Siphonaria perexigua, B. W. Jenkins & Köhler, 2024

Jenkins, Bruce & Köhler, Frank, 2024, Hidden in plain sight: Systematic review of Indo-West Pacific Siphonariidae uncovers extensive cryptic diversity based on comparative morphology and mitochondrial phylogenetics (Mollusca, Gastropoda), Megataxa 13 (1), pp. 1-217 : 170-172

Siphonaria perexigua sp. nov.

( Figs 68A–C, K , 69A–B )

Siphonaria ‘ laciniosa group, unit 10’— Dayrat et al. 2014: 261, fig. 3L.

Material examined. Type material. Holotype, from Clifton Beach , Karachi, 24°45.500’N, 67°05.968’E, Pakistan; coll. S. Mushtaq, S. Amir and B.W. Jenkins, PA02-1, 4 July 2018 ( AM C.585859 [M233], Fig. 68A ) . Paratypes: same data as holotype ( AM C.585891 17p, C.585104 p [M232], Fig. 68C , C.585105 p [M234], Fig. 68B ); French Beach Karachi , Pakistan; coll. S. Mushtaq, S. Amir and B.W. Jenkins, 24°50.367’N, 66°49.387’E PA01-1 4 July 2018, ( AM C.585183 p [SK187]) .

Other, non-type material. Pakistan: Karachi , Clifton Beach 24°45.500’N, 67°05.968’E PA02-1, 4 July 2018 ( AM C.595985 [SK569]); French Beach 24°50.367’N, 66°49.387’E PA01-1 ( AM C.585894 3p, C.595949 p [SK376]) .

Taxonomic remarks. Sequences of a specimen of ‘unit 10’ from Hikman Peninsula, Oman first published in Dayrat et al. (2014) have subsequently been referenced by Gonzàlez-Wevar et al. (2018) as ‘ Siphonaria sp. (Caroline Island)’ under an identical GenBank registration number.

External morphology. Foot sole, foot wall, cephalic folds and pneumostome evenly cream, without dark pigmentation; mantle narrower than foot wall, edge thickened, weakly lobed.

Shell ( Figs 68A–C ; Table S9). Small sized (max sl mean = 4.6 mm, SD = 0.58 mm, n = 4), elongate ovate; height tall, medium thickness; apex offset strongly posterior and left, apical sides very flat to weakly convex, protoconch direction homostrophic (n = 2), apex curled to posterior, shell whorl dextral; growth striae indistinct; rib count (mean = 21, SD = 0.43, n = 4), primary ribs indistinct, pale white, slightly bent, broaden to shell lip; shell lip weakly corrugated, uneven; no distinct secondary ribs, rib interstices darker brown; siphonal ridge indistinct, radial shading bands absent. Interior shell lip white, pale to dark brown rays on shell margin to spatula align under primary/secondary ribs, siphonal groove indistinct, same colour as shell edge, points to right anterior; spatula golden brown to mottled brown; ADM scar distinct, CMS concave, paler than shell lip; thickening of shell lip occurs in some specimens; infills and reduces lip corrugations.

Reproductive system ( Fig. 69A; n = 3). Positioned within right side of coelom, against foot wall on foot muscle, under the respiratory cavity and intestine; epiphallic parts positioned between BM and RAM. GA very small with singular GP through foot wall; AO small, narrow, bluntly pointed, joined to inner side of GA; ED short, elongated, centrally bent, joins to outer side of GA; GA, AO, ED all white muscular fibrous tissue; EG medium, soft whitish tissue, slightly folded, joins ED; single centrally folded broad flagellum (F1), shorter but similar width to ED, appears as an extension of ED; BD and CD connect closely but in opposing directions into GA between ED join and GP; CD short, broad, slightly bent, smooth, whitish, featureless; BD long, narrow, featureless; both ducts pass closely together through outer side of RAM ( BD over CD); MG / AG complex medium; CD connecting to ducts, BC embedded in folds of AG / MG; BD without distal loop and MA; BC relatively large, spherical, thin whitish translucent test; HD short, thickened, coiled, links ducts in soft white folded tissues of AG to yellowish granulated HG; outer edge of MG lobbed; AG / MG larger than HG, sides match curvature of inner foot wall.

Spermatophore ( Fig. 69B ). Thread-like (length = 3.07 ± 0.03 mm, n =2), translucent, test thin; head section bluntly rounded, evenly cylindrical, containing a core white gelatinous mass, tapers along the transparent flagellum to a thin tip; both sections smooth, featureless. Head section longer wider than flagellum (head length = 1.47 ± 0.06 mm, ~ 48% of SPM length, head width = 69 ± 0 μm, flagellum width = 17 ± 0 μm, n = 2); 2 SPM tightly coiled embedded in whitish gelatinous mass in BC [SK562].

