Granulilittorina Habe & Kosuge, 1966

Reid, David G., 2007, The genus Echinolittorina Habe, 1956 (Gastropoda: Littorinidae) in the Indo-West Pacific Ocean, Zootaxa 1420 (1), pp. 1-161 : 15-16

Subgenus Granulilittorina Habe & Kosuge, 1966

Granulilittorina Habe & Kosuge, 1966b (17 May, see Petit & Bieler 1996): 313–314, 328 (type by monotypy Granulilittorina philippiana Habe & Kosuge, 1966a = E. vidua ).

Taxonomic history: The name Granulilittorina was first used in the combination Granulilittorina philippiana Habe & Kosuge, 1966a (15 January). Nevertheless, since there was no indication that the generic name was new, it must be regarded as a nomen nudum (as noted by Petit & Bieler 1996). It was validated by the same authors later the same year, when they published a longer description of ‘ Granulilittorina philippiana Habe et Kosuge (gen. et sp. nov.)’ (ICZN 1999: Art. 13). The evidence given for the new name was the unusual ‘cogwheel’ egg capsule and the slightly reduced chromosome count. In fact the egg capsule, so far unique, represents only a minor variation on the common type for Echinolittorina (Reid 2002a) , while information on chromosome numbers is very incomplete (reviews by Reid 1989a; Thiriot-Quiévreux 2003). This generic name has since been used at generic rank only in the Japanese literature (Habe 1973; Higo 1973; Higo & Goto 1993). Rosewater (1970) employed it as a subgenus of Nodilittorina , for those species with granulose sculpture, and included six IWP species and two doubtful Atlantic ones. Later, one of the Atlantic members was removed and another added (Rosewater 1981).

A molecular phylogenetic analysis of all known Echinolittorina species showed conclusively that all the IWP species belong to a monophyletic group within the genus (Williams & Reid 2004; Fig. 1 ). It is appropriate that this significant biogeographic and phylogenetic group should be recognized taxonomically, and the only available name is Granulilittorina .

Diagnosis: As for the genus, with the following modifications. Shell aperture dark with pale spiral band at base. Penis with bifurcate base bearing a single mamilliform gland; penial vas deferens an open groove. Copulatory bursa always opening in anterior position within pallial oviduct (as in Fig. 3B , or further towards anterior end). Distribution exclusively Indo-West Pacific.

Remarks: The existence of an IWP clade of ‘ Nodilittorina ’ species was hinted at by Reid’s (1989a) separation of N. ( Echinolittorina ) with the copulatory bursa in a posterior position within the pallial oviduct from N. ( Nodilittorina ) with an anterior bursa; members of the former were present only in the Eastern Pacific, Atlantic and southern Africa, whereas the latter group were mainly of IWP distribution. The position of the bursa was later redefined (an anterior state being an opening less than two-thirds of the way back from the anterior end of the pallial oviduct to the anterior extremity of the capsule gland; see Fig. 3B ), but this character did not contribute to the resolution of a more thorough morphological phylogenetic analysis of ‘ Nodilittorina ’ (Reid 2002a). Now that the phylogeny of Echinolittorina is better resolved, it is clear that all members of the IWP clade show the anterior condition, but that both states occur in the basal clades. A morphological synapomorphy for the subgenus Granulilittorina therefore remains elusive.

In the absence of a useful fossil record for Echinolittorina , the explanation of the monophyly of the IWP species can only be speculative. Williams & Reid (2004) used the earliest probable fossil ( E. lozoueti (Dolin & Pacaud, 2000) from the middle Eocene of France) to calibrate the rate of molecular evolution, and thus estimated the origin of the IWP clade at 32.7 million years ago (Ma). This considerably predates the final closure of the Tethyan Seaway at about 18 Ma. If extinction has been insignificant, the basal paraphyly of the Eastern Pacific plus Atlantic clades ( Fig. 1 ) is consistent with an origin of the genus in this region, followed by eastward expansion through the Tethyan Seaway and ultimate isolation of a single lineage in the IWP. However, it is more likely that Neogene extinctions in the Atlantic removed the sister taxa of several IWP lineages, resulting in apparent monophyly of a single IWP clade of greater age than the final closure of the seaway (Williams & Reid 2004). The extreme homoplasy of shell features among littorinids means that the very meagre fossil record of Echinolittorina -like shells is difficult to interpret.

Figures

FIGURE 1. Molecular phylogeny of Echinolittorina species produced by Bayesian analysis of concatenated gene sequences from 28S rRNA, COI and 12S rRNA genes (outgroups removed). Branches with posterior probability (PP) <90% are grey, branches with PP 90% are black. The three measures of nodal support are: posterior probability / bootstrap (neighbour-joining algorithm) / bootstrap (maximum likelihood). Support values are only shown when at least two support values are>50%. Geographical occurrence is shown, with Indo-West Pacific taxa (subgenus Granulilittorina) being enclosed in a grey box. Also shown are the informal groups used in the Systematic Descriptions. (Phylogeny after Williams & Reid 2004).

FIGURE 3. Terminology of Echinolittorina penes and pallial oviducts. A. Penis of E. wallaceana. B. Pallial oviduct of E. australis, with transverse section. The same conventions of shading and line are followed throughout subsequent figures. Abbreviation: bt, structure visible by transparency.