The horseshoe crabs belong to an ancient family of marine arthropods, with lineage dating back to 450 million years ago. They are considered a living fossil because they have remained practically unchanged in terms of shape and size for those millions of years.
In the past I have seen empty shells of the horseshoe crabs in the various coastal areas, but have not seen a live one. Traditionally they were harvesting of their blood, which is used for the detection of bacterial endotoxins in medical applications. In my undergrad years, there was excitement in my field when a Singaporean team managed to cloned the enzyme responsible for the endotoxins detection, thereby negating the need to harvest the horseshoe crabs in the wild.1
In Singapore, there are two species of horseshoe crabs, the Coastal Horseshoe Crabs and the Mangrove Horseshoe Crabs.
On a Sunday morning while birding at Mandai Mudflats, while searching for early returning waders, I chanced upon a spot where there were quite a number of overturned horseshoe crabs. At first, I thought they were all dead. But one of them was waving its tail around. As the mud was soft (that’s why it’s a mudflat), I carefully walked over to set it right side up. It then proceeded to ‘walk’ away. My first live horseshoe crab! As I scanned the surrounding, I noticed even more of them, some partially submerged in the mud. As their colours did not stand out, I did not notice them from a distance. I wanted to photograph them, but my long birding lens made things very difficult indeed, as I had to stand way back to get them in frame.
I think some of them got caught in the rapid retreat of the waters due to low tide, and although technically they can right themselves, most were just motionless. I managed to turn a few over, but reckon there were at least 30-40 of them, and the soft mud was doing me no favours. But before I could do more, turning back, I could see thick smoke bellowing from a close distance away. Natural human curiosity compelled me to get to the source of the smoke instead. Apparently there was a fire at a factory at Sungei Kadut and I drove over to have a closer look. 2
When I got back home, and having done some more reading up, the horseshoe crabs species encountered that morning was the Mangrove Horseshoe Crab (Carcinoscorpius rotundicauda).