There are four species of horseshoe crab living today:
(a) Limulus polyphemus;
(b) Tachypleus tridentatus;
(c) Tachypleus gigas; and
(d) Carcinoscorpius rotundicauda.
Limulus occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia.
Horseshoe crab ecology
Horseshoe crabs have specific and specialized habitat requirements at different stages of their lives, related to:
(a) spawning;
(b) juvenile development; and
(c) adulthood.
These factors, particularly the habitat requirements for juvenile development are critical to understanding the palaeoenvironments in which horseshoe crab fossils are found.
Spawning habitats and early development
All four species of horseshoe crab have specific spawning requirements related to a combination of factors including tides, substrate type and sand grain size.
L. polyphemus, T. gigas and T. tridentatus usually nest on the high tide mark of protected sandy beaches, and each of these species appears to have very specific preferences as to range of sand grain size for nesting.
Unlike the three other species, C. rotundicauda, which is the smaller species, does not spawn on sandy beaches. Instead, it lays its eggs in subsurface, muddy substrate in streams, rivers or mangrove stands (C. rotundicauda has a different lifestyle to the three species above and this will be discussed in a separate blog post with specific reference to the palaeoenvironment of the Carboniferous Mazon Creek deposits)..
In areas where suitable beaches are not available, or have been diminished, L. polyphemus has been observed to lay eggs within the sediments of offshore sandbars that parallel beaches, in the vicinity of the deltas of tidal creeks or on the sandy banks of tidal marsh creeks.
Adult female horseshoe crabs carry thousands of eggs in a breeding season, which are laid in small batches over a period of time. Spawning is tied to the lunar cycles, taking place on high tides, so that the eggs can be laid specifically in the high tide zone, where it appears the correct microclimate involving a combination of temperature, moisture and oxygen content is critical to successful embryonic development and hatching (Brockman 2003).
Photographs
Spawning habitat - inside the spawning area
Development inside the egg
After spawning, embryonic development starts. Eggs take up to 45 days to develop.
(Image courtesy: City University, Hong Kong)
Development of trilobite larvae - inside the spawning area
Once hatched, small trilobite larvae are released, and according to the species, these develop in the spawning area, or very close to it. The two rows of images on the right are newly hatched 1st and 2nd instar trilobite larvae.
(Image courtesy: City University, Hong Kong)
Juvenile development habitat - on a "nursery beach"
About one year after hatching, trilobite larvae of the three large species move to a nursery beach in the inter-tidal zone, which is usually a shallow water mudflat in close proximity to the spawning site, where they will spend up to sixteen years as juveniles, feeding, growing and regularly moulting as they grow in size. This is the horseshoe crab nursery in seagrass beds (Halophila beccari) at Ha Pak Nai, Deep Bay, Hong Kong. The juveniles forage and develop in the pools between the seagrass mounds.
This is a series of juvenile Tachypleus tridentatus moults collected from the high tide lines at Ha Pak Nai in 2009 (the largest moult is 12.5 centimetres across the prosoma and represents the last stage of juvenile development). Depending on the species, juvenile horseshoe crabs may moult up to eighteen times before reaching maturity (Sekiguchi 1988).
Adult habitat - on the seabed
After reaching maturity, the young adults move into deeper water to forage and grow. This is a mating pair of adult Tachypleus tridentatus captured in the stream mouth at Ha Pak Nai in 2009. The larger female is on the left side of the photograph.
References
Brockman, H. J., (2003). Nesting behavior: A shoreline phenomenon. In: Shuster CN, Barlow RB, Brockman HJ, (Eds). The American Horseshoe Crab. Harvard University Press. Cambridge.
Sekiguchi. K. (1988). Biology of Horseshoe Crabs. Science House Co., Ltd, Tokyo.
