Valettietta trottarum, Stewart & Bribiesca-Contreras & Weston & Glover & Horton, 2024

Stewart, Eva C. D., Bribiesca-Contreras, Guadalupe, Weston, Johanna N. J., Glover, Adrian G. & Horton, Tammy, 2024, Biogeography and phylogeny of the scavenging amphipod genus Valettietta (Amphipoda: Alicelloidea), with descriptions of two new species from the abyssal Pacific Ocean, Zoological Journal of the Linnean Society 201 (4), pp. 1-23 : 13-18

Valettietta trottarum sp. nov.

Figs 8–11

Valettietta gracilis — Ritchie et al., 2015: 122–128, fig 2, table 1, 3.

Valettietta anacantha — Ritchie et al., 2015: 122–128, fig 2.

Valettietta cf. anacantha Va 1— Bribiesca-Contreras et al., 2021: 1–15, figs 1, 4–6, table 1.

Valettietta cf. anacantha — Mohrbeck et al., 2021: 1–12, figs 3, 5, 7, tables 2, 3.

ZooBank registration: http://zoobank.org/ 901CA16B-ECE3- 4886-A72D-74960AA34027

Holotype: Immature, 10.5 mm, carcass and 22 slides, ECDS-AMP10 , NHMUK 2024.37 , Clarion-Clipperton Zone, Pacific Ocean (16.891° N, 123.004° W), expedition JC120, Station 008, depth 4313 m, genseq-1 COI ( PP841401 ), genseq-1 H3 ( PP855319 ), genseq-1 28S ( PP848488 ).

Paratypes: Immature, 8.79 mm, carcass and two slides, ECDS-AMP13 , NHMUK 2024.53 , Clarion-Clipperton Zone, Pacific Ocean (17.321° N, 122.833° W), expedition JC120, Station 039, depth 4230 m, genseq-2 COI ( PP841403 ), genseq-2 16S ( PP849004 ), genseq-2 H3 ( PP855320 ), genseq-2 28S ( PP848489 ). Immature, 8.65 mm, carcass and 1 slide, ECDS-AMP17 , NHMUK 2024.39 , Clarion-Clipperton Zone, Pacific Ocean (16.891° N, 123.004°W),expeditionJC120,Station 008,depth 4313m,genseq-2 COI ( PP841405 ). Immature, 9.09 mm, carcass, ECDS-AMP18 , NHMUK 2024.40 , Clarion-Clipperton Zone, Pacific Ocean (16.891° N, 123.004° W), expedition JC120, Station 008, depth 4313 m, genseq-2 COI ( PP841406 ), genseq-2 16S ( PP849006 ). Immature, 7.9 mm, carcass, ECDS-AMP21 , NHMUK 2024.43 , Clarion-Clipperton Zone, Pacific Ocean (16.891° N, 123.004° W), expedition JC120, Station 008, depth 4313 m, genseq-2 COI ( PP841409 ), genseq-2 16S ( PP849009 ). Immature, 7.92 mm, carcass and three slides, ECDS-AMP22 , NHMUK 2024.44 , Clarion-Clipperton Zone, Pacific Ocean (16.891° N, 123.004° W), expedition JC120, Station 008, depth 4313 m, genseq-2 COI ( PP841410 ), genseq-2 16S ( PP849010 ). Immature, 7.81 mm, carcass, ECDS-AMP44 , NHMUK 2024.45 , Clarion-Clipperton Zone, Pacific Ocean (16.891° N, 123.004° W), expedition JC120, Station 008, depth 4313 m, genseq-2 COI ( PP841430 ) .

Type locality: The Clarion-Clipperton Zone , central Pacific Ocean (16.891° N, 123.004° W), expedition JC120, Station 008, depth 4313 m .

Etymology: This species is named for the Trott family of Deal, Kent ( UK). In their service as sailors of luggers, they saved many lives before the introduction of formal lifeboats (c.1750— c.1856). It is particularly named for Robert and Suzanne Trott, who spent their childhood in the city of Valetta, Malta. Used as a noun in apposition, gender feminine.

