Experimentally derived estimates of growth by juvenile Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura) from nursery beaches in Hong Kong

Juvenile Tachypleus tridentatus and Carcinoscorpius rotundicauda with prosomal widths of between 17.1 and 91.1 mm were obtained from their nursery beaches in Hong Kong. They were kept in the laboratory and fed a mixture of squid, prawn and fish. Prosomal width and wet weight were measured weekly to obtain growth data. Over half of the individuals molted during a five and a half month captivity period. After every ecdysis, prosomal width and wet weight of T. tridentatus grew by averages of 24.2% and 71.5% over pre-molt measurements, respectively. Similar values were obtained for C. rotundicauda, i.e. 24.0% and 77.3%, respectively. Three T. tridentatus with prosomal widths of between 26.5 and 35.0 mm molted twice between 89 and 149 days, leading to an average growth rate of 0.1 mm·day−1 and 0.04 g·day−1 in terms of prosomal width and wet weight, respectively. A positive growth allometry (b coefficient=2.97) was identified, indicating that weight gain for T. tridentatus, and possibly C. rotundicauda, was faster than prosomal width growth after each ecdysis. The effect of temperature on growth was also determined by comparing the percentages of T. tridentatus which molted at ∼28 °C and ∼18 °C. Fifty percent of individuals molted at the former, but only 10% at the latter. This study indicates that Hong Kong horseshoe crabs take a shorter time to reach sexual maturity, as compared with conspecifics in Japan, because they can molt more frequently at higher sediment/water temperatures (∼28 °C) if food is available.