Comparing the fossil juvenile horseshoe crabs found at Solnhofen with juvenile horseshoe crabs in their natural habitat on intertidal mudflats 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 modern intertidal mudflats at Ha Pak Nai, Deep Bay, New Territories; Yi O on Lantau and Luk Geng in the New Territories, Hong Kong and proposes an intertidal mudflat origin for the examples cited.

This post compares references to the presence of juvenile horseshoe crabs found at Solhofen - Eichstatt with photographs of juvenile horseshoe crabs in their natural habitat on three intertidal mudflats in Hong Kong.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“… the juvenile horseshoe crab, Mesolimulus and the crustacean Mecochirus were still alive for a short time on the lagoon floor.” (Barthel et al - Page 58).

“The individuals which crawled and died on the plattenkalk sediment in the Solnhofen- Eichstatt area are all juveniles” (Barthel et al - Pages 77 & 79).

“Almost all fossil limulids from the western area of Solnhofen and Eichstatt are thought to be juveniles” (Barthel et al - Page 140).

Comments on juvenile horseshoe crab habitats and ecology 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.

Each of the living species of horseshoe crabs has different habitat requirements during their lives, related to spawning, juvenile development and adulthood.

In all living horseshoe crab species, spawning takes place on the high tide mark. After spawning, eggs take up to 45 days to develop. Once hatched, small trilobite larvae are released and according to the species, these develop in the spawning area, or very close to it.

About one year after hatching, small juveniles move to a nursery area 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. Depending on the species, they may moult up to eighteen times before reaching maturity (Sekiguchi.1988).

In Hong Kong those nursery areas are on open intertidal mudflats, often associated with seagrass beds in the case of T. tridentatus and in and around mangroves in the case of C. rotundicauda. The juveniles of these two species in Hong Kong also appear to have preferences for substrate type in their intertidal areas where they forage.

Intertidal mudflats in Hong Kong can be broadly categorized as intertidal mudflats with a predominant sand component and intertidal mudflats with a predominant mud component.This is important in understanding juvenile horseshoe crab ecology because in Hong Kong juvenile T. tridentatus favour intertidal mudflats with a high sand component, whilst juvenile C. rotundicauda favour intertidal mudflats with a high mud component

In addition, because of these preferences, whilst juveniles of these two species occur sympiatrically at a number of locations, their differing spawning and juvenile habitat requirements suggest their substrate foraging preferences mean they do not directly compete with each other.

Photographs of juvenile horseshoe crabs and their habitats in Hong Kong

Intertidal mudflat at Ha Pak Nai, with Halophila beccari seagrass beds

Juvenile T. tridentatus foraging trail on intertidal mudflat at Ha Pak Nai

Series of T. tridentatus juvenile moults collected at Ha Pak Nai in 2009

Photograph showing the size range of the juvenile moults collected at Ha Pak Nai in 2009

Intertidal mudflats with juvenile C. rotundicauda

Yi O, Lantau

Luk Geng, New Territories

References

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

Sekiguchi. K. (1988). Biology of Horseshoe Crabs. Science House Co., Ltd, Tokyo