LITTHABITELLIDAE Falniowski and Hofman, 2023

Falniowski, Andrzej, Jaszczyńska, Aleksandra, Osikowski, Artur & Hofman, Sebastian, 2023, Litthabitellidae: a new family of the Truncatelloidea (Mollusca: Caenogastropoda), Journal of Natural History 57 (5 - 8), pp. 299-329 : 305-307

Family LITTHABITELLIDAE Falniowski and Hofman , fam. nov.

urn:lsid:zoobank.org:act:A8E1F6A7-EEAE-4B8D-A82D-19A7144283C6

Type genus

Litthabitella Boeters, 1970

Diagnosis

Minute truncatelloid snails with the following synapomorphic character states: osphradium broadly ovate; pallial oviduct gland complex short; gonoporus deep inside the mantle cavity; the ventral channel not on the ventral side of the capsule gland, but somewhat lateral; thickened oviduct circular; the distal termination of the penis wide and blunt, with sharp and short filament harbouring the vas deferens situated laterally, and with two flat, cockscomb-like, four-folded lobes.

Description

Shell ( Figures 3 and 4 ) ovate conical, convex in outline, thick-walled; the border between the proto- and teleoconch is easily discernible, the initial part of the protoconch narrow, on the protoconch surface only delicate irregularities ( Figure 5 (a–c)), aperture pyriform, teleoconch smooth with delicate growth lines. Operculum ( Figure 6 ) thin and horny, colourless and translucent, elongate-ellipsoidal, spiral, paucispiral, with submarginal nucleus. Head and tentacles white, with no pigment, no hyperciliation (long and dense cilia at one or two cephalic tentacles, characteristic of e.g. some Hydrobiidae ); mantle partly black pigmented. Osphradium ( Figure 7 (a)) broadly ovate; ctenidium well developed.

In the radula ( Figures 5 (d–i) and 8(b)) the central tooth with deep sinus at the proximal part of the cutting plate and one pair of large and prominent basal cusps. The median cusp narrow and slender, more than twice as long as the adjacent cusps, similar in shape. On each side of the median cusp are four to five cusps; the cutting edge is straight, with no cusps. In the lateral tooth, the biggest cusp is triangular, twice as long and broad as the adjacent ones. The formula of the tooth is (5)4–1–5; on the inner marginal tooth 26–28 long and slender cusps. The caecum is absent ( Figure 8 (c)).

The pallial oviduct gland complex ( Figure 7 (a)) is relatively short, lying far posteriorly of the mantle margin, thus the gonopore is at the terminal part of the capsule gland and situated deep inside the mantle cavity, far behind the anus. The ventral channel is not on the ventral side of the capsule gland, but lies somewhat laterally ( Figure 7 (b,d)). There is a bursa copulatrix and two receptacula typical of the Truncatelloidea : a distal one (rs 1 after Radoman 1983) situated near the junction of the oviduct and the duct of the bursa, and a proximal one (rs 2 after Radoman 1983) situated at the proximal end of thickened oviduct; in Litthabitella the proximal receptaculum is vestigial. The thickened oviduct is circular, forming neither a spiral nor a horseshoe-like loop ( Figure 7 (c)). The prostate is small, nearly vestigial, observable as slight outgrowth of the vas deferens. The penis is big and massive ( Figures 9 and 10 ), filled with glandular tissue ( Figure 10 ), with a distal termination that is wide and blunt, with a sharp and short filament harbouring the vas deferens, which is situated laterally, and two flat, cockscomb-like, four-folded lobes.

Molecular phylogeny

We obtained 26 new sequences of COI (457 bp, GenBank accession numbers ON661272–ON661297), 16 of H3 (310 bp, GenBank accession numbers ON661565– ON661580) and 24 of 18S (401 bp, GenBank accession numbers ON667998– ON668023). The tests by Xia et al. (2003) revealed no saturation. Results from the substitution saturation analysis showed an Index of substitution saturation (ISS) (0.71 for COI; 0.48 for H3) significantly smaller than the critical ISS value (ISSC: 0.97 for COI; 0.59 for H3), indicating that all sequences are useful in phylogenetic reconstruction. In all analyses, the topologies of the resulting phylograms were identical in the ML and BI phylogram analyses; thus, we present the phylogram computed with RAxML.

The COI phylogram ( Figure 11 ) clearly shows all the populations of Litthabitella as a distinct, highly supported group (bootstrap 99%). As usual with COI, deeper nodes are not well supported; thus, the relationships among the Litthabitella remain unresolved. In the 18S phylogram of Szarowska (2006), Emmericia was a sister clade to all the Hydrobiidae (including Hydrobiinae and Sadlerianinae sensu Szarowska 2006). The time of divergence of the Litthabitellidae was estimated as 8.69 ± 1.87 Mya in the COI phylogram.

