Distribution and diet of Carcinoscorpius rotundicauda in Sungei Mandai ,

Germaine Leng & N. Sivasothi Department of Biological Sciences National University of Singapore Where is Singapore? 23 km (14 miles )

Area : 712 km2 (275 sq. miles) 1/3 Tokyo or ½ New York City 41 km (25 miles ) Horseshoe in Singapore

• Coastal horseshoe , • Mangrove , Carcinoscorpius rotundicauda • IUCN Red List: DD • IUCN Red List: DD • Singapore Red List: • Singapore Red List: Endangered Vulnerable Sheltered shores of the northern coast Wetlands Reserve Unprotected Mandai mangrove and mudflat

Mandai mangrove and mudflat Mandai mangrove and mudflat Mandai mangrove and mudflat Tidal streams in Mandai mangrove Microhabitats of a tidal stream

Mud Sand Seagrass

Stream Streamlets • > 15 cm wide • Width < 15cm •Clear demarcation of •Unclear demarcation of bank bank • Steady flow of water •Trickle of water Objectives

1. What are the population characteristics of C. rotundicauda in the tidal stream?

2. How is C. rotundicauda distributed across microhabitats in the tidal stream? Methods • Surveys: – 10 surveys between Dec 2012 to Feb 2013 – Every quarter moon phase during day low – Area of tidal stream = 4,011 m2

Recorded microhabitat (single recorder)

Located (with field assitants) Measurements (single recorder) Population size in the tidal stream

• Population mean was 272 ± 89.7 individuals • 0.068 individuals per m2 (or 6.8 / 100 m2) • Weekly counts ranged: 133 to 398 individuals Population structure – Size range Population structure • 10.2mm – 160.2mm in tidal stream – Males • 51 ± 18.9 (Range 29 – 88) – Females • 31 ± 15.1 (Range: 17 – 67) 19% male – Juveniles 12% female • 186 ± 76.8 (83 – 287) juvenile 69% – Mating pairs • 30 ± 18.9 (2 – 32) Population size in the tidal stream

1. Population density (0.068/m2) much lower than mudflat? • Mudflat population nearby = 0.5 individuals per m2 (Cartwright-Taylor et al., 2009) • Few figures are available to evaluate site quality; standard sampling method would be helpful

2. Weekly population variance • Tidal stream population is mobile/oscillates with mudflat? • In tidal streams, one-off surveys may not reflect population number Objectives

2. How is C. rotundicauda distributed across microhabitats in the tidal streams? Distribution of C. rotundicauda across microhabitats

600

500

400

No. of C. rotundicauda 300 adults juveniles

200

100

0 Mud Stream Streamlets Seagrass Sand

• C. rotundicauda not evenly distributed in tidal stream microhabitats Distribution of C. rotundicauda in microhabitats 1. Sand is least preferred; adults cluster in mud (species preference)

2. Juvenile associated with streamlets – Require hydration during low ?

3. Heterogeneity of tidal stream habitats important – supports use by different growth stages of C. rotundicauda

1 Morton & Lee, 2011

2. Diet of the mangrove horseshoe crab C. rotundicauda at Sungei Mandai Kechil mangrove (tidal stream and mudflat) Diet of C. rotundicauda

• Only two studies in the literature – Two juveniles in HK (Zhou and Morton, 2004) – 42 adults in (John et al., 2012) Objectives

1. What is the diet of C. rotundicauda?

2. Does C. rotundicauda feed selectively? Methods • Weekly collection of adults and juveniles – Sept 2013 to Feb 2014 (five months) – Three individuals per collection: 1 male, 1 female and 1 juvenile (total = 64) – Dissected and sorted gut content

• Infauna sampling for potential prey – 315 cm3 of sediment – n = 18 – Sieved through 500 µm mesh

• Calculation of selective feeding – Strauss’ Linear Food Selection Index (used in several HSC diet studies) What did gut content analysis reveal?

• 80% was substrate • 20% was identifiable – 91.4% was of origin – 8.6% was plant matter What were Frequently occurring (FO)

and relatively abundant (Ri) items? 1 2

FO: 87.7% FO: 29.8%

1mm Ri: 65.1% 1mm Ri: 2.0%

4 3

2mm

FO: 63.2% FO: 33.3% 0.5 mm 2 mm R : 8.2% Ri: NIL i Does C. rotundicauda feed selectively?

• Strauss’ Linear Food Selection Index – relative abundance in gut to relative abundance in sediment

• > 0 indicates selection • 0 indicates random feeding • < 0 indicates avoidance Does C. rotundicauda feed selectively? 1

0.8 Polychaeta

0.6

Preference 0.4 Chironomidae 0.2 Ostracoda Plants

0

Selective Selective index -0.2 Gastropoda

-0.4 Nematoda

Avoidenc e -0.6 Adult Juvenile -0.8

-1 C. rotundicauda is a subsurface, deposit-feeding vermivore? 1. C. rotundicauda consumes substrate (80%) + mainly polychaetes – T. gigas: substrate (5 - 50%) + molluscs & polychaetes1 – L. polyphemus: substrate4 + bivalves2, 3

2. Sediment as food source? – Year round availability – Low organic content compensated with high intake

3. Need for selectivity – Sorting allows higher efficiency – Substrate selection? 1 Chatterji, 1992 2 Botton, 1984; 3Botton & Loveland, 1989 4Botton et al., 2003 Selection for polychaetes

1. Polychaetes are highly profitable prey – High flesh weight – Not protected by shells – Fairly common; abundant in some areas

2. Polychaetes are a prey item in other studies – C. rotundicauda in Malaysia1 2 3 – L. polyphemus (FO: <3%) and T. gigas (Ri: 5.3%) 1 John et al., 2012 2 Botton & Rope, 1984 3 Chatterji, 1992 Avoidance

1. Nematodes – Hard protective cuticle; difficult to process – C. rotundicauda and T. tridentatus juveniles avoided nematodes in 1

2. Bivalves and gastropods – Selected for by T. gigas2 and L. polyphemus3 – Long handling time for C. rotundicauda, not energetically feasible? 1 Zhou & Morton, 2004 2 Chatterji, 1992 3 Botton, 1984 Crustacea: an important component of diet?

• Digested beyond enumeration and identification • May be amphipods/tanids? 0.5 mm

• High FO: 63.2% – C. rotundicauda and T. tridentatus juveniles in Hong Kong (FO: 33.3%)1 – L. polyphemus (FO: 3.3%; 16.7%) 2,3

1 Zhou & Morton, 2004 2 mm 2 Botton, 1984 3 Botton &Loveland, 1989 Application

This resumption of horseshoe crab studies prompted by the last meeting is actively contributing to conservation effort for Mandai Mangrove & Mudflat.

Further research is required...

Thank you