Population Structure and Breeding Pattern of the Mangrove Horseshoe Crab Carcinoscorpius Rotundicauda in Singapore
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Vol. 8: 61–69, 2009 AQUATIC BIOLOGY Published online December 29 doi: 10.3354/ab00206 Aquat Biol OPENPEN ACCESSCCESS Population structure and breeding pattern of the mangrove horseshoe crab Carcinoscorpius rotundicauda in Singapore Lesley Cartwright-Taylor*, Julian Lee, Chia Chi Hsu Nature Society of Singapore (NSS), 510 Geylang Road, #02-05 The Sunflower, Singapore 389466 ABSTRACT: The first year-long survey of the mangrove horseshoe crab Carcinoscorpius rotundi- cauda was conducted at the Mandai mudflats at Kranji in Singapore to determine if breeding is year round or seasonal and to provide qualitative and quantitative baseline data to monitor the health of the population. At spring tide from September 2007 to July 2008, volunteers collected horseshoe crabs along the exposed mudflats as the tide receded. The carapace width was measured, and the sex and breeding status of each individual were determined. The proportion of juveniles in different size groups varied in each month. In November and January, 25 and 30%, respectively, were 2 to 3 cm in width, while in June and July, 8 and 4%, respectively, were in this size group. The size cohorts showed recruitment to the smallest size classes from November to March and recruitment to the larger size classes from March to July. Juveniles less than 2 cm were not found in June, suggesting that there may be a rest period of low or no breeding activity from May to July resulting in none of the smallest sizes mid-year. Ratios of males to females varied from 0.85 to 1.78 throughout the year, and although pairs in amplexus were found year round, no spawning activity was seen. Males mature at about 8 cm while females may mature later at 10 cm. A mean of 33% growth during ecdysis in juveniles was observed, suggesting that juveniles of approximately 2 cm may moult about 5 more times before maturity. KEY WORDS: Mangrove horseshoe crabs . Carcinoscorpius rotundicauda . Breeding patterns . Population structure . Growth . Sex ratio Resale or republication not permitted without written consent of the publisher INTRODUCTION been well described by Mikkelsen (1988), but his book was based on work carried out before the late 1980s. Horseshoe crabs have changed very little over 500 Since then, land reclamation and coastal development million years and have survived 2 mass extinctions or degradation have destroyed much of the habitat of (Stormer 1952), but now, according to the IUCN Red horseshoe crabs (Savage 2001, Ng & Sivasothi 2002, Data Book, one species is near threatened (Limulus Mishra 2009). Moreover, the biology, ecology and polyphemus), and data on the other three are deficient breeding patterns of C. rotundicauda have not been as (IUCN 2008). well documented as those of the North American spe- One of the 4 species of horseshoe crab, the mangrove cies Limulus polyphemus (Linnaeus, 1758). According horseshoe crab Carcinoscorpius rotundicauda (Latreille, to Mikkelsen (1988), the breeding cycle of C. rotundi- 1802), occurs only in Asia: around India, Indonesia, cauda in Thailand is thought to be continuous, whereas Malaysia, Philippines, Singapore, Thailand and Hong in Borneo it is thought to occur from March to July, but Kong (Lee & Morton 2005, IUCN 2008). The Singapore there are no quantitative data to support this. For other Red Data book released in November 2008 classifies parts of Southeast Asia, including Singapore and this species as vulnerable (Davidson et al. 2008). The Malaysia, there is a paucity of information, and it is still global distribution of all species of horseshoe crab has not clear if this tropical species has a seasonal breed- *Email: cartwrighttaylor@gmail.com © Inter-Research 2009 · www.int-res.com 62 Aquat Biol 8: 61–69, 2009 ing pattern, as occurs with L. polyphemus, along the The aim of this project was to provide qualitative and temperate shores of USA, or if breeding occurs year quantitative baseline data over a period of 1 yr to round. Abundance, population structure and spawning determine if breeding is year round or seasonal. From patterns of the Atlantic horseshoe crab off the east these data, the structure and stability of the population coast of the USA have been well studied (Botton et al. can be monitored over time. 2003, Carmichael et al. 2003, James-Pirri et al. 2005, Leschen et al. 2006), but very little has been published on the spawning activity of C. rotundicauda. The mas- MATERIALS AND METHODS sive seasonal migration up the beach for spawning that is a well known part of the breeding pattern of Study site. The study site is on the north coast of the L. polyphemus in the USA has never been described main island of Singapore at 1° 26’ N and 103° 45’ E, at for C. rotundicauda. Kranji. About 