Diagnosis: Coxa 1 not shorter or slightly shorter than coxa 2 and not strongly tapered distally. Gnathopods 1 and 2 weakly subchelate or simple, tapering distally, palms weakly defined or lacking. Gnathopod 2 propodus extremely slender and elongate, length more than 6 × width, propodus with weak convex dentate palm. Uropod 3 outer ramus article 2 elongate, subequal to article 1. Maxilliped inner plate reaching two-thirds along palp article 1, with two short nodular robust setae, and three smaller nodular setae at sub-apical medial margin. Telson with three robust setae in large sub-apical notch.

Description ( Figs 8–11 ) Based on holotype, immature female, 10.5 mm length, NHMUK 2024.37

Head: Head large, rostrum absent. Eyes not documented at collection and not apparent in preserved specimen ( Fig. 8 ). Lateral lobe sub-rectangular. Antennae 1 ( Fig. 9 ) elongate, 0.39 × as long as body length, flagellum 22-articulate, sparsely setose; accessory flagellum 4-articulate, reaching to end of basal conjoint article of flagellum; conjoint article subequal to length of peduncle article 1. Antennae 2 ( Fig. 9 ) 0.84 × the length of Antennae 1, 0.33 × as long as body, flagellum 22-articulate, proximal flagellar articles with slender setae.

Mouthparts: Upper lip ( Fig. 10 ) asymmetrically rounded with small apical notch, distal surface minutely setose. Lower lip ( Fig. 10 ) as for genus. Mandibles ( Fig. 10 ) asymmetric. Left incisor 7-dentate and closely applied to 7-dentate lacinia mobilis, spine row with 4 large spines interspersed with long plumose setae, molar strongly triturative; palp robust, article 1 small, article 2 elongate, broadened medially/distally, 11 A2 setae present, D2 setae present along distal three-quarters of medial margin. Article 3 oval, tapering distally; two A3 setae and four plumose E3 setae present, 15 plumose D3 setae on the posterodistal half of the margin. Right incisor 8-dentate, closely applied to 5-dentate lacinia mobilis, spine row with nine large spines interspersed with long plumose setae, molar strongly triturative; palp robust, article 1 small, article 2 elongate broadened medially/distally, 14 A2 setae present, D2 setae present along distal three-quarters of medial margin. Article 3 oval, tapering distally; three A3 setae and five plumose E3 setae present, 13 plumose D3 setae on the posterodistal half of the margin. Maxilla 1 ( Fig. 10 ) inner plate with 22 marginal plumose setae, outer plate with 6/5 crown arrangement; palp large, article 2 lateral margin with two long slender setae, distal margin with 13 robust setae and row of seven long submarginal setae. Maxilla 2 ( Fig. 10 ) inner and outer plates subequal, distally setose, inner plate also with oblique row of 22 plumose facial setae. Maxilliped ( Fig. 10 ) inner plate reaching two-thirds along palp article 1, with two short nodular robust setae, and three smaller nodular setae at sub-apical medial margin; outer plate reaching halfway along palp article 2, inner margin with row of 15 short nodular robust setae grading to four elongate and plumose setae distally; palp 4-articulate, dactylus well developed.

Pereon: Gnathopod 1 ( Fig. 9 ) subchelate; coxa 1 shorter than coxa 2, slightly tapering distally, proximal margin rounded, distal margin sub-rectangular and setose; carpus shorter than propodus, propodus not strongly elongate, tapers slightly distally; palm acute, palmar margin slightly rounded and dentate, delimited by three bifid robust setae, dactylus overlapping palm, inner margin smooth, outer margin with a few fine setules. Gnathopod 2 ( Fig. 9 ) subchelate; coxa rectangular, proximal margin rounded, distal margin setose, basis curved, ischium extremely elongate (equal to basis); carpus slightly shorter than propodus (1:0.81), both slender, propodus length seven times width; palm acute, palmar margin convex and minutely dentate, dactylus overlapping palm, inner margin toothed with a few fine setules, one robust palmar defining seta. Pereopod 3 ( Fig. 11 ) coxa rectangular, distal margin setose, ventral proximal margin corner angular rather than rounded; basis curved; merus longer than propodus, and both longer than carpus; all articles with long slender marginal setae. Pereopod 4 ( Fig. 11 ) coxa very broad with large posteroventral lobe, posterodistal margin broadly rounded and setose; merus and propodus subequal in length and longer than carpus. Pereopods 5–7 ( Fig. 11 ) coxa 6 smaller than coxa 5, both weakly bilobate; coxa 7 smaller than coxa 6, not bilobate; basis broadly expanded distally, increasing from P5 to P7.

