Comparing the fossil juvenile horseshoe crab moults found at Solnhofen with juvenile horseshoe crab moults and moulting 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 to the presence of horseshoe crab moulting and moulted exoskeletons found at Solhofen - Eichstatt with photographs of juvenile horseshoe crab moults and moulting in their natural habitat on intertidal mudflats in Hong Kong.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“The other type of trace is the spiral death track with the body at the end. It has been suggested that these represent moulting activity but this cannot be the case as there are no marks made by the animal after it had crawled out of its skin” (Barthel et al - Page 79).

“Some of the fossils are the remains of moulted exoskeletons rather than body fossils. The moults are easily distinguished because they tend to be incomplete, distorted and lack the relief of the body fossils. The moulting activity must have taken place outside the lagoon and the moults transported to the lagoon independently.” (Barthel et al - Page 140).

Comments on juvenile horseshoe crab moulting activity 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.

Being arthropods, juvenile horseshoe crabs moult to grow and depending on the species, they may moult up to eighteen times before reaching maturity (Sekiguchi.1988). Juveniles horseshoe crabs develop in nursery areas in the intertidal zone, where they will spend up to sixteen years, according to species, feeding and growing, regularly moulting as they grow in size.

Where juvenile moulting (ecdysis) has been observed in the wild in Hong Kong it takes place once the tide retreats. As the tide retreats, the juvenile will emerge from the substrate and remain still while the entire exoskeleton is moulted. Once separated from its’ old exoskeleton (exuviae), the newly emerged animal will expand in size, then take up to one hour for the chitin in its’ new exoskeleton to harden before it moves off.

The discarded moult (exuviae), being lighter than water is usually picked up on the next incoming tide and deposited on the high tide line.

Video of juvenile T. tridentatus moulting (in a pool)

https://www.youtube.com/watch?v=HZjuCc22NoI

Juvenile T. tridentatus moulting - sub-aerial on moist sediment

T. tridentatus moult floating in a pool of water on an incoming tide

T. tridentatus moults found along the high tide line at Ha Pak Nai

Size range of juvenile T. tridentatus moults found along the high tide line at Ha Pak Nai in 2009

References

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