MEPS 167:301-306 (1998)  -  doi:10.3354/meps167301

Shorebird predation may explain the origin of Holocene beds of stout razor clams in life position

Oscar Iribarne*, Juan Valero, Mariano M. Martinez, Luis Lucifora, Silvina Bachmann

Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata, CC 573 Correo Central (7600), Mar del Plata, Argentina

Several hypotheses have been proposed to explain the origin of death assemblages of stout razor clams Tagelus plebeius buried in life position found in Holocene sediments in coastal Argentina and Uruguay: (1) a rapid decrease of sea level, (2) obstruction of the mouth of a coastal lagoon and episodic deposition of fluvial sediments in the mouth of a creek, and (3) a 'taphonomic feedback' process. Hypothesis 1 has been discarded, Hypothesis 2 only has a local explanation, while based on current information Hypothesis 3 is unlikely. A bird predation hypothesis is proposed here on the basis of a study of an extant population. The density of an extant population at the Mar Chiquita coastal lagoon, Argentina (37°46'S, 57°27'W), is higher in the lower intertidal (up to 200 clams m-2) and decreases towards the upper intertidal, and clam size is correlated with burial depth. However, live individuals there coexist with large densities (up to 1500 clams m-2) of dead clams in life position, with densities peaking at the middle-upper intertidal. These shells showed no size-depth of burial correlation. Stout razor clams in fossil deposits (1500 yr BP) also showed large densities of shells in life position (528 clams m-2, SD = 16, n = 20) with no clam size-burial depth relationship. Predation by the American oystercatcher Haematopus palliatus is an important cause of mortality year-round, taking 28 clam h-1 during 3:43 h d-1. Half of the successful attacks left shells in life position buried in the sediment. A computer simulation based on this information showed that predation by 1 oystercatcher produces 9.2 life-position shells m-2 yr-1 (SD = 3.7 life-position shells m-2 yr-1, n = 10000). Thus, 89.5 yr (95% CL = 50-422 yr) are required to generate a mean shell density similar to the one found on our study site (824 clams m-2). Thus, predation by oystercatchers generates a pattern similar to fossil assemblages and is the most likely explanation for the Holocene shell assemblages of the stout razor clams.

Intertidal · Predation · Paleoecology · Stout razor clams · Oystercatchers

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