Saving the ‘Old Antiques’ – Horseshoe Crabs Research and Rescue

November 5, 2009 at 12:33 am | Posted in itchy fingers | Leave a comment
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I can’t remember when was the first time I ever heard of the name ‘Horseshoe Crab’, but I vividly remembered one of my friends from the Bird Group of Nature Society reported rescuing some stranded Horseshoe Crabs in abandoned fishing nets while birdwatching in some ulu parts of Singapore some years ago. That was the time I started to know more about these animals and their plights. Shortly since then, there have been regular trips organised by volunteers to try to save these animals. Again, like most busy city dwellers always full of excuses, Itchyfingers had long wanted to join in and do our little bits to help but it was only recently that I really found the time to sign up as a volunteer.

Despite the name, Horseshoe Crabs are not crabs but are more related to spiders and scorpions. They belong to the large group of invertebrates (animals without backbones) called Arthropods. Horseshoe crabs are always called the ‘living fossil’ as they have remained basically unchanged for millions of years – fossils over 400 million years look almost identical to the species living today!

The venue of the Horseshoe Crab rescue was at some ‘nameless track’ near Kranji. When we reached the end, we could see the mudflat ahead, but just to be sure we were at the right ‘nameless track’, I checked with the people nearby at some factory/workshop to confirm. Seeing us, the uncle immediately told us, “they are all down there!”. We were kinda surprised how they knew the reason we were there for, but actually, we shouldn’t be, cos, like what they said, who else would come to this place then? :p

As we were about to join the others, the people suddenly mentioned about some 老古董 (old antique). Being Itchyfingers, of course I was curious to find out what were they talking about. Then the uncle showed us this…

A whole terra cotta pot of horseshoe crabs! (Photo by my friend, Butterfly.
Read here
for her side of the story…)

They said these Horseshoe Crabs were collected when they were ‘washed up the shore’ and they were going to eat them. I remembered watching tv about some people in Ansila, Thailand, grilling them to remove toxins from their eggs, but I have not heard of people here eating them. In my mind, I was thinking, “Goodness, here we were, trying to rescue the animals from the net and these people were telling us they were going to eat them when they knew that there were people just down at the mudflat doing the rescue!” How ironic! So I just said they were posionous and cannot be eaten, but the folks told us that they would grill them to eat the eggs… 😦

The uncle promptly flipped open one of the females’ book gills to show us
the eggs
. According to the tv programme, normal grilling may not remove
the toxin completely, so I was kinda curious how did they do that…But I
refrained from asking…in case they start to show us on the spot how to
grill…:o Now I wonder what do people do to the males, as there is hardly
any flesh for consumption…

The uncle also showed us some mussles and clams that they collected. Then he told us to check with the researchers if they want to buy the Horseshoe Crabs at 50 cents a piece…er…gotta go join the rest since we were behind meeting time…

The folks down at the mudflat had already started their rescue. We met the lady in charge and were given a few tips on how to handle Horseshoe Crabs – by the side of the shells and never by the tail in case we break them. The telson (tail) may look menacing especially when it is pointed up, but actually it is harmless and not used as a weapon. Instead it is used as a rudder when swimming as well as to right itself when the animal is flipped upside down. The Horsehoe Crab may bend its abdomen at the point where it joins the main shell (carapace) and dig into the sand with the tail to support itself while it turns over. So, without the telson, the animal may not be able to survive. I was kinda excited to hold a life specimen for the first time. Besides captives and dead ones, this was the second time I saw and hold a life adult Horseshoe Crab. The first time was at the Pulau Semakau trip but it was a tiny juvenile one…can’t see much details…hahah… :p

A female Horseshoe Crab has 5 pairs of legs with similar sized claws

Can you see the difference? The male has bulbous front pincers on the first
two pair of legs which they used to cling onto the females whilst mating.
Males are smaller than the females. Juveniles do not display this difference
until they are much older

Locally, we have two kinds of Horseshoe Crabs – the larger Coastal Horseshoe Crabs (Tachypleus gigas) the Mangrove Horseshoe Crabs (Carcinoscorpius rotundicauda) that we were rescuing. Besides being smaller, it can also be recognised by the tail – there is no spike and the cross-section of the tail is round (The coastal species has triangular cross-sectioned tails) And oh, Horseshoe Crabs are romantic creatures.. :p They mate during high tides under the new and full moonlight. According this site, it was said that “the advantage of a full moon is that by laying eggs in the highest tidal point, subsequent high tides won’t wash away the eggs before they’re hatched and juveniles are ready to take their places in the deep bays or continental shelves a few weeks later.”

