Comparing comments on fossil juvenile horseshoe crabs found at Solnhofen spending “short times out of water” with sub-aerial activity of juvenile horseshoe crabs in their natural habitat on an intertidal mudflat in Hong Kong.

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at a modern inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong and proposes an inter-tidal mudflat origin for the examples cited.

This post compares references of juvenile horseshoe crabs found at Solhofen - Eichstatt spending short periods out of the water with photographs of sub-aerial foraging and movement by juvenile horseshoe crabs in their natural habitat on the intertidal mudflats at Ha Pak Nai.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“Limulids burrow in the mud to shallow depths in search of small invertebrates for food. They may spend short periods out of the water, and they are particularly tolerant to fluctuations in the salinity or temperature of the water.” (Barthel et al - Page 140).

Comments on juvenile horseshoe crabs spending short periods out of the water in Hong Kong

There are four species of horseshoe crab living today, Limulus polyphemus,Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda. L. polyphemus  occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia. T. tridentatus and C. rotundicauda occur in Hong Kong.

In Hong Kong juvenile horseshoe crabs forage on intertidal mudflats and in areas in and around mangroves.

The foraging activity may take place in shallow water in stream beds or in the pools of water that form between seagrass bed mounds or sub-aerially when the tide has fully receded. Sub-aerial foraging and  movement is normal practice for juveniles of both T. tridentatus and C. rotundicauda and the juveniles can spend extended periods of time engaged in sub-aerial activity, as long as the surface of the sediment is moist or they are or they are not exposed for prolonged periods to high temperatures or direct sunlight. In addition, quite a lot of foraging activity is conducted on the surface of the sediment, not buried in it.

Photographic evidence from Ha Pak Nai. 

Juvenile T. tridentatus running across the surface of an intertidal mudflat.

Juvenile T. tridentatus foraging, then walking across the surface of an intertidal mudflat.

Sub-aerial juvenile T. tridentatus foraging trails, made on the surface of an intertidal mudflat.

References

Barthel, K.W., Swinburne, N.H.M., and Conway Morris, S. (1994). Solnhofen: A Study in Mesozoic Palaeontology. Cambridge University Press, Cambridge.