BULLETIN OF MARINE SCIENCE, 63(1): 11–20, 1998 POPULATION CHARACTERISTICS OF THE MOLE CRAB, HIPPA ADACTYLA FABRICIUS, IN THE INTERTIDAL SEDIMENT AT KAVARATTI ATOLL, LAKSHADWEEP ISLANDS B. S. Ingole, R. A. Sreepada, Z. A. Ansari and A. H. Parulekar ABSTRACT Some population characteristics of a little known mole crab Hippa adactyla Fabricius from the sandy intertidal habitat of Kavaratti atoll, Lakshadweep islands, were studied for understanding the resource potentials besides some features of breeding behavior. The study is based on the detailed examination of 1140 specimens collected from the intertidal beach during November 1993. The specimens were in the size range of 8.84– 22.56 mm carapace length (CL). The population of mole crab consisted of three modal size classes, dominated by the newly recruited or the ‘0’ year individuals. The size at maturity of female crabs (estimated from the smallest egg bearing females) was 10.14 mm CL. The egg bearing population constituted more than 60% of the total observed specimens of mole crab, and over 80% of the ‘I’ and ‘II’ year population carried eggs. Mean fecundity was estimated to be 1628.61 ± 853.13 eggs/female/clutch, with a large variation in the total number of eggs per female. Regression analysis of fecundity with length and weight (wet weight) revealed that fecundity correlated better with length (r = 0.86) than weight (r = 0.82). However, the differences between regression values were insignificant. This mole crab species possess great potential for commercial exploitation especially in oceanic environment, like Lakshadweep islands, where food resources are limited. The information on the population parameters provided here therefore would be useful for sustainable exploitation. Anomuran mole crabs belonging to the super-family Hippidae are widely distributed on the intertidal sandy beaches along the west coast of India (Patil, 1951; Sankolli, 1965; Ansell et al., 1972; Nagabhushanam and Chandrasekhara Rao, 1972; Murugan and Balakrishnan Nair, 1990). They are an important group of intertidal sand fauna and con- tribute significantly to the macrofaunal standing stocks. The most common species found is Emerita holthuisi, followed by E. asiatica (= E. emerita), Hippa adactyla, H. pacifica and H. indica. Ever since Emerita holthuisi was described from the intertidal area of Ratnagiri along the central west coast of India, by Sankolli (1965), it has received much attention (Ansell et al., 1972; Achuthankutty and Wafar, 1976; Nagabhushanam and Kulkarni, 1977). However, as compared to Emerita, very little is known about Hippa adactyla, a species known to occur in greater densities at Phuket island, Thailand (Janekorn, 1982). In India, H. adactyla is reported from the intertidal beach at Madras (Gravely, 1941) and from the Lakshadweep island (Narayanan and Sivadas, 1986). Despite the wider distributional range and prominence in the macrobenthic assemblage of intertidal fauna (Ansari et al., 1990) little is known about the population biology of H. adactyla from the Indian waters. Hippa adactyla is locally known as “Ourudom” at Kavaratti atoll, Lakshadweep is- lands, where it has been reported as being the dominant macrobenthic burrowing species (Narayanan and Sivadas, 1986); thus, ecologically it can play a very significant role in sediment reworking and nutrient recycling. In addition, the crab population is exploited a little for its flesh, and it is also traditionally believed to possess some medicinal properties 11 12 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 1, 1998 Figure 1. Location of the study area. Table 1. Environmental parameters recorded at Kavaratti beach during Hippa adactyla sampling. Temperature (° C) Air 31.5 Water 31.0 Sediment 30.3 Lagoon surface water Salinity (PSU) 35.3 pH 8.2 Dissolved Oxygen (ml l−1) 3.25 Chlorophyll a (mg m−3) 0.35 Sediment texture (%) Pebbles (> 4 mm) 1.30 Coarse sand (1–0.25 mm) 95.30 Fine sand (0.25–0.062 mm) 3.20 Silt & clay (< 0.062 mm) 0.2 Organic carbon (%) 0.26 INGOLE ET AL.: POPULATION STRUCTURE OF MOLE CRAB HIPPA ADACTYLA 13 Figure 2. Size-frequency distribution of H. adactyla, from Kavaratti beach, Lakshadweep islands (curve fitted by eye and represent an expected size distribution) (personal discussions with islanders). Similarly, Emerita emeritus is harvested for food at Phuket island, Thailand (Janekorn, 1982). This underlines the potential of H. adactyla for commercial utilization. Considering the ecological and socio-economic importance of this species, the present study was undertaken to provide basic information on population structure of H. adactyla on an intertidal habitat at Kavaratti atoll. MATERIALS AND METHODS STUDY SITE:—Kavaratti is an atoll of the Lakshadweep group, situated some 230 km off the Kerala coast of India (Fig.1) and located along 10°33'N and 72°38'E, with an island on the east and lagoon on the west. The Lakshadweep group of islands developed on the Chagos Lakshadweep