Comparative remarks. Siphonaria perexigua sp. nov. ( normalis group, unit 10) forms a well-differentiated lineage in the mitochondrial tree (Clade E, Figs 1 , 4 ). It differs from other species by COI distances of ≥ 18% (Table S8). Specimens from Pakistan examined and sequenced herein form a sub-clade with an unidentified species from Oman ( Siphonaria sp. ‘unit 10’) published by Dayrat et al. (2014). We have not examined the specimen from Oman but presume that it is conspecific based on the close genetic relationships with the samples from Pakistan. In Pakistan, we found S. perexigua sp. nov. in sympatry with four other congeners: For comparisons refer to comparative remarks under these species ( S. asghar , S. belcheri , S. kurracheensis , and S. crenata ). Bosch et al. (1995: 185) identified individuals of this species as S. compressa Allanson, 1958 . However, S. compressa (not reviewed herein) differs in shell geometry (fragile shell, apex weakly offset, not overlapping posterior shell edge; see Chambers & McQuaid, 1994a: 266, fig. 1A) and distribution (west coast of S. Africa; Chambers & McQuaid, 1994a) from the the specimens figured in Bosch et al. (1995: fig. 861).

Distribution and habitat. Recorded from Karachi, Pakistan and Hikman Peninsula, Oman ( Fig. 70 ). In this study, found in sheltered positions on moderately exposed rocky shores, mid littoral level, amongst small barnacles ( Fig. 68K ).

Etymology. From ‘perexiguus’ (Latin = very small), for the very small shell size of this species; adjective.

Figures

FIGURE 1. Maximum Likelihood phylogram based on analyses of a concatenated sequence data set of 16S and COI. Branches are collapsed at the species level. Branch labels give unit numbers and accepted species names. Numbers on branches indicate branch support employing 10,000 ultrafast bootstraps.Available genus-group names are shown next to their type species. Scale bar indicating modelled sequence divergence.

FIGURE 4. Maximum Likelihood phylogram (partial, species not collapsed). Clades C–F (normalis, lateralis and pectinata groups) of the tree shown in Fig. 1. Branch labels give specimen identifiers for new sequences or Genbank accession numbers for imported sequences from other studies and geographic regions (see Tables S1–S2 for details). Identical haplotypes are merged into single tips. Numbers on branches indicate branch support by employing 10,000 ultrafast bootstraps. Clade names give unit numbers and accepted species names. Scale bar indicating modelled sequence divergence.

FIGURE 68. Shells of S. perexigua sp. nov., S. planucosta sp. nov. and S. bourailensis sp. nov. A–C, K. S. perexigua sp. nov., Pakistan, Karachi. A. Holotype AM C.585859 [M233]. B. Paratype AM C.585105 [M234]. C. Paratype AM C.585104 [M232]. K. In situ. D–E, L–N. S. planucosta sp. nov., Timor-Leste, Dili. D. Holotype AM C.584826 [M446, SK229]. E. Paratype AM C.585442 [M451, SK165]. L. In situ. N. Protoconch, AM C.585925 [SK231]. F–J, O–P. S. bourailensis sp. nov., NC, La Roche Percee. F. Holotype AM C.585017 [M368]. G. Paratype AM C.608182 [SK126]. H. Paratype AM C.585466 [M372]. I. Paratype AM C.585438, Lifou, Chateaubriand [M390]. J. Protoconch, AMS C.585331 [SK125]. O. NC, Poum, animal. P. Poum, in situ. Unlabelled scale bars = 10 mm.

FIGURE 69. Reproductive morphology of S. perexigua sp. nov., S. planucosta sp. nov., S. bourailensis sp. nov. and S. pravitas sp. nov. A–B. Paratype of S. perexigua sp. nov. AM C.585183 [SK187]. C–D. Holotype of S. planucosta sp. nov. AM C.584826 [M446, SK229]. E–F. Holotype of S. bourailensis sp. nov. AM C.585017 [M368]. G–H. Holotype of S. pravitas sp. nov. AM C. 585040 [M192, SK118]. Scale bars = 1 mm.

FIGURE 70. Known occurrence records of S. ouasseensis sp. nov., S. caledonica sp. nov., S. perexigua sp. nov., S. planucosta sp. nov. and S. bourailensis sp. nov.