Horseshoe Crab Rescue and Research with the Nature Society of Singapore

A few months ago, I joined the Nature Society in Singapore. I mostly did it to join their events because there is only so much I can do by myself. It’s nice to have enthusiastic nature people to hang around with and who know much more than I do. The organisation holds talks by local scientists, adventurers and plans events including bird watching, hikes and even oversea trips. One event that caught my eye involved horseshoe crabs conservation. Known as a living fossil, horseshoe crabs are an extremely interesting species of animal because of their ancient origins. Their closest relative is actually the trilobite, which is extinct. They predate the dinosaurs and even flowering plants. And horseshoe crabs are not really crabs, either, as crabs have four sets of legs. Horseshoe crabs of give sets. According to the people with the Nature Society, horseshoe crabs are actually more related to scorpions and spiders! The horseshoe crab has survived major climate events, like the ice age, to survive until modern times. The rescue and research events are ongoing and takes place once a month, coinciding with low tide, near Kranji on the northwest shore of Singapore.

The mudflat at low tide.

We were told to bring especially sunscreen, water and, specifically, covered shoes. The location was emailed to us and we were instructed to go to Kranji MRT, take the 925 bus, hop off at the Chinese temple and walk to a mysteriously named Unnamed Track. The site itself sits amidst an industrial area. On the bus, I met fellow horseshoe crabbers, all of us looking mighty out of place amongst the construction workers. Several construction companies lined the Unnamed Track and the small lane was filled with mechanical knick knacks and something that curiously looked like a small submarine. Upon arrival, it became apparent how important the pack list items would be. Firstly, it was a scorcher of day with not a cloud in the sky. In the 30C sun, my water and sunscreen were quickly put to use. Luckily the assembly area was somewhat shaded on the edge of the mangroves. The low tide exposed the mudflat and along with it, all kinds of litter. Broken glass, plastic bags, tires, old nets – how can these ancient creatures possibly live here with all this man-made litter?

A very strange submarine thingy abandoned near the mudflats - what is it? Does anyone know?
A very strange submarine thingy abandoned near the mudflats – what is it? Does anyone know?
Guardians of the mud flat?

Several people had already shown up by the time I got to the edge of the mudflat. We were a motley crew of young and old, experienced and first-timers. There were two fathers brought their small children, two couples who regularly attended, a few pairs of friends, some students and a few random people like myself who came alone. This particular sunday happened to be Mother’s Day so there were only about 20 of us. Apparently, some sundays can attract over 100 people!

Demonstration on how to set up the quadrants down the transect line.
A demonstration on how to set up the quadrants along the transect line.

Van gathered us to prep us for the day’s activity. Normally, volunteers came armed with scissors to free crabs unfortunate enough to get stuck in fishing nets. But since we were such a small group, the representative from the Nature Society decided to carry out a density population survey of the crabs. The subsequent information gathered would help conservation efforts. To do the population survey, strings that measured 30m were laid on onto the mudflat. The strings started from about 10m from the shore and needed to be parallel to each other. They stretched from the shore and out towards the sea. Five metre intervals were staked by bamboo skewers and it is from these skewers that we are suppose to lay down square metre quadrants. After the quadrant was laid out, we needed to probe the mud with our hands in search of the horseshoe crabs. If we found one, we needed to document its location, measure the shell for size and specify whether its male, female, juvenile, moulting or dead. There are four different species of horseshoe crabs in the world and two of them live in Singapore: Coastal and Mangrove. The ones we dealt with are the Mangrove Horseshoe Crabs so they like to hang out in the mud. Apparently, Singapore is the most densely populated place in the world for horseshoe crabs, not including mating season.

Tons of these little guys in the mud.
Tons of these little guys in the mud.

The heads-up for covered shoes was really appreciated as the mudflat was difficult to walk in. Any loose shoe would have been sucked off your foot and lost in the muck. Broken glass lurked about so it was only natural that sandals were a no-no. There were lots of interesting animals in the mud at low tide. Plenty of worms – red ones and green ones – lived in the sediment. It seemed like every handful of mud was home to three or four. Other creatures included flower crabs, bivalves of all sorts, snails, little tiny fish etc.

Probing the mud with our fingers to search for our little friends.
Probing the mud with our fingers to search for our little friends.

The survey proved to be a bit disappointing. We were only able to spot horseshoe crabs at the 25m mark and it was the only quadrant that contained the animal. We spotted some outside of our quadrants but weren’t allowed to count them. Apparently, hundreds of the creatures would be documented on normal rescue days. Alas, science requires a consistent methodology of surveying population density.

Found one!
Found one!

We finished pretty quickly because of the low count and packed up the transect line and skewers. We washed up on shore, met up for a short debrief before the event drew to a close.

A flat worm.
A flat worm.

After the event I did a bit of research on horseshoe crabs and their importance for both humans and nature is pretty impressive. In the past, horseshoe crabs have been used for fishing bait and fertilizer. But their most important contribution comes in the medical field.

Another juvenile horseshoe crab.
Another juvenile horseshoe crab.

The blood of the horseshoe crab is very sensitive to bacterial endotoxin, especially of the gram negative variety. Humans are also sensitive to endotoxin. It makes us ill, especially if enters our body directly. Think IV drips, injectable drugs, vaccines and surgical instruments etc. Blood from horseshoe crabs is used to detect the presence of bacteria in these medical items to prevent contamination thus, safer. The blood will clot or gel up if it tests positive. In more recent times, there have been alternative testing methods developed to move away from harvesting the blood of horseshoe crabs. Their blood is actually blue. Instead of using hemoglobin to carry oxygen, horseshoe crabs have hemocyanin, which contains copper, giving its blood the blue colour.

The underbelly.

Another anatomical marvel of the horseshoe crab are their many eyes. They have ten in total. TEN. That’s a lot. These eye vary in ability to ‘see’ as some are more like simple light sensors. There are two compound eyes and two simple eyes on top of the shell. Six other eyes are scattered around the body, including one on the tail and two near its mouth! Speaking of tails, it’s crucial for horseshoe crabs to have a tail as it helps them flip over in case they are turn upside down in the surf. We were told to never pick up a horseshoe crab by the tail. If we break their tails by accident, their inability to right themselves over would greatly increase their vulnerability to predators.

This little guy measured one 1.2cm across!
This little guy measured only 1.2cm across!

Predators like shorebirds and sea turtles especially rely on horseshoe crabs as a food source. The birds eat the eggs as a protein-rich food source. Loss of horseshoe crabs would greatly effect the balance of ecosystems around the world.

Crabs discarded by fishermen from their nets.
The underbelly. Notice the mouth amidst the legs?
The underbelly. Notice the mouth amidst the legs?
Horseshoe crabs may be pointy, but not venomous.

Hope you’ve enjoyed my little adventure with the horseshoe crabs. If you want to find out more, you can click on this link for the Nature Society. And more info on horseshoe crabs can be found HERE (mostly North American information) and HERE (Singapore). See you next time and don’t forget to go for a hike 🙂 Cheers, J


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