Horseshoe Crabs of Kranji

The trials and tribulations of an undergraduate research project on mangrove horseshoe crabs.

Monday, February 23, 2004

18th February Wednesday (Yenling and me)

Worked from 2.45pm to 4.45pm at the KR4 West stream (check out location on map)



- found 10 adults altogether out of which are 3 mating pairs and 2 freshly moulted
- freshly moulted ones found in very discrete places among roots. I had to literally dig them out.
- based on smallest mating male so far, 89mm is an adult for male horseshoe crabs
- 60mm - 80mm juveniles seem to prefer slightly moist moderate to soft mud, an intermediate between the streamlets for juveniles, and drier mud for adults (but all speculations still)

I got bitten by a pea-sized green insect several times nearer the end of the stream and it seemed to be attacking me on purpose. The bites are painful and don't leave any swelling or scars. But it was enough to chase me out of the site.

Stock Check! Equipment borrowed so far

From Ecolab:

- 2 pairs of small size booties
- Refractometer
- 2 compasses
- 2 fifty metres measuring tape + 2 hundred metres measuring tape
- Several nalgene and plastic bottles
- 1 measuring tape (the kind you use to measure waistline)
- 2 small fish tanks

From Siva:

- green bag and everything inside (first aid kit, flagging tape, white spray paint)

Monday, February 16, 2004

15th February Sunday - Wendy and Me

On Sunday, dearest Wendy accompanied me to the mangroves as Yenling had to return to Malaysia. We continued from where I last stopped at the KR4 East stream last friday and completed the search up to where we stopped mapping.

Tough luck at first and we only found a number of large moulted shells. Perhaps some warming up is needed before we can spot them, that's why every field trip until now had seemed to be less eventful at the beginning. The method I use is basically to scan the site and touch any suspicious bumps and/or scrap the surface layer of mud to expose the hard carapace.

Nearer the end of the search, I actually found a new way to locate the horseshoe crabs. By using a stick (can be thin as long as it doesn't bend or break too easily) to poke into the mud, camouflaged or fully buried horseshoe crabs can be found. It is also safer and faster than to use fingers. And most importantly, it causes less damage to the substrate than if I were to excavate the mud. I found 2 adults in half an hour using this method. Both of them I did not manage to spot by pure sight.

Just before we left, we found a mating pair in the shallow stream. This could mean that they don't only mate during high tides, like their cousins (other species of horseshoe crabs) do.

Lesson Learnt: DO NOT carry lose objects, ensure that everything should be attached to some fixed part of the body at any one time.

Saturday, February 14, 2004

Friday the 13th with shrimps and horseshoe crabs

Last friday, Siva, Leng and I went for our fieldtrip in the morning for the first time throughout this project. Needless to say the morning traffic caused delays and we managed to get there only at 8.30am. I have lessons at 12 that day so there was a bit of a rush to quickly finish what we were set for.

Objective: To find adult horseshoe crabs
Before friday, my findings have been primarily juveniles and I had only spotted 2 adults in different areas. Thus it was high time I locate the adult horseshoe crabs since my hypothesis does deal with a comparison in distribution between the two size classes

Once at the water edge below the bridge, I noticed a dead horseshoe crab and a mating pair next to it! The male clasped on to its mate tightly while i tried to do the prosoma measurements. The poor guy eventually had to let go. After finishing my business, I tried to put them back in the same spot, with the male on top of the female, but they just swam off separately.



After some time spent searching at the mudflats further out to the sea, I realised the mating pair I saw was plain luck. My search image was bad and I don't really know where exactly to look.

After what seemed like half an hour, I returned to the main stream. The visibility of the water is bad and feeling the bottom of the stream with my hands seem in vain as it does not cover enough area. I then decided to look at the banks and WALA!, I see one, and then another half buried on the muddy slopes.

The rest of the search earned a couple more adults and some 60mm-70mm juveniles. I ignored recording juveniles smaller than about 60mm. It would take up all the time I had because they were everywhere (note: only in suitable sub-habitats).

At 10.30am sharp, my stomach growled an alarm to pack up and go.

Sunday, February 08, 2004

Field trip 5th February-Wendy, Yenling(leng) and me

On 5th feb, we worked only from 3pm to 4.30pm. We had a short hour and a half of light drizzle before the pouring started again (was raining when we were on the way to sungei buloh) and forced a stop to whatever we were doing.

