A NEW OVOVIVIPAROUS SPECIES OF TECTARIUS (GASTROPODA, LITTORINIDAE) FROM NIUE, SOUTH-PACIFIC, WITH A MOLECULAR PHYLOGENY OF THE GENUS

A new ovoviviparous littorinid gastropod, Tectarius (Echininiopsis) ni uensis, from Niue, west of the Cook Islands, is described. This is dis tinguished from the only other ovoviviparous member of the genus, T. ( E.) viviparus (Rosewater, 1982) from the Mariana Islands, here redescr ibed in detail. The new species is remarkable for its high-level habit at in the littoral fringe on wave-exposed karstic limestone cliffs, fo r 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, inc luding 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 phylog enetic analysis of the same species, based on the sequence of a portio n of the 16S ribosomal RNA mitochondrial gene. Neither analysis unequi vocally confirms the monophyly of Tectarius. The divergence of DNA seq uences within Tectarius suggests that the genus arose in the Upper Cre taceous, much earlier than the oldest (Upper Eocene) fossils. Only 4 o f 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 be en explained by early extinction of poorly-dispersed brooding taxa. Ho wever, ovoviviparity may have persisted in Echininiopsis for at least 35 million years, and has not precluded colonization of islands 6300.k m apart.