Galathea stilla, Macpherson & Rodríguez-Flores & Machordom, 2025

Galathea stilla sp. nov.

( Fig. 11 View FIGURE 11 , 12 View FIGURE 12 )

Material examined. Holotype: Kiribati, Phoenix Islands, 4°31’S, 172°13’W, 10.5 m, 18 September 2015: M 2.8 mm ( UF51343 ). GoogleMaps

Paratype: same data than holotype: ov. F 3.0 mm ( UF51346 ) .

Etymology. From the Latin, stilla , drop, in reference to the small size of the species.

Description. Carapace: As long as broad; anterior cervical groove indistinct. Seven ridges on gastric region: 1 epigastric ridge, with 2 epigastric spines, medially convex and medially uninterrupted; 2 protogastric ridges, anterior one uninterrupted, with 1 parahepatic spine at each side, posterior ridge uninterrupted or scale-like, with 2 median long thick simple setae; 2 mesogastric ridges, anterior ridge uninterruptedly extending laterally to anteriormost of branchial marginal spines, posterior one short; 2 metagastric ridges, anterior ridge uninterrupted and fused with anterior branchial ridge, posterior ridge short. Mid-transverse ridge uninterrupted, preceded by shallow cervical groove, followed by 5 ridges, 2–3 of them uninterrupted. Lateral margins medially convex, with 7 spines: 2 spines in front of and 5 spines behind indistinct anterior cervical groove; first anterolateral, well-developed, at same level of lateral limit of orbit, accompanying another spine ventral to between first and second, second spine small or minute; 2 spines on anterior branchial region, last small, and 3 spines on posterior branchial margin, last small. External orbital limit ending in small spine; infra-orbital margin with strong spine. Rostrum broad triangular, 1.2 times as long as broad, length 0.5 postorbital carapace length and breadth 0.4 that of carapace, nearly horizontal in lateral view; distance between distalmost lateral incisions 0.2 distance between proximalmost lateral incisions; dorsal surface with some short setae; lateral margin with 4 deeply incised sharp teeth.

Pterygostomian flap rugose, with facial spine on anterior part, anterior margin ending in small spine.

Thoracic sternum: As long as broad, lateral extremities gently divergent posteriorly. Sternite III with median shallow notch, anterior margin at each side of notch slightly convex. Sternite IV with anterior part as wide as sternite III, with some minute striae. Sternites IV–VI with a few striae on lateral sides. Sternite III ~ 2.2 × as wide as long; sternite IV nearly 1.6 × as wide as long, and 1.4 × as wide as sternite III.

Pleon: Tergites II and III each with 2 transverse ridges on tergite, anterior ridge more distinctly elevated than posterior ridge; tergite of somites IV and V with posterior ridge medially interrupted; somite VI with 2 medially interrupted ridges, posteromedian margin straight. Males with G1 and G2.

Eye: Ocular peduncles 1.2 times longer than broad, maximum corneal diameter 0.7 rostrum width.

Antennule. Article 1 with 3 well-developed distal spines, distodorsal larger. Ultimate article with a few short setae not in tuft on distodorsal margin.

Antenna. Article 1 hardly visible from dorsal view, with depressed ventral distomesial process exceeding distal margin of article 3. Article 2 with distomesial spine smaller than distolateral, exceeding midlength of article 3. Articles 3 and 4 unarmed.

Mxp3. Ischium with well-developed distal spine on flexor margin; extensor margin unarmed; crista dentata with 26–28 denticles. Merus subequal in length to ischium, with 3 spines on flexor margin, proximal one located at midlength, distal one at terminal end, median spine smaller than others; extensor margin with distal spine. Carpus with distal spine on flexor margin.

P1. 2.2 times carapace length. Merus 0.8 times carapace length, 1.5 times as long as carpus, with rows of spines, mesial and distal spines strong. Carpus 0.9 length of palm, 1.5, times longer than broad, lateral and mesial margins subparallel, dorsal surface with some spines in longitudinal rows; mesial surface with row of well–developed spines; and row of small spines along lateral margin. Palm 1.8 times longer than broad, lateral and mesial margins subparallel; small spines roughly in rows on dorsal; lateral row continued on to lateral margin of fixed finger; mesial row continuing on the mesial margin of movable finger. Fingers 0.7 as long as palm, each finger distally with two rows of teeth, spooned.

P2–4. Relatively slender, somewhat compressed. P2 1.7 times carapace length. Meri successively shorter posteriorly (P3 merus 0.9 length of P2 merus, P4 merus 0.8 length of P3 merus); P2 merus 0.7 carapace length, 3.0 times as long as broad, 1.5 times longer than P2 propodus; P3 merus 2.5 times as long as broad, 1.3 times length of P3 propodus; P4 merus 2.5 times as long as broad, 1.3 length of P4 propodus. Extensor margins with row of 7–9 proximally diminishing spines on P2–3, 6 spines on P4; lateral surface unarmed on P2–3, 1–2 spines on P4; flexolateral margins with strong terminal spine and 1–2 additional spines on terminal half; flexomesial margin with terminal spine on P2–3. Carpi with 4–5 spines on extensor margin; lateral surface with row of 2-4 small spines or acute granules paralleling extensor row; flexor distal margins with spine. P2–4 propodi each 3.5–4.5 times as long as broad; extensor margin with 4 proximal spines on P2–4; flexor margin with 4–5 slender movable spines, terminal spines paired; 2–3 proximal spines on lateral side. Dactyli subequal in length, 0.7 length of propodi, ending in incurved, strong, sharp spine; flexor margin with prominent triangular terminal tooth preceded by obsolescent 5 teeth.

Epipods present on P1-3.

Setae: Three types of setae were observed, (1) short plumose setae, with setules along both sides of shaft, densely located on ridges of carapace and pleon and sparsely on P1–4; (2) long thick iridescent simple setae, sparsely located on carapace, pleon and P1–4; (3) long thick plumose setae, with long setules along one side of shaft, sparsely located on P1–4.

Genetic data. COI.

Remarks. Galathea stilla belongs to the group of species having an uninterrupted ridge between the anteriormost branchial marginal spines, the carapace lateral margin having one small spine between the anterolateral spine and the anteriormost branchial marginal spine, one pair of epigastric spines, the pterygostomian flap with 1 facial spine, and epipods on P1–3. The new species is closely related to G. paulayi Macpherson & Robainas-Barcia, 2015 from the French Polynesia and Line Islands.

The new species is easily distinguished from G. paulayi by the shape of the P2-4 meri, slenderer in G. paulayi than in the new species. For instance, the P2 merus is 3.5–3.9 times as long as broad in G. paulayi , whereas it is nearly 3.0 times in the new species.

The genetic divergence between these two species is 12.07% (COI).

The new species is also genetically close to G. amamiensis Miyake & Baba, 1966 , and G. boucheti Macpherson & Robainas-Barcia, 2015 , both from the Indian and Western Pacific areas. However, the new species has epipods on P1–3, whereas G. amamiensis and G. boucheti only have epipods on P1. Genetically, they are also very different from the new species (11.96% and 11.55%, respectively for COI).

Distribution. Kiribati, Phoenix Islands, 10.5 m.