Pleon and urosome: Epimeron ( Fig. 9 ) 1 weakly rounded, 2 subquadrate, 3 produced to a subacute tooth. Uropods ( Fig. 11 ) biramous, lanceolate. Uropod 1 ( Fig. 11 ) inner ramus shorter than outer (1:0.8), outer margin with three short robust setae, inner margin with two on distal half, proximal half of inner margin minutely serrate; outer margin of outer rami lined by short robust setae, inner margin with two setae on central margin, inner margin minutely serrate; apex of both rami with inset small setule; peduncle with row of robust setae on both margins, one large robust seta present on peduncle at base of inner ramus. Uropod 2 ( Fig. 11 ) inner ramus shorter than outer (1:0.75), both margins of inner and outer ramus lined by short robust setae, adjacent margins minutely serrate; apex of both rami with inset small setule; peduncle damaged, one large robust seta present on peduncle at base of inner ramus and two at base of outer ramus. Uropod 3 ( Fig. 11 ) inner ramus shorter than outer (1:0.9), distal and proximal article of outer ramus subequal in length, apex of proximal article with robust setae, inner margin of distal article minutely serrate, centre of outer and inner margin of outer rami with one robust seta; inner and outer distal margin of inner rami minutely serrate, outer margin with three robust setae on proximal half of margin; peduncle with long fine setae on inner margin, two robust setae present at base of outer rami. Outer ramus article 2 elongate, subequal to article 1. Telson ( Fig. 11 ) triangular, over 75% cleft, apices with three short robust setae in subapical notch; lobes with three lateral robust setae.

Habitat and ecology: Valettietta trottarum is an abyssal benthopelagic scavenger species. It is currently known to occur in high numbers in both the east and west Clarion-Clipperton Zone (abyssal central Pacific), at depths of 4082–4313 m. Based on available genetic sequences, the species is also present in both the Mariana and New Hebrides Trenches, in the western and southern Pacific, respectively, at depths down to 5467 m ( Ritchie et al. 2015).

Remarks: Valettietta trottarum morphologically most closely resembles V. gracilis in the distally narrowing propodus of gnathopod 1, the sub-rectangular shape of coxa 2, and the presence of three spine groups along the telson. However, V. trottarum can be distinguished from V. gracilis by the convex dentate palm of gnathopod 2 (concave in V. gracilis ), and the large sub-apical notch on the telson with three robust setae (vs. a small notch with one robust seta in V. gracilis ). Key morphological differences distinguishing V. trottarum from V. synchlys and V. anacantha can be found in the key to the genus (below). Ritchie et al. (2015) identified specimens of V. gracilis and V. anacantha morphologically in the west and south Pacific Ocean; however, based on our phylogenetic analyses of the associated genetic sequences, some of these specimens actually belong to the species V. trottarum (Supporting information, Figs S1S 2 ). Modified records can be found in Supporting information, Table S1 . Body size in examined specimens ranged from 7.81 to 10.50 mm, which was notably smaller than the body size range of measured specimens of V. synchlys . This, however, should not be deemed as a defining characteristic of the species, as we only collected immature individuals and so the full body size range is not currently known.

