Assimilation Efficiency of Sanderlings (Calidris alba) Feeding on Horseshoe Crab (Limulus polyphemus) Eggs

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Abstract
Sanderlings (Calidris alba) feed extensively on horseshoe crab (Limulus polyphemus) eggs during their 4-wk spring migratory stopover on beaches in Delaware Bay. To assess metabolizable energy (ME) present in such a diet, we studied the assimilation efficiency of seven captive C. alba fed a Limulus egg diet for 3 wk. Each bird ingested on average 30.9 g eggs d⁻¹ (or approximately 8,300 eggs d⁻¹). The average metabolic efficiency was 38.6% ± 1.0%. Associated with this low metabolic efficiency was a rapid gastrointestinal passage rate of 63 ± 2.5 min (n = 4). While on the Limulus egg diet, the birds maintained body weight (51.3 ± 5.2 g); when switched to a diet of mealworms (Tenebrio larvae), they rapidly gained weight (1.8 ± 0.20 g d⁻¹) and exhibited an average metabolic efficiency of 75.9% ± 8.0%. We attributed the poor assimilation efficiency exhibited by C. alba when feeding on Limulus eggs to the resistance of the egg cuticle to chemical and enzymatic digestion; 72% of the ingested eggs are passed through the gastrointestinal tract unbroken. Calidris alba compensate for the low metabolizable energy of Limulus eggs by processing daily nearly three times the dry mass equivalent eaten on a diet of Tenebrio larvae.