PACIFIC, WITH A MOLECULAR PHYLOGENY OF THE GENUS

A new ovoviviparous littorinid gastropod, Tectarius (Echininiopsis) niuensis, from Niue, west of the Cook Islands, is described. This is distinguished from the only other ovoviviparous member of the genus, T. (E.) viviparus (Rosewater, 1982) from the Mariana Islands, here redescribed in detail. The new species is remarkable tor its high-level habitat in the littoral fringe on wave-exposed karstic limestone cliffs, for its variation in shell shape according to tidal level, shell colour polymorphism, calcified operculum and penis with a single mamilliform penial gland. In a cladistic analysis of morphological characters, including single representatives of each of the three other subgenera of Tectarius (Tectarius, Echininus, Tectininus), these two ovoviviparous species appear as sister-taxa. This is confirmed by a molecular phylogenetic analysis of the same species, based on the sequence of a portion of the 16S ribosomal RNA mitochondrial gene. Neither analysis unequivocally confirms the monophyly of Tectarius. The divergence of DNA sequences within Tectarius suggests that the genus arose in the Upper Cretaceous, much earlier than the oldest (Upper Eocene) fossils. Only 4 of the 175 species of Littorinidae are known to be ovoviviparous (with brooding through metamorphosis) and the possible adaptive significance of this type of development is discussed. Hitherto, its rarity had been explained by early extinction of poorly-dispersed brooding taxa. However, ovoviviparity may have persisted in Echininiopsis for at least 35 million years, and has not precluded colonization of islands 6300km apart