Fossarilittorina Rosewater, 1981

Reid, David G., 2009, The genus Echinolittorina Habe, 1956 (Gastropoda: Littorinidae) in the western Atlantic Ocean 2184, Zootaxa 2184 (1), pp. 1-103 : 15-16

Subgenus Fossarilittorina Rosewater, 1981

Littorina (Fossarilittorina) Rosewater, 1981: 29 (type species by original designation Phasianella meleagris Potiez & Michaud, 1838 ).

Diagnosis: Shell smooth, with microstriae and a single incised line at periphery; pseudumbilicus adjacent to columella. Penis simple, lacking mamilliform glands and glandular disc, sperm duct closed as a tube. Pallial oviduct contains an additional gland around egg groove at posterior end of straight section (anterior to translucent capsule gland); copulatory bursa opens at anterior end of straight section. Radula lacks flange on outer side of base of outer marginal tooth. Caribbean Sea, Gulf of Mexico, Gulf of Guinea to Angola.

Remarks: This subgenus was introduced by Rosewater (1981) for the type species ‘ L. (Fossarilittorina) meleagris ’, based on the distinctive character of the umbilicate shell with a resemblance to the genus Fossarus . The radula was not described in detail, but was said to resemble that of Melarhaphe neritoides , the implication being that it differed from those of other species now included in Echinolittorina . Rosewater suggested that other possible members might include the species now classified as E. mespillum and the members of the E. porcata group in the eastern Pacific (see Reid 2002a). Subsequently, Fossarilittorina was treated as a full genus by Bandel & Kadolsky (1982), who included a description of the penis among its defining characters. Reid (1986) also accepted this as a full genus, and in an early cladistic analysis concluded that it was the sister of Melarhaphe , based on synapomorphies of the penis. As a result of a more detailed phylogenetic analysis of Littorinidae based on anatomical and shell characters, Reid (1989) placed Fossarilittorina as a subgenus of ‘ Nodilittorina ’ (then including species now assigned to Echinolittorina , Austrolittorina , Afrolittorina and Nodilittorina s.s.), and included within it only E. meleagris , E. mespillum and doubtfully E. modesta ( Philippi, 1846a) . This result was further refined by a species-level cladistic analysis of ‘ Nodilittorina ’ ( Reid 2002b), which restricted the subgenus Fossarilittorina to E. meleagris and E. mespillum . Although this analysis produced a poorly-resolved phylogeny, in all trees this pair of species appeared in a relatively basal position, sister to all other ‘ Nodilittorina ’, Littorina and some other genera. This exclusion from ‘ Nodilittorina ’ was a consequence of the simple penis and outer marginal teeth of the radula, interpreted as plesiomorphic traits.

Three molecular studies have included one or more species of Fossarilittorina . In a Bayesian analysis of all littorinine subgenera based on four gene fragments, E. meleagris fell within a clade of six exemplars of Echinolittorina ( Williams, Reid & Littlewood 2003) . This was confirmed by an analysis of all 59 Echinolittorina species then known ( Williams & Reid 2004), but the basal branching order within the genus was not resolved. In the latest analysis (Bayesian inference using Beast program) Fossarilittorina appeared as the basal branch of Echinolittorina , although with only weak support (posterior probability of clade of remaining Echinolittorina species = 76%; Williams & Duda 2008; Fig. 1 ). If clustered within Echinolittorina , the absence of penial glands and form of the outer marginal radular teeth are reconstructed as synapomorphies of Fossarilittorina ( Williams et al. 2003) , but if the subgenus is basal these are equivocally symplesiomorphic. The combination of molecular and morphological evidence does tend to support the view that this subgeneric clade is sister to the remainder of the genus.

Although the absence of penial glandular disc and mamilliform penial gland, and the absence of the outer flange on the base of the outer marginal radular teeth are equivocal synapomorphies of Fossarilittorina , based on the topology of the latest molecular phylogeny the unequivocal synapomorphies can be listed as: shell sculpture smooth with single incised line; pseudumbilicus (shared with E. porcata group); penial sperm duct closed as a tube by superficial epithelial fusion; additional gland around egg groove at posterior end of straight section of pallial oviduct.

This subgenus is known only from the tropical Atlantic Ocean. In addition to E. meleagris and E. mespillum there is a third species, unnamed but sister to E. meleagris , in West Africa ( Williams & Reid 2004; Williams & Duda 2008). No detailed account of the western Atlantic species has appeared since that by Bequaert (1943).

Unusually for the genus, these species occupy damp microhabitats and occur as low as the mid-eulittoral. Rosewater (1981) noted the resemblance of the shells to those of the E. porcata group in the eastern Pacific. The latter are likewise small, umbilicate, and sometimes develop smooth shells, although spiral ribs are usually prominent ( Reid 2002b). They can be found nestling among barnacles (like E. meleagris ) and in shallow pools (like all Fossarilittorina ), which may explain their convergent shell form. In the E. porcata group it has been argued that smooth shells are an ecophenotypic response to permanent submergence in pools ( Reid 2002b). There is no indication of a similar effect in Fossarilittorina species , but this does hint that smooth shells may be adaptive in these species that occur relatively low on the shore.

Figures

FIGURE 1. Molecular phylogeny of all 60 known species of worldwide Echinolittorina (after Williams & Duda 2008), with geographical distributions and subgeneric groups. This is a chronogram, produced by BEAST analysis of sequence data from 28S, 12S and COI genes, which employs Bayesian inference and an uncorrelated, relaxed, log-normal clock; error bars are 95% highest posterior density intervals, and numbers are Bayesian posterior probabilities (see Williams & Duda 2008 for methods). The time calibration of the phylogeny may be underestimated; see Remarks on subgenus Echinolittorina.