BULLETIN OF MARINE SCIENCE, 41(2): 214-220,1987 XANTHID CRABS IN THE CORALS, POCILLOPORA DAMICORNIS AND P. VERRUCOSA OF SOUTHERN TAIWAN Kun-Hsiung Chang, Yu-Shan Chen and Chang-Po Chen ABSTRACT Thirty-six species of xanthid crabs were collected from two coral species; of these, 356 individuals of 25 species and 444 indi viduals of 21 species were from Poci/lopora damicornis and P. verrucosa, respectively. Trapezia formosa, T. cymodoce, T. septata, T. digitalis, T. garthi, T. ferruginea, and Pilodius sp. a were abundant in both corals, but T. cheni was abundant in the coral P. verrucosa collected from the low-tide area off Orchid Island. Trapezia digitalis, T. formosa, and T. garthi were confined to the low-tide corals. About 30 to 75% of the adults of T. cymodoce, T. ferruginea and T. septata existed singly or formed a single pair in a given coral head in both corals collected from the subtidal areas, but it never occurred in corals collected from the low-tide areas. The carapace width of females was significantly larger than that of paired males in most of Trapezia spp. The carapace width of the Trapezia collected from low-tide corals is smaller than that collected from subtidal corals. The pocilloporid reef corals harbor a high diversity of macro-invertebrate com- munities (Patton, 1974; 1976; Abele and Patton, 1976; Coles, 1980; Edwards and Emberton, 1980; Black and Prince, 1983). The crabs of the family Xanthidae often play an important role in these communities. For example, the crabs com- posed 40% of individuals associated with Pocil!opora damicornis at Heron Island, Australia (Patton, 1974). Two crab genera Trapezia and Tetralia of the family Xanthidae are believed to be obligately host-specific to Pocilloporidae corals and Acroporidae corals, respectively (Garth, 1964; Knudsen, 1967; Castro, 1976; Patton, 1976). Coral reefs are abundant in Taiwan, particularly in the south (Randall and Cheng, 1977; 1979). A general survey of the crustaceans associated with the branching corals has been conducted in the waters of Taiwan (Chen, 1983; Soong and Chang, 1983; Jeng and Chang, 1985). Two new species of Trapezia, i.e., T. cheni and T. garthi, has been reported from Taiwan (Galil, 1983). Here we report on the xanthid crabs associated with the pocilloporid reef corals of southern Taiwan. MATERIALS AND METHODS Forty-two coral heads of Pocillopora damicornis were collected in the low-tide zone down to a depth of 7 m from Pen-ghu (Pescadores Is.), Hsiao-Liuchiu, and Nan-Wan (Fig. I). The corals collected from the subtidal areas belong to the variety of P. d. bulbosa or P. d. caespitosa. These are typical varieties of semi protected and well-protected biotopes (Veron and Pichon, 1976). Fifty coral heads of Pocillopora verrucosa were collected in the low-tide zone off Orchid Island and at 4-10-m depths off Nan- Wan. The coral heads were wrapped individually with plastic bags in situ, broken from the substrate with a rock hammer, brought back and then fragmented. The xanthid crabs were collected and preserved and their carapace width were measured. The xanthid crabs collected were identified under the consultancy of Drs. Garth and Galil. The taxonomic status of Trapezia septata is used after Galil and Lewinsohn (1985). RESULTS Species Composition and Abundance. - Table 1 lists the species and number of individuals of the xanthid crabs collected from these two coral species. The 356 214 CHANG ET AL.: XANTHID CRABS IN POCILLOPORID CORALS OF TAIWAN 215 Figure 1. The collecting sites. individuals collected from the coral P. damicornis were made up of 25 species, while the 444 individuals collected from P. verrucosa contained 21 species. About 90% of the crabs collected were from seven or eight species in P. damicornis, or P. verrucosa, respectively. All the dominant species belonged to the genus Tra- pezia, with one exception, Pilodius. Trapezia cymodoce, T. ferruginea and T. septata were common in both corals that occurred in low-tide and subtidal areas, but T. formosa, T. garthi, T. digitalis, and Pilodius sp. a were abundant only in corals that occurred in the low-tide area. Trapezia cheni was common in P. verrucosa collected from the low-tide off Orchid Island. Two juveniles of Tetralia glaberrima were