How the Horseshoe Crab is Being Conserved and Re-Populated

There are 4 different species of horseshoe crabs in existence today. One of these species called Limulus polyphemus is found in North America and the other 3 species are found in Southeast Asia. Fun fact: the horseshoe crab is not actually a crab. It’s a sea spider. It is more closely related to an arthropod then it is to a crab. One of the coolest things about this species is that despite existing for hundreds of millions of years on planet Earth – the horseshoe crab that exists today is nearly identical to its ancient ancestors. They are literally the living saying of “if it ain’t broke – don’t fix it.”

Ancient Horseshoe Crab Fossil – Image Source: Sacred Heart University

The horseshoe crab is an important part of the ecology of coastal communities. Their eggs are a major food source for shorebirds who migrate north including the federally-threatened red knot. These shorebirds have evolved to time their migrations to coincide with peak horseshoe crab spawning activity, especially in the Delaware and Chesapeake Bay areas. They use these horseshoe crab beaches as a gas station, to fuel up and continue their journey according to the Florida Fish and Wildlife Commission.

Red knot Birds with Horseshoe Crabs – Image Source: The Center for Conservation Biology

Many other species – marine and terrestrial – feed on horseshoe crab eggs in Florida. Adult horseshoe crabs serve as prey for sea turtles, alligators, horse conchs and sharks. For humans, the horseshoe crab is extremely important to the biomedical industry. Their unique, copper-based blue blood contains a substance called “Limulus Amebocyte Lysate”, or “LAL”.

Horseshoe Crab Blood – Image Source: Mark Thiessen – National Geographic

This compound coagulates or clumps up in the presence of small amounts of bacterial toxins and is used to test for sterility of medical equipment and virtually all injectable drugs. This ensures that any injection is free of contaminated bacteria. So if you have ever had any kind of an injection, vaccination or surgery – you have benefited from horseshoe crabs. Additionally, research on the amazing and complex compound eyes of horseshoe crabs has led to a better understanding of human vision. In short, each of the horseshoe crab’s lateral eyes contains about 1,000 photoreceptors, known as ommatidia, and each one is about 100 times bigger than the cones and rods found in the human eye, making the limulus eye an enticing model for vision researchers.

Horseshoe Crab Eye Diagram – Image Source: ScienceDirect.com

The horseshoe crab population numbers have been in decline since the 90s due to overharvesting and habitat degradation. This is bad news for all of the species who depend on them for a food source as well as for us humans and the medical industry.

So what is being done in order to conserve them as well as re-populate them? On the fishery side, there are surveys and even a citizen’s science program where they all collect data and population numbers. These numbers are then reported back to conservancy organizations who then work with the fisheries in order to determine the number of crabs that are allowed to be harvested each season. This allows the wild populations a chance to recover while still providing the crabs for human needs.

For the Southeast Asia species, in Singapore, the Wildlife Reserve Conservation Foundation is supporting Laura-Marie Yap Yen Ling’s data and creating a refuge project. Through this project enough data was collected to warrant the local horseshoe crab’s breeding and spawning areas to be designated protected areas.

Wildlife Reserves Singapore Horseshoe Crab Conservation Project – Image Source: Wildlife Reserves Singapore Conservation Fund

They have also created a breeding facility. What they found was that aeration and moisture were two critical factors needed in order for the proper development of the eggs to occur. As they collect more and more data from their breeding project, scientists are developing a conservation program which will help to manage the populations and habitats of the horseshoe crabs. This can then be duplicated for all of the other species with the necessary tweaks. This is the first study of it’s kind for the Asian species.

Horseshoe Crab Breeding Facility – Image Source: Wildlife Reserves Singapore Conservation Fund

For the American Horseshoe Crab, The Wetlands Institute collected fertilized Horseshoe Crab eggs – with the proper permits – from spawning beaches along the Delaware Bay. They then reared the eggs under controlled conditions in an aquarium. After about a month, the eggs hatched and the newly born Horseshoe Crabs were then raised in culture tanks.

Horseshoe Crab Breeding Facility – Image Source: Wildlife Reserves Singapore Conservation Fund

In this controlled environment, free of predation, aquaculture dramatically increases Horseshoe Crab survival both before and after the first molts. Before this occurs, the crab’s exoskeleton is very weak and thin. As the crab grows they shed their exoskeleton revealing a new, much tougher, more durable one underneath. The newly hatched crabs – now with their proper armor – are kept in culture tanks until they are ready to begin feeding. From there, they are then released at their respective egg collection locations.

Horseshoe Crab embryo development – City University, Hong Kong

The hope of this project is to ensure that the maximum number of eggs hatch and then that the baby crabs are protected until they are able to have a better chance of survival – at which point they are then released back into the wild. This technique along with the surveys and the fisheries’ help is helping to conserve and expedite the process of the repopulation of this species.

Horseshoe Crab Molting – Image Source: Unknown

Horseshoe crabs reach maturity after about 16 molts which is about 9 to 12 years of age. So by protecting them during the most vulnerable 2 molts, their chances of survival are higher. Being released at this age then allows them to adopt their instinctual migratory and spawning patterns which in turn creates even more horseshoe crabs. Over time, this will be a huge help in recovering this species’ numbers in the wild. A HUGE conservation win for these ancient dwellers of the earth who play a critical role in the coastal food chain as well as for human medicine.

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