Royal Society Publishing

The Comparative Morphology, Phylogeny and Evolution of the Gastropod Family Littorinidae

D. G. Reid

Abstract

An account is given of the comparative morphology of the family Littorinidae, based on examination of 122 species, grouped into 32 subgenera. The shell, operculum and principal organ systems are described, and their phylogenetic significance assessed. A total of 53 characters, coded as 131 character states, were chosen for inclusion in a cladistic analysis of the phylogenetic relationships of the subgenera. This was performed by the program PAUP, using the principle of maximum parsimony. The outgroup for the analysis comprised representatives of the Pomatiasidae and Skeneopsidae. A consensus tree was obtained from cladograms with consistency indices of 0.408 (autapomorphies excluded). The analysis supports the monophyly of the Littorinidae, and the family can be formally defined by the two synapomorphies of a spiral pallial oviduct and an anterior bursa copulatrix. Three principal clades are identified and given subfamilial rank. The Lacuninae and Laevilitorininae show more plesiomorphic character states, are specialized for life in temperate and polar waters, and occupy the low eulittoral zone and continental shelf. In contrast, the Littorininae occur mainly on tropical and temperate shores, and their synapomorphies of pelagic egg capsules, complex penial glands, paraspermatic nurse cells and sculptured shells can be interpreted as adaptations for their typical habitat in the high eulittoral zone and littoral fringe. The reconstruction of character states on the cladogram provides hypotheses about the evolution of individual characters. Primitively, the male reproductive tract appears to have been an entirely closed duct, opening at the penial tip. Progressive opening of the anterior part of the tract occurred, and was correlated with the appearance of paraspermatic nurse cells to prevent premature dispersal of euspermatozoa. The littorinid capsule gland, responsible for the production of pelagic egg capsules, is believed to be a new structure, not homologous with that of related families. In three cases there is evideiice, from both parsimony and protoconch morphology, of reversion from non-planktotrophic to planktotrophic development. The cladogram is used as a basis for a new classification of the Littorinidae, in which three subfamilies and 14 monophyletic genera are recognized. This is summarized in an appendix, with diagnoses of supraspecific taxa, including descriptions of one new subfamily and four new subgenera, and a list of the 173 recognized Recent species. The poor fossil record of the family is reviewed, and its biogeography discussed in the light of the phylogenetic hypothesis. Of particular interest is the bipolar distribution of the marine Lacuninae, the possible origin in Gondwanaland of the Indian freshwater genus Cremnoconchus, the presence of several relict taxa of Littorininae in the tropical and temperate Atlantic and the probable dispersal of the genus Littorina from the Tethys Sea to the northwestern Pacific and thence to the northern Atlantic in the late Pliocene. Some ecological implications of the phylogenetic hypothesis are considered, with special reference to the diverse types of spawn and life-history strategies. The primitive benthic gelatinous spawn can be viewed as a phylogenetic constraint on the range of habitat and latitudinal distribution of the Lacuninae and Laevilitorininae. The pelagic egg capsules of the Littorininae may have been an important adaptation permitting their exploitation of the littoral fringe and tropical regions, but preventing radiation into terrestrial and freshwater habitats. Non-planktotrophic, non-planktonic development in benthic egg masses is found only at high latitudes, and has appeared independently in Lacuninae, Laevilitorininae and Littorina. The only other non-planktotrophic littorinids are two ovoviviparous tropical species of restricted distribution and probably recent origin.

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