ridge during several sea-level changes that caused colonization of various coral communities be- longing to different families. Bordering the western margin of the lagoon is a coral reef with a width of about 250 to 300 m except at the south-west point, where the width is more than 400 m. In general, lagoon depth varies from 1.5 to 1.8 m at low water and 2.4 to 3.6 m at high-water. The lagoon shore of the island consists of a sandy beach, though at some places the beach rocks are exposed during low tide. The south eastern seaward shore of the island is marked by storm beaches consisting of coral pebbles and boulders piled up well above the high tide mark. The la- 14 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 1, 1998 Figure 3. Relationship between body weight (wet weight) and carapace length (a), and its logarithmic form (b) of H. adactyla. goon side of the beach slopes from about low water neap tide has a luxuriant growth of macro- phytes. At the ebb, a seagrass bed becomes partially exposed and often decayed pieces of blades accumulate on the beach. Topographically the intertidal beach at Kavaratti is almost flat and composed of coarse to fine coralline sand. The sediment in the lagoons and surrounding area is biogenic in nature with no terrigenous contribution. SAMPLING.—Sampling for the mole crab population was conducted on the exposed sandy beaches around the Kavaratti atoll during 3–5 November 1993. All the sampling stations were located in the mid tide level. A metallic quadrant (625 cm2 area; penetration depth, 10 cm) was forced into the sediment; sand was scooped and sieved through a 1 mm sieve. All the specimens of mole crab were sorted alive as egg bearing and non-egg bearing and then preserved separately in 5% neutralized formalin solution and later transferred to 70% alcohol. Environmental parameters such as tempera- ture (air, water and sediment), salinity, dissolved oxygen (DO), and pH were also recorded. Sedi- ment samples were collected from randomly selected sites for sediment texture analysis and or- ganic carbon content. INGOLE ET AL.: POPULATION STRUCTURE OF MOLE CRAB HIPPA ADACTYLA 15 Table 2. Contribution of each size class to the breeding population of Hippa adactyla. Year Sfize class NumberoOgvigerousfemalesFemalepopulationtakin ssalc specimens partinbreeding(%) N)umbers(% 082.1–13.1419182198. 24.5 1193.2–19.1318341458. 79.9 2199.2–23.1295280392. 80.6 LABORATORY ANALYSIS AND DATA PROCESSING.—The total carapace length (from tip of the rostrum to the posterior margin of the cardiac region,) and carapace width was measured using a Vernier calipers to the nearest of 0.1 mm. Wet weight of the specimen was obtained on a single pan electric balance up to 0.1 mg (Mettler AE 160). All the eggs of the ovigerous specimens were removed carefully and counted to evaluate the individual fecundity. Mean weight of an individual egg was calculated taking the dry weight of 100 eggs. All length measurements were sorted into 1 mm size classees to construct the body-size histo- gram. Different recruitment classes and year classes were separated from length-frequency polymodal Figure 4. The asymptotic length (L∝) of H. adactyla. 16 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 1, 1998 Figure 5. Regression of egg number per female on carapace length. groups. Regression analysis (Pearson correlation coefficient) was carried to understand the rela- tionship between brood variable and female carapace and total length. ESTIMATION OF THE SIZE AT SEXUAL MATURITY FROM THE LENGTH-WEIGHT DATA.—Length-weight data were repetitively divided into two size-delimited subsets, one for animals with a total length less than and the other for animals with total length greater than a hypothesized transition point (Lovett and Felder, 1989). A separate regression function was calculated for each of the two subsets after each relocation of the hypothesized transition point. Such calculations were performed repeti- tively between the hypothesized transition points with sequential relocation at 1.0 mm intervals. The size at which the relative growth rate changed most precipitously was defined as the total length value (transition point) at which the regression function for the two subsets best fit the combined data set. Trisection point was selected theoretically on the basis of highest correlation coefficient (r) value. RESULTS PHYSICO-CHEMICAL PARAMETERS.—The islands of the Lakshadweep group has a tropical climate and the present study period falls within the post-monsoon season (October– January) (Table 1). Weather conditions during the sampling period were relatively calm. The salinity, temperature, pH, and DO did not vary much and were comparable to the earlier studies of Narayanan and Sivadas (1986) and Ansari et al. (1990) for the same area and during comparable season (Table 1). Sediment consisted of coarse to fine coralline sand with a mixture of pebbles, molluscan shell pieces and silt clay.