When I told the lady in charge about the terra cotta pot of Horseshoe Crabs, she went up immediately to talk to the folks. Oops…hopefully there won’t be a clash, as my friend said she sounded furious. Meanwhile, we tried our hands in helping to free our first Horseshoe Crab of the day! I felt so sorry for the poor animal entangled helplessly in the net…Horseshoe Crabs use their five pairs of book gills to breathe underwater. Though these book gills also allow the animals to breathe on land for short periods of time, they have to remain moist. Thus, if the animal is being trapped in net for an extensive period especially under our hot weather and unable to free itself back to the water, it will soon die. 😦 We had to carefully cut all the fine wires in between the legs…I could almost feel its pain…I must admit my eyes were slightly moist when we finally got all the wires cut off from the many legs of the poor animal.. :p By then the lady in charge had returned with a big grin, her hand holding that big pot of Horseshoe Crabs. She had saved them from the dinner plate…yeah! 🙂

Besides saving the stranded Horseshoe Crabs from abandoned nets, we also helped to look and collect life specimens for data recording so that more studies can be done. First, I went with another lady to collect pails of water under the hot afternoon sun. Then, we split into groups with the more experienced volunteers helping newbies like us to look for Horseshoe Crabs in the mudflat. After our intertidal walk at Semakau last month, we decided to get a pair of booties to help us wade through water better. But our booties were not that useful here at the mudflat cos half the time I found my feet stucked deep in the soft mud and had to use all my strength to pull them out so as to make the next step! At the same time, we had to look for the animals and also be mindful not to step on other smalls animals. Should have taken pictures of our clumsy moves but we were carrying both our backpacks and a pail of water to put the Horseshoe Crabs found and also a plastic bag to do a mini coastal cleanup…so busy…hands full…not very convenient to keep snapping away, especially when the others were hard at work… :p

Only managed to take a pic with this guy we found in the mudflat…Initially
it was kinda hard to look for one, but observe carefully for movements and
soon we found our pail filled with big and small Horseshoe Crabs to be
brought to the guys for measurement and recording

In total, within the two hours of so under the scorching sun, the group of volunteers collected 168 Horseshoe Crabs. They measured the width of the carapace, recorded the number of males and females before releasing them back again. For us, we rescued three Horseshoe Crabs from the net, but too bad we didn’t take pictures of them. Must be serious at work mah, hahah… 😀

We were shown a moult of the Horseshoe Crab and the lady in charge asked if I would like to bring it back as a keepsake. Since it is just an empty shell, there won’t be bad smell and I was also curious to look at the anatomy more closely. So happily, we packed it in a recycled plastic bag…

The moult really looked like a life animal! Especially when you flipped it
around, the legs also flipped over, as if alive.. hahah

Like insects and all over invertebrates, Horseshoe Crabs moult a few times
in their life time as the hard exsoskeleton do not grow. To grow, it splits
open along the front edge as seen here, and crawls out. When it first emerges
from the old shell, the animal is soft, making it vulnerable to predation.
It then takes in lots of water and stretches itself until it is about 30% larger
than the old shell. The new shell hardens in a few hours. The normal crab species
we are more familiar with moult by slipping out from the end of its abodmen.
The soft-shelled crabs that people eat are actually crabs that have just moulted

Back home, I soaked the moult overnight in a pail of water to get rid of the mud. Just like what the lady told me, when the moult dried, it felt like paper. The legs turned stiff so it is no longer movable as if it was a life animal. So interesting!

Top view of the Horseshoe Crab moult. There are two compound eyes along
the ridge. Other simple eyes not visible to our naked eye are located  along
the front, while other senory organs are along the side

Can you see one of the compound eyes here? Will be quite interesting to
take a super macro closeup shot… :p

I wanted to take a picture of the front edge where it spilt opened but the moult had turned stiff. Amazingly when soaked back in water, it turned soft again…hahah…

The underside. There are five pairs of walking legs, with the first two pairs
of appendages for shovelling food into the mouth. Under the abdomen are
5 pair of flaps known as book gills used for respiration. Their flapping action
helps propel the animals through water. I noticed the fifth walking leg of
the moult looked rather different and undeveloped…They are called the
‘pusher legs’ which are used for cleaning gills on its abdomen and pushing
the animal across sandy beaches or the seabed

Now, if I didn’t bring back the moult, no way I would have noticed this difference on a life Horseshoe Crab! :p

The mouth is in the center. Can’t see any suggestion of a mouth here though…
hahah…They feed on small worms, shrimps and other mud dwelling creatures

Do you know that the blood of the Horseshoe Crabs has medical uses? For a long time, the blood has been used to test drug for endotoxins, a byproduct of bacteria that remain even after sterilization. The blood of the Horseshoe Crab is copper based instead of iron based like ours. Upon exposure to oxygen it turns blue! The blood gels when it comes in contact with bacteria or endotoxins – if a horseshoe crab sustains an injury such as a cut, when bacteria tries to enter it’s body, its blood gels, hence creating a barrier against the bacteria. Biomedical companies extract the compound that makes the blood gel, and used it as a test to ensure the vaccines or drugs produced are not contaminated with endotoxins. Even though it was claimed that care is taken while harvesting the blood from live specimens, there will always be a certain percentage of casualities. A good news is that our own researchers at NUS has achieved “a bioengineering feat in successfully cloning the enzyme that clots the blood of the horseshoe crab, making it one of nature’s most sensitive sensors for toxic contaminants.” Certainly a great news since now we won’t have to traumatise the Horseshoe Crabs then! 😀

After the ‘workout’ at the mudflat, I found myself having muscles aches…Had been so busy rushing deadlines that I missed my routine exercise for more than a month…But I think the effort was worth it. Horseshoe Crabs certainly are amazing creatures. 8) To learn more about this ‘old antique’, do visit this very informative site! 😀

Also see related post:
> Between the Tides – Semakau Intertidal Walk


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