Key to Valettietta species:

1. Coxa 1 much shorter than coxa 2 and strongly tapered distally......................... Valettietta cavernicola (Anchialine cave habitat) Coxa 1 not shorter or slightly shorter than coxa 2 and not strongly tapered distally .......................................................................2

2. Gnathopods 1 and 2 subchelate, robust, with sub-rectangular broadened carpus and propodus, palms weakly oblique .......... ............................................................................................................................................................................................................................3 Gnathopods 1 and 2 weakly subchelate or simple, tapering distally, palms weakly defined or lacking ........................................4

3. Tegument ornamented with small conical setules, single tooth on the posterodistal margin of coxae 1, 2, and 3....................... ............................................................................................................................................................................................ Valettietta punctata Tegument without ornamentation, two teeth on the posterodistal margin of coxae 2 and 3............................. Valettietta lobata

4. Gnathopod 2 propodus extremely slender and elongate, length more than 6 × width....................................................................5 Gnathopod 2 propodus slender, length less than 5 × width..................................................................................................................6

5. Gnathopod 2 propodus lacking palm, very strongly narrowed distally; uropod 3 outer ramus article 2 shorter than article 1 (0.74 × length of article 1)............................................................................................................................................... Valettietta gracilis Gnathopod 2 propodus with weak convex palm; uropod 3 outer ramus article 2 elongate, subequal to article 1 ....................... ........................................................................................................................................................................................... Valettietta trottarum

6. Gnathopod 1 propodus robust, tapering distally, broadly rounded proximally, carpus, short and broad, clearly shorter than propodus ....................................................................................................................................................................... Valettietta anacantha Gnathopod 1 propodus slender, tapering distally, carpus and propodus subequal ........................................... Valettietta synchlys

Figures

Figure 1. Distribution of Valettietta Lincoln & Thurston, 1983 species.A, all published records of Valettietta species. Stars represent type localities and circles all other records. Filled shapes indicate specimens identified by morphology and DNA, and empty shapes indicate specimens identified by morphology only.Individuals only tentatively identified by morphology (e.g. V. cf.gracilis), or only identified to genus level are not shown. Black boxes indicate location of map sections B and C. B, type locality for V. trottarum and V. synchlys within the Clarion- Clipperton Zone. C, paratype locality for V. synchlys within the Clarion-Clipperton Zone. See Supporting information, Table S2 for full specimen collection details.

Figure 2. Bayesian tree showing the relationships between amphipod species based on a concatenated dataset of 16S and COI sequence data. Bayesian posterior probability values greater than 0.5 (before /) and maximum-likelihood bootstrap values (after /) are shown on branch nodes. Branches are labelled by species name, with collection location and depth (m) in brackets.Asterisks indicate sequences added by this study.Species delimitation results and family are denoted on the right.Scale indicates time relative to root age.Location abbreviations are as follows, CCZ—Clarion-Clipperton Zone; ANS—Afanasy Nikitin Seamount; WZFZ—Wallaby Zenith Fracture Zone; NHT—New Hebrides Trench; MT—Mariana Trench; PAP—Porcupine Abyssal Plain; PCT—Peru-Chile Trench; AnB—Angola Basin; ArB—Argentine Basin; BB—Brazil Basin; KT—Kermadec Trench; SEIR—South East Indian Ridge; MAR—Mid-Atlantic Ridge. For complete references and accession numbers for sequences see Supporting information, Tables S2, S3.

Figure 8. Valettietta trottarum holotype, immature, NHMUK 2024.37, preserved specimen. Scale bar = 1mm.

Figure 9. Valettietta trottarum holotype, immature, 10.5 mm, NHMUK 2024.37, Clarion-Clipperton Zone, 4313 m. A = Antenna; GN = Gnathopod.

Figure 10. Valettietta trottarum holotype, immature, 10.5 mm, NHMUK 2024.37, Clarion-Clipperton Zone, 4313 m.MX = Maxilla; Md = Mandible; MXP = Maxilliped; LL = Lower lip; UL = Upper lip; l = left; r = right. Unfilled circles indicate setal bases.

Figure 11. Valettietta trottarum holotype, immature, 10.5 mm, NHMUK 2024.37, Clarion-Clipperton Zone, 4313 m. P = Pleopod; U = Uropod; T = Telson.P7 and U3 are illustrated from the paratype, immature, 8.79 mm, NHMUK 2024.53.