Abstract

Grazing trails of the Valviferan isopod Chiridotea coeca are examined to test the relationships between trail morphology and the distribution of food (organic carbon). These isopods burrow up to 1 cm beneath the surface within ripple troughs and planar-bedded sand in the upper intertidal zone. The burrows are grouped into three forms based on the tortuosity of their course and degree of looping and trail crossover in plan view. Sediment samples taken directly from the trail furrows are used to establish the total organic-carbon content associated with each burrow morphology. There is an increase in organic-carbon content from burrows of low tortuosity (linear burrows) to burrows of higher tortuosity (convolute burrows with many crossovers), suggesting that benthic food content directly influences the behavior of C. coeca. Detailed study of trace emplacement further reveals a relationship between C. coeca and food content leading to the recognition of three grazing styles. These are directly related to the plan-view morphology of the trail and reflect the depth at which the isopod tunnels in the sediment. Graphical analysis of weight percent organic carbon against grazing style shows a positive correlation between deep burrowing and high benthic food content. The morphology of C. coeca trails differs from deep-sea turbidite and flysch deposits. In the deep-sea environment resources are replaced slowly, and food distribution is comparatively low and uniform, leading to regular meanders and complex traces that do not crossover existing trails. In the intertidal zone, C. coeca encounters sporadically distributed resources that are replenished semidiurnally. The resultant burrows exhibit increasing crossovers with increasing food content, representing a once-over feeding strategy designed to rapidly harvest a high-value, renewable resource.

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