHN-IU-2022-852, GenBank Acc. PV749084; MNHN-IU-2021-5756, GenBank Acc. PV749083).

Remarks

Trapezionida pluto sp. nov. belongs to the group of species having five branchial spines on the carapace, lateral spines on the second pleomere, granules on sternites 6 and 7, and dorsal spines on the carapace other than epigastric spines. Additionally, the Mxp3 has a small distal spine on the extensor margin, and P2–4 dactyli have movable spines along the entire length of flexor margin. The new species resembles T. limula (Macpherson & Baba, 1993) from the SW Indian Ocean ( Madagascar, MNHN-IU-2014-13483, GenBank Acc. PV749086) and T. taenia ( Macpherson, 1994) from New Caledonia. However, the new species can be distinguished from these species by the following characters:

– The thoracic sternum of the new species and T. taenia has numerous granules on the sternites 6 and 7, whereas the granules are only on the sternite 7 in T. limula . Furthermore, the sternum has more scale-like ridges in T. limula and T. taenia than in the new species.

– The pleonal tergites 2 and 3 each have only one uninterrupted transverse ridge behind the anterior transverse ridge in the new species, whereas these sternites possess three or four transverse ridges in T. limula and T. taenia .

– The distomesial spine of the antennular peduncle article 1 is distinctly longer than the distolateral spine in T. limula and T. taenia , whereas these spines are subequal in the new species.

– The distomesial spine of the antennal peduncle article 2 reaches or exceeds the end of article 3 in the new species, whereas it overreaches the distal end of the antennal peduncle in T. limula and T. taenia .

– The P1 has clearly more spines in the new species than in T. taenia .

– The new species is genetically very different from the other two species, a COI divergence of 9.3% from T. limula and>15% from T. taenia . See Machordom et al. (2022) for the sequences of T. taenia .

Distribution

Saya de Malha Bank, at depths of 150– 210 m.