collected from P. verrucosa. Biological Characteristics of the Xanthid Crabs. -For crabs of the genus Trapezia, collected from both coral species, 70 to 80% of the individuals collected were adults, with one exception (T.ferruginea being 20 to 30% only), 88% of the adults formed heterosexual pairs, 85% of the adult female crabs were ovigerous, the sex ratios were I: I (Tables 2, 3). About 35 to 70% of the adults of T. cymodoce, T. ferruginea and T. septata existed singly or formed a single pair in a given coral head of both corals. They occurred in the subtidal areas, but not in the low-tide areas (Tables 2, 3). In the subtidal areas, 78 and 48% of the coral heads of P. damicornis and P. verrucosa, respectively, were occupied by one adult pair of any Trapezia species. Coral head occupation by Trapezia was 0 and 8%, respectively, in the low-tide areas. The average carapace width of the Trapezia collected from the low-tide corals was smaller than that collected from the subtidal corals (Tables 2 and 3). The carapace width of females was significantly larger than that of paired males in T. cymodoce, T. digitalis, T. formosa, T. ferruginea, T. garthi, but not in T. cheni and T. septata (Table 4). 216 BULLETIN OF MARINE SCIENCE, VOL. 41, NO.2, 1987 Table 1. List ofXanthidae species and number of individuals collected from Pocillopora damicornis and P. verrucosa in different depths No. individuals P. damicornis P. verrucosa 0-1 m 2-7 m 0-1 m 4-9m Species (N = 24) (N ~ 18) (N = 25) (N = 25) Actaea speciosa (Dana) 0 0 I 2 Actaea sp. a 2 0 0 0 Cyc/oxanthops (?) sp. a 0 I 0 0 Etisus (?) sp. a 0 2 0 0 Etisus (?) sp. b 0 0 I 0 Paraxanthias sp. a 2 0 0 0 Pilodius sp. a 32 3 25 0 Pilumnus sp. a 5 0 8 0 Pilumnus sp. b 0 0 1 I Pilumnus (?) sp. c 1 5 0 0 Tetralia glaberrima 0 0 2 0 forma rubridactyla Patton Trapezia cheni Galil 4 0 26 0 Trapezia cymodoce (Herbst) 56 19 55 56 Trapezia digitalis Latreille 40 0 32 4 Trapezia ferruginea Latreille 12 18 7 21 Trapeziaformosa Smith 60 1 92 9 Trapezia garthi Gabl 40 0 33 0 Trapezia rufopunctata (Herbst) 0 0 0 1 Trapezia septata Dana 22 14 29 27 Trapezia tigrina Eydoux & Souleyet 0 0 0 3 Unknown genus sp. a 5 I 0 0 Unknown genus sp. b I 0 90 0 Unknown genus sp. C 0 0 I I Unknown genus sp. d I 0 0 I Unknown genus sp. e 0 0 2 0 Unknown genus sp. f I 0 0 0 Unknown genus sp. g 0 2 0 0 Unknown genus sp. h 0 0 0 1 Unknown genus sp. i 1 0 0 0 Unknown genus sp. j 0 0 1 0 Unknown genus sp. k 0 0 1 0 Unknown genus sp. I 1 0 0 0 Unknown genus sp. m 1 0 0 0 Unknown genus sp. n 1 0 0 0 Unknown genus sp. 0 0 1 0 0 Unknown genus sp. p 0 1 0 0 No. of species 20 12 17 12 Total no. xanthids 288 68 317 127 Total no. associated epibionts 467 202 446 265 DISCUSSION Host Specificity. - From the waters of Taiwan, Chen (1983) collected 69 specimens of the species Tetralia glaberrima from Acroporidae corals, but only 2 from Pocilloporidae corals; 858 specimens of the genus Trapezia from Pocilloporidae corals and 4 from Acroporidae corals. These data indicate that the crab genera Trapezia and Tetralia ofXanthidae are obligately host-specific to Pocilloporidae corals and Acroporidae corals, respectively (Garth, 1964; Knudsen, 1967; Castro, 1976). Moreover, the present study reveals that most Trapezia crabs collected are common in both corals, except that T. cheni is more abundant in P. verrucosa CHANG ET AL.: XANTHID CRABS IN POCILLOPORID CORALS OF TAIWAN 217 Table 2. Characteristics of adult Xanthidae associated with P. damicornis collected from low-tide (N = 24) and subtidal (N = 18) Carapace width (mm) % %in Depth Ovigerous % coral Species (m) Female- Male* females Pairing headt T. cheni 0-1 4.7(0.2) 2 4.2 (-) I 100 67 0 T. cymodoce 0-1 6.1 (1.2) 23 5.7 (1.0) 23 83 96 0 2-7 9.7 (1.4) 5 7.9(2.0) 7 100 83 67 T. digitalis 0-1 6.3 (0.8) IS 5.8 (1.0) 17 73 94 0 T. ferruginea 0-1 7.5 (-) I 6.6 (-) I 100 100 0 2-7 9.5 (1.4) 8 7.7 (0.8) 8 50 100 75 T·formosa 0-1 6.1 (1.1) 24 5.7 (1.0) 23 100 89 0 T. garthi 0-1 5.9 (1.1) 18 5.4 (0.9) IS 100 85 0 T. septata 0-1 6.3 (1.3) 9 5.4 (0.9) 8 89 59 0 2-6 11.1 (0.4) 5 9.9 (1.8) 6 100 100 73 Pilodius sp. a 0-1 5.4 (1.6) 5 5.2 (1.0) 5 60 60 • (Mean (standard error) N) are given. t % of adult Trapezia occurring singly or fonning a single pair in a given coral head.