Interoceanic differences in adaptation: effects of history and productivity

Abstract

Previously published evidence indicated that the degree of habitat specialization and enemy-related adaptation among shallow-water tropical marine species was greatest in the Indo-WestPacific (IWP), intermediate in the Eastern Pacific (EP), and lowest in the Western Atlantic (WA) and Eastern Atlantic (EA). New data on the incidence of unsuccessful shell breakage (measured a s the frequency of shell repair) and on the importance of shell breakage as a cause of death for shallow-water gastropods complicate, but do not invalidate, this picture. Columbellid and planaxid gastropods conforn~ to the expected interoceanic pattern of shell repair in that the lowest frequencies of repair were found in the Atlantic; but the Cerithiidae and Thaididae showed no interoceanic differences in repalr, and the Conidae exhibited a contradictory pattern in which repair was most frequent in the WA and least frequent in the IWP. Great intraregional variation existed in the contribution that breakage made to overall gastropod mortality, as inferred from analyses of dead shells. There was a tendency for breakage to be relatively more important as a cause of death in the IMrP and EP than in the WA, but the differences were not statistically significant. A review of other recent literature generally supported the hypothesis that the IWP biotas have undergone more specialization with respect to hab~ ta t and eneinyrelated adaptation than have marine biotas in tropical America. Available historical evidence suggests that these d~fferences are attributable to contrasting geological histories. In the IWP, extensive speciation, especially among well-armored molluscs, took place in the absence of significant extinctions. In the WA, widespread extinctions selectively eliminated armored gastropod taxa, which were not replaced through diversification of Pliocene and Pleistocene survivors. The apparent decline of planktonic productivity in many parts of the WA following uplift of the Central American land bridge during the Pliocene may have contributed to the observed pattern of molluscan extinctions. In the EP, where productivitv has remained h ~ g h , extinction rates were lower, and some highly adapted species have invaded from the IWP. The paper closes with a discussion of future directions in research.