Effects of sediment type and tank shape on horseshoe crab (Limulus polyphemus) growth and survival in culture

Abstract

We measured the effects of two conditions, sediment type (natural or clean sand) and tank shape (rectangular or kreisel style), on growth, survival, and movement patterns of juvenile American horseshoe crabs (Limulus polyphemus). During the 9-week experiment, horseshoe crabs reared in natural sediment and/or in rectangular tanks had higher growth rates than crabs reared in kreisel style tanks with clean sand. Horseshoe crabs reared in natural sediment also had lower #x0025; mortality compared to crabs in clean sand. Hence, by the conclusion of the experiment, horseshoe crabs reared in kreisel style tanks with clean sand showed no growth and 100 #x0025; mortality. To determine whether sediment type affected growth and survival by affecting nutrition, we measured N and C stable isotope ratios in horseshoe crab tissues compared to organic matter in sediments and gel diet fed to crabs during the study. Sediment-derived organic matter made a significant contribution to the diet of crabs reared in natural sediments. Enriched N stable isotope ratios, along with the lack of growth and poor survival, suggested that crabs reared in clean sand experienced starvation. The movement patterns of juvenile horseshoe crabs within each tank also varied with sediment and tank type, suggesting links between these attributes and settlement or foraging behavior. Natural sediment may contribute to horseshoe crab success by providing an additional food source to juvenile crabs, and tank shape may have a secondary effect on growth, survival, and movements related to hydrodynamics that merits further study.