Does Human Infringement At The Spawning Grounds Challenge Horseshoe Crab Eggs And Their Embryogenesis

Abstract

Horseshoe crabs come ashore to search for surf-protected areas with loosely packed sediments to spawn. This enables the buried horseshoe crab eggs to have sufficient oxygenation and moisture to complete their embryogenesis. Under these circumstances, Balok, on east Peninsular Malaysia (South, South China Sea) was frequently visited by Tachypleus gigas (Müller, 1785) for their spawning. It became doubtful whether horseshoe crab embryogenesis could complete under stressful habitat conditions onset human infringement through physical infrastructure placement on the beach. In addition, the absence of biological evaluating tools triggered the idea to evaluate the health of horseshoe crab spawning grounds using parasitic worms as sensitive indicators. In this study, field visits were made between 2009 and 2014 to trace horseshoe crab nests. Upon successful excavation, the eggs were collected carefully from the nests using plastic hand shovel. Meanwhile, nematodes (Ascaris spp. and Dolicholaimus spp.) and polychaetes (Glycera spp. and Lumbrineris spp.) as well as poor conditioned eggs including those in black (unfertilized/desiccated) or red (bacteria infected eggs), were also recorded from the horseshoe crab nests. The assumed healthy green-yellow green horseshoe crab eggs were brought back to the laboratory and acclimatized for 2 days under 28 % salinity and 28 °C prior the laboratory culture. The number of days in reaching third embryonic moult (Stage-20), hatching of first-instar trilobite larvae and moulting into second-instar was recorded for 40 days. While the field visits of 2011 documented 5-6 worms (nematode and polychaete) per horseshoe crab nest, during 2013, it increased to 6-11 worms. It was clear that physical alterations that took place at the beach during 2011 such as construction of erosion barriers, parking lot and then, the fish jetty during 2013 were responsible for the increased worm intrusion into horseshoe crab nests. The higher abundance of nematodes and polychaetes were significantly correlated (P < 0.05) with the increasing number of black and red eggs after the year 2011, when human infringement became active at Balok. Together, the duration for horseshoe crab embryogenesis to reaching Stage-20 and the duration to hatching into the trilobite larvae were both, delayed up to 5 days and, the egg hatching viability was reduced up to 63% after nematodes and polychaetes trespassed the horseshoe crab nests. With this, poor environment health affects the quality of buried horseshoe crab eggs. Hence, conservation and management efforts are necessary to prevent loss of T. gigas population and their spawning area at Balok, the most documented horseshoe crab spawning ground on east Peninsular Malaysia