Horseshoe Crabs of Kranji

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

Friday, June 11, 2004

A finale

Visit my website: Fiona's Mangrove Haven for the full report and other information.

Despite the bloopers throughout this study, I did not fail this module. So it is true that UROPs is a good place for beginners like me to make a headstart into research, whether it is to prepare ourselves for honours or masters, or just to find out if research is the field for us.

I have learnt tremendously through this project, especially from Siva and Yenling, and of course the horseshoe crabs. And so here I'll like to highly recommend UROPs to NUS science students who would like to give research a try. If you would like to know more about personal experiences whatsoever, please feel free to email me at my singnet account: hong0655

In the mean time, get muddy and wet!

Where are the eggs stored?

A few days ago, I chanced upon a Taiwanese documentary on cable (channel 52 if I'm not wrong). Unfortunately, I only caught the last 15 minutes.

The documentary was on the captive breeding of Tachypleus (not entirely sure of species; probably T. tridentatus). Males and females were caught and checked regularly for presence of gravid in the females. The interesting thing that I saw, was that the eggs are actually located in a sort of storage sac near the book gills and a gentle squeeze at the opening releases the eggs. Previously, I thought that gravid was stored near the anterior of the prosoma/shell.

This may mean that females I thought to be 'non-gravid' may posibly have been fertile and eeven ready for spawning! This is definitely a question to be answered in future field trips.

During the last sampling trip on 25th March 2004, I encountered this dead female, with eggs seemingly spilling out from around its book gills.



Back to the documentary - the scientists brought the males and gravid females to the beach and released them (temporarily) for pairing and spawning. After that the horseshoe crabs were picked up again and returned to the lab.

A Brief Recap

After the last sampling trip on the 25th of March, both Yenling and I began working on our final report to be submitted to the department. The sections in the report, like most scientific journals include a short Abstract, Introduction, Methodology, Results, Discussion, Conclusion and of course Appendix. It was also during that time, that I realised I had more loose ends to tie than I expected. Loopholes in results, missing data, and improper sampling techniques made the analysis of results tedious.

Eventually, the conclusions that could be made from my study are:
1. Juveniles (<60mm) can be found in high numbers in specific micro-habitats (flat mudbanks of streams imperforated with small pools and shallow streamlets).
2. Horseshoe crabs >60mm may bury themselves fully in mud
3. January to March is either not the mating season, or there is no season at all.
4. Mudflats are an important area to look into next due to presence of fully buried horseshoe crabs

The night before the submission of the report, all three of us (Yenling, Siva and me) skipped our night's rest to rush out it out. On top of the full report, we also had to submit a 4 page abstract for the department's record. To add on to the stress, I also had a language oral exam on the day of submission. It is definitely an experience not to be forgotten.

Sunday, March 28, 2004

Stress... ...

At the last stage of writing the report, which is dued on 2nd April.
Still have many lose ends to tie and lots of write-ups to complete.
Stress... ...

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