Widely distributed Pacific plate endemics and lowered sea-level

Glacial sea-Ievellowerings resulted in greatly increased land area in the Malayan, Indonesian, southern New Guinea-northern Austalian region. During the glacial lows, cold, coastal upwelling greatly increased in the region and depressed surface water temperatures significantly. We propose that loss of marine habitats and cooler seawater temperatures resulted in extinction of Indonesian-Malayan populations of continuously distributed Indo-Pacific marine organisms, and contributed to the existence of widely distributed Pacific plate endemics. Springer (1982) made special note of endemism among the shore fishes and other organisms of Pacific plate islands. In particular he called attention to widely distributed endemics, such as the halfbeak fish Hyporhamphus acutus (Gunther) (Fig. 1) and the gastropod Strombus maculatus Sowerby (Fig. 2). Springer was unable to explain the factors that might have contributed to the evolution of widely distributed Pacific plate endemics. Herein, we wish to call attention to a type of distribution pattern that offers clues to the origin of widely distributed Pacific plate endemics. During the course of our ongoing revision of the marine, shallow-dwelling (often intertidal), blenniid fish genus Istiblennius Whitley, we noted that the distributions of two species, 1. gibbifrons (Valenciennes) and 1. bel/us (Giinther), exhibited disjunct distribution patterns (Figs. 3, 4). For each species, one portion of the distribution was confined to the Pacific plate (including margins) and the other to the Indian Ocean. On cursory examination of the literature, we found several fishes and mollusks that exhibit similar, although not always quite as sharply delimited, distributions (literature references refer to distribution maps). Among the fishes are: Stegastes albifasciatus (Schlegel and Miiller) (Allen and Emery, 1985: fig. 1);Pervagor aspricaudus (Hollard) (Hutchins, 1986: fig. 10);Liopropoma lunulatum (Guichenot) and L. tonstrinum Randall and Taylor (Randall and Taylor, 1988: figs. 3, 7); Cirripectes perustus Smith (Williams, 1988: fig. 10). Among the mollusks are: Strombus haemastoma Sowerby, S. dentatus Linne, and S. terebel/atus Sowerby (Abbott, 1960: figs. 57, 59, 62); Littorina pintado (Wood) (Rosewater, 1970: pI. 348); Chlamys coruscans (Hinds) (Waller, 1972: fig. 2); Rhinoclavis diadema Houbrick (Houbrick, 1978: pI. 35); Clypeomorus nympha Houbrick (Houbrick, 1985: fig. 59; reproduced here as Fig. 5). It is readily apparent from these fish and mollusk examples that a widely distributed Pacific plate endemic (or an endemic almost entirely restricted to the Pacific plate) would result if the disjunct Indian Ocean portion of the taxon became extinct or differentiated taxonomically. The reverse is also true: an Indian Ocean endemic would result if the Pacific population became extinct or differentiated taxonomically. A possible example of this circumstance is illustrated by the distributions of the blenniid fishes Cirripectes Juscoguttatus Strasburg and Schultz, a widely distributed Pacific plate endemic, and C. gilberti, an Indian Ocean endemic (Fig. 6), which Williams (1988 and in prep.) hypothesized cladistically to be sister taxa prior to our recognizing the significance of Pacific plate-Indian Ocean disjunct distributions.