Adaptive Radiation in Sand-Burrowing Amphipod Crustaceans

Abstract

Over millenia of geological history, the highly oxygenated, unstable substratum of wave-exposed intertidal sands has been a physically harsh environment and a challenge to survival for animal life attempting to utilize its relatively rich planktonic and interstitial nutrient resources. The Atlantic Ocean originated presumably in the late Mesozoic, yet since that time, certain endemic groups of gammaridean amphipod crustaceans of different feeding and reproductive morphology and behavior have apparently developed, convergently, two main types of fossorial body form necessary to survival in this environment. The obligate-sand-burrowing subfamilies Pontoporeiinae and Haustoriinae (family Haustoriidae) illustrate the phenomenon of adaptive radiation within these two body forms. The wide-bodied hydraulic-tunnelling subfamily Haustoriinae contains eight distinct genera of which seven genera and approximately 28 known species occur (many sympatrically) along the non-tropical North American Atlantic coast. The species occupy definitive but somewhat overlapping niches of the sandy beach environment. Analysis of amphipod feeding morphology and distribution in the North Pacific and North Atlantic regions indicates that (1) filter-feeding and reproductive mechanisms are most specialized within the Haustoriinae; (2) the most specialized haustoriin species inhabit the highest intertidal and most landward estuarine environments; (3) evolution and adaptive radiation are proceeding most fully and rapidly in warmtemperate (southern) parts of the subfamily range; and (4) the Haustoriinae may be replacing the more primitive and less specialized colder-water Pontoporeiinae in regions from which the former is now excluded by ecological and distributional barriers.