2 km to the east is the stone causeway There is little recent information on abundance, sex across the Strait to Johor Baru in Malaysia. There is ratios, population structure or densities of Carcinoscor- flow of the sea through the strait, but flushing is pius rotundicauda, or whether numbers are stable or de- restricted to some extent by the causeway. The Strait of creasing. A 2008 report in Marine News (www.merinews. Johor is an estuary, and salinities vary depending upon com/article/horseshoe-crabs-galloping-towards-extinction/ the state of tide, weather and terrestrial runoff. The 136265.shtml) indicated that 2 species, C. rotundi- salinity in the strait typically exhibits 2 high levels, in cauda and Tachypleus gigas (Müller, 1785), are being March/April and October/November, and 2 low levels, poached in their thousands by children engaged by local in January and June/July (Ng & Sivasothi 2002), fishermen to collect the crabs, which are then sold to depending upon the state of tide, weather and terres- pharmaceutical companies. In addition, harvesting for trial runoff. commercial purposes is considered a serious threat to the The tidal amplitude varies from about 3 m during the survival of these species in Malaysia (Christianus & Saad spring tides at the full moon to about 1 m at the neap 2007). With the increasing interest in C. rotundicauda for tides. The distance, as measured for the study, from the medical research (Ding et al. 2005, Ng et al. 2007) and high tide to the lowest tide mark is about 200 m. The the additional threats of habitat loss due to coastal intertidal zone is almost flat, has no mangroves, and development in Southeast Asia, such information is the mud is very sticky, at least ankle deep in most urgently needed to assess the ecological requirements parts. Part of the area is bounded by a sea wall. The and conservation status of this species and to manage search area was approximately 26 680 m2, 115 m from and conserve it. Loss of habitat is listed as one of the 2 the high to low tide area by 232 m along the shore line, main threats to horseshoe crabs in Singapore (Davidson with a firm man-made earth bund or quay midway et al. 2008); the other is pollution. along the shore stretching out to sea. This bund is In Singapore, the Mandai mudflats at Kranji on the almost level with the mudflats, so it is exposed at low north coast of the main island are the largest mudflats re- tide but covered as the tide comes in. The site, particu- maining in the country and are the habitat of a population larly near the high tide area, is strewn with various of Carcinoscorpius rotundicauda. The Nature Society of types of debris at low tide, including large lumps of Singapore (NSS) has been organising volunteers to visit concrete, driftwood, logs and sticks, and pieces of this site every 3 to 4 mo to rescue horseshoe crabs that metal or plastic, and it is here in the intertidal zone that have become entangled in deployed or abandoned fish- the mangrove horseshoe crab Carcinoscorpius rotundi- ing nets. In 2007, the volunteers also started collecting de- cauda can be found. mographic information on the horseshoe crabs found dur- Search times and dates. The tide heights were the ing this operation. These unpublished data showed a overriding factor in selecting search times and dates. large proportion of small juveniles in March with very few The searches required uncovered mudflats, so the seen in June, suggesting some seasonal pattern to spawn- spring tides were chosen for the search days, which ing, contrary to opinion in Singapore and Malaysia (R. all occurred within 3 d of full moon. To determine Tan, A. Christianus pers. comm.; www.wildsingapore. population structure over time, searches were made com/wildfacts/arthropoda/limulidae/limulidae.htm). Be- over the whole of the study area from September 2007 ginning in September 2007, more regular visits were through to July 2008, except for December 2007 and made to the site at the spring tide each month for 1 yr, and February 2008, when the optimal search times fell on data on C. rotundicauda were collected more systemati- public holidays, and August 2008, when the optimal cally to determine breeding patterns and provide infor- tide time was after dark. As trained volunteers were mation on population structure. This is the first systematic, recruited to provide sufficient manpower, searches quantitative survey over a period of 1 yr on a population of were all done at weekends, which meant that the this species anywhere in Asia. searches were not always conducted on exactly the Cartwright-Taylor et al.: Breeding pattern of Carcinoscorpius rotundicauda 63 same day of the lunar cycle. Visits to the site were adult horseshoe crabs, and the females were distin- timed to start when the receding tide height was about guished from juveniles by their size alone.