Wendy helped me and we sampled at the channel between KR4 and buloh east, where a streamlet runs through and the mud is relatively soft.

I had originally wanted to sample around the junction of the main streams near the bridge but the tide was still high, and the earlier rain probably increased the surface runoff as more water from inland rushed downstream into the sea.

Despite the short duration we worked, we managed to record 40 juvenile sightings, and most importantly, I got to try out the datasheet.

Lessons learnt and other things to work on:
1. Need to standardize the naming of the sites both me and yenling are working on
2. When sampling, must have at least one person with clean hands to record, and the other doing the dirty work
3. Bring umbrellas if in suspect of rain (not for ourselves, but for the data and equipment)
4. The bus at Kranji reservoir carpark comes only every 30 minutes

Thursday, February 05, 2004

4th Feb Field trip - Grace, Leng and me (3pm to 6pm)

By chronological order:

1. Took low tide photos from bridge

2. Sketched map of mangroves west of bund with emphasis of streams, but...
- streams branch out extensively from sea to landwards and I had problems drawing the streams with relation to the other streams [result: did not finish the whole area due to confused mind and orientation]
* Sketched till I saw the gazebo that has the painted tiles (from commonwealth secondary)

3. Moved back to KR4 stream at bridge 2 and headed out to the mudflats seaward - saw (not sampling) an adult horseshoe crab (110mm) half buried in soft mud (that means I'll have to search in that area on all fours with a rake some time soon )
- saw moving juvenile (~30mm) which left a distinct trail behind it.



- it was a beautiful feel out in the mudflats so much so that I exchanged precious field work time for a few moments of fresh air and pleasant scenery

4. Retreat back to junction where KR4 stream branches into KR4 East and KR4 West
- presence of flats with soft mud on the banks exposed due to low tide
- found several juveniles in mud(sufficiently moist) and in small streamlets that run through the flats


Other Stuff:
- Learnt from taxi uncle that we should not use the public phone at the visitor centre to call for cab. The handphone leaves a contactable number behind in case the driver needs to ask for directions
- Call 6552 1111 for comfort cabs first (they got the most no. of cabs) then try others like 6552 2222 which is for citycab (but almost always need to wait very long for plate no.).
- Saw a huge beautiful fish in the KR4 East stream probably trapped due to the receding tide. Grace took photos

Tuesday, February 03, 2004

Field trip 2nd February

On the 2nd of Feb, Leng and I together with the companion of experts Prof Ng, Yixiong and Siva and volunteer Huaqin, went to our working site to learn how to search for our animals. Yixiong and Huaqin helped Leng with her shrimps while Siva helped me with the horseshoe crabs. Prof went around with his son, Paul and all of us moved into the mangroves west of the Kranji Nature Trail bund.

I was told that the horseshoe crabs can be found usually in streamlets and thus the search began. Initially, I could not establish the search image yet and missed a few that were spotted by Prof and Siva. After some adjustments and getting-used-to, I started picking them out. Only the maximum width of the prosoma was measured (using measuring tape and recorded in mm) that day.
Siva sketches the area, labelled the streams and recorded the widths which i call out to him. Moulted skins were also measured and indicated. He also marks the approximate location on his sketch where each horseshoe crab was found.
Soon I noticed a pattern about the substrate and the depth of water which the horsecrabs prefer. Together with the prompt from Siva, I began to work faster.

All in all about 45-50 juvenile horseshoe crabs were found with body widths measuring between 6mm to 65mm. One dead adult was also found and measured 130mm.

From this trip, the lessons learnt are:
1. Bring only essential equipment during mangrove fieldtrips so as to reduce weight and bulk as much as possible
2. Juveniles like very shallow streams and silty substrate which is not too coarse nor too soft. (This hypothesis can be further worked on later)
3. There is a slight relation between size of juvenile and size of stream; Larger horseshoe crabs need larger streams (deduced by Siva)
4. Just counting or quantifying is not enough in science. A measurement or qualification of the specimens can tell more and initiate related questions that further develop the understanding of the ecology.
5. However do not be too overtaken by measurements as it might slow down the procedure a lot.
6. There should be a data sheet for each stream worked on for effective recording and easier reference later.


In the subsequent trip, I would have to map this area with emphasis on the streams before I can begin my own sampling.

Would like to thank Prof Ng, Yixiong, Siva and of course Huaqin for their indispensable help. Leng and I have benefitted a lot from this fieldtrip.