Our 18S phylogram ( Figure 12 ) shows a similar picture: a well-supported clade (bootstrap 83%) includes all the Hydrobiidae and Litthabitella , with the latter as the sister clade of all the Hydrobiidae . The set of available sequences of histone H3 was very restricted, but the relationships ( Figure 13 ) are similar to those in the 18S phylogram; the well-supported clade (bootstrap 97%) includes Litthabitella as a sister clade of all the Hydrobiidae , but also of Heleobia (Cochliopidae) . The phylogram inferred for the three concatenated loci ( Figure 14 ) also shows the Litthabitellidae as a sister taxon to all the Hydrobiidae , but the clade joining the Litthabitellidae and Hydrobiidae is not well supported (only 62%); the clade Litthabitellidae is well supported (bootstrap 100%).

Figures

Figure 3. Holotypes of Litthabitella: (a) L. aquaadcrucem; (b) L. cetinensis; (c) L. levkadensis. Scale bar: 1 mm.

Figure 4. Shells of Litthabitella. (a–l) L. chilodia: (a–d) locality 3, (e–j) locality 5, (k,l) – locality 2; (m–s) L. aquaadcrucem: (m–q) locality 6, (r) locality 8, (s) locality 9; (t–v) L. cetinensis, locality 1. Sequence numbers: a, 2G39; b, 2G40; c, 2G43; d, 2G45; e, 2G4; f, 2G1; g, 2G3; h, 2G2; i, 2F99; j, 2F100; k, 2D3; l, 2D5; m, 1T10; n, 1T11; o, 1T12; p, 2G15; q, 2G16; r, 8F3; s, 1M25; t, 8G1; u, 8G2; v, 6D8. Scale bar: 1 mm.

Figure 5. Protoconchs and radulae of Litthabitella. (a–c) Protoconchs: (a) L. chilodia, locality 5; (b) L. aquaadcrucem, locality 6; (c) L. cetinensis, locality 1; (d–i) radulae: (d,g) L. chilodia, locality 5; (e,h) L. aquaadcrucem, locality 6; (f,i) L. cetinensis, locality 1. Scale bars: a–c = 400 μm; d–f = 30 μm; g–i = 10 μm.

Figure 6. Operculum of Litthabitella chilodia. Scale bar: 300 μm.

Figure 7. Female reproductive organs of Litthabitella aquaadcrucem (Sotonići,locality 8): (a) female organs with rectum, anus, ctenidium and osphradium; (b) pallial section of the organs; (c) complex of bursa copulatrix, two receptacles and oviduct; (d) perpendicular sections through the capsule gland. Abbreviations: a, anus; bc, bursa copulatrix; cbc, bursal duct; ct, ctenidium; ga, albumen gland; gn, capsule gland; gp, gonoporus;osp,osphradium;ov, oviduct; rec, rectum;rs1, seminal receptacle close to junction of bursal duct and albumen gland; rs2, seminal receptacle adjoining renal oviduct; vc, ventral channel; vc̍, ventral channel separated with broad folds, imitating spermathecal duct) (after Szarowska 2006).

Figure 8. Histology of L. aquaadcrucem: (a) head; (b) radula and ctenidium; (c) stomach; (d) eye. Abbreviations: bc, buccal cavity; c, cuticle; cce, ciliated columnar epithelium; ct, ctenidium; e, eye; es, oesophagus; l, lens; m, mantle; o, odontophores; r, retina; ra, radulae; sg, salivary glands; ss, style sac; vb, vitreous body. Scale bars: a = 150 μm; b–d = 100 μm.

Figure 9. Penes of Litthabitella: (a) L. chilodia, Trsteno, locality 5, scale bar: 500 μm; (b–e) L. aquaadcrucem: (b–d) Sotonići, locality 8; (e) Budva, locality 6; (f) L. cetinensis, Kostanje, locality 1; (b, d,e) bent, in relaxed habitus. Arrows: glandular swellings.

Figure 10. (a–c) Penes of L. aquaadcrucem – histology: asterix, lobes; f, filament. Scale bars: 100 μm.

Figure 11. Maximum likelihood phylogram computed for COI sequences; bootstrap support and Bayesian posterior probabilities are shown when bootstrap supports>65%. Results of delimitation analysis are also shown.

Figure 12. Maximum likelihood phylogram computed for 18S sequences; bootstrap support and Bayesian posterior probabilities are shown when bootstrap supports>65%.

Figure 13. Maximum likelihood phylogram computed for H3 sequences; bootstrap support and Bayesian posterior probabilities are shown when bootstrap supports>65%.

Figure 14. Maximum likelihood phylogram computed for all concatenated sequences; bootstrap support and Bayesian posterior probabilities are shown when bootstrap supports>65%.