BULLETIN OF MARINE SCIENCE, 41(2): 214-220,1987

XANTHID IN THE CORALS, DAMICORNIS AND P. VERRUCOSA OF SOUTHERN TAIWAN

Kun-Hsiung Chang, Yu-Shan Chen and Chang-Po Chen

ABSTRACT Thirty-six 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. 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- 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 genera Trapezia and Tetralia of the family Xanthidae are believed to be obligately host-specific to 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 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 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 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 (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. than in P. damicornis. This host specificity may also be caused by the difference in localities, since that P. verrucosa is the only available pocilloporid in the low- tide areas off Orchid Island. However, Preston (1973) revealed that there is no evidence that particular species of Trapezia prefer particular species of Pocil/opora. Thus, further studies are needed to confirm this host-specificity of Trapezia cheni within the pocilloporid corals. Abundance and Body Size between Shallow and Deeper Locations. - Trapezia cheni and T. garthi have been found only in the corals collected from the low- tide areas, and T. digitalis, and T. formosa are significantly more abundant in corals of the low-tide than in those of the subtidal, suggesting that some crabs of the genus Trapezia may be confined to a certain depth of Pocilloporidae corals. This restriction of distribution has also been found in some Trapezia spp. in other

Table 3. Characteristics of adult Xanthidae associated with P. verrucosa collected from low tide (N = 25) and subtidal (N = 25)

Carapace width (mm) % %in Depth Ovigerous % coral Species (m) Female- Male> females Pairing headt T. cheni 0-1 5.7 (1.0) 8 4.9 (0.6) 7 88 80 0 T. cymodoce 0-1 6.4 (1.3) 25 6.1 (0.8) 20 64 80 0 4-10 9.4 (2.8) 19 9.1 (2.6) 19 89 95 32 T. digitalis 0-1 6.6 (0.8) 12 6.2 (1.0) II 75 78 0 5-10 10.2 (1.1) 2 9.4 (0.9) 2 100 100 0 T. ferrllginea 0-1 7.0 (2.1) 2 6.5 (0.2) 2 100 100 0 4-10 10.9 (2.5) 7 8.2 (1.6) 7 71 100 43 T.formosa 0-1 6.3 (1.2) 32 5.7 (1.0) 37 84 87 6 5-10 7.8 (0.5) 4 7.4 (1.4) 4 100 100 0 T. garthi 0-1 6.3 (1.3) 13 5.9 (1.1) 12 85 96 0 T. septata 0-1 7.1 (0.7) 9 6.0 (1.1) 13 67 82 0 4-10 9.8 (2.8) 9 9.5 (2.5) 8 67 82 35 Pilodius sp. a 0-1 5.9 (1.0) 5 6.1 (1.2) 5 60 40 > (Mean (standard error) N) are given. t % of adult Trapezia occurring singly or forming a single pair in a given coral head. 218 BULLETIN OF MARINE SCIENCE. VOL. 41, NO.2, 1987

Table 4. Size differences (carapace width of female minus that of male, mm) between paired indi- viduals associated with Pocillopora damicornis and P. verrucosa (Mean, I SE and number of pairs are given)

Corals

Crabs Depth P. damicornis P. \'errucosa T. cheni low-tide 0.60 ± 0.61 (6) T. cymodoce low-tide 0.42 ± 0.64 (22) ** 0.52 ± 0.46 (18) ** subtidal 0.84 ± 1.85 (5) 0.43 ± 0.67 (I8) * T. digitalis low-tide 0.62 ± 0.30 (I5) ** 0.41 ± 0.62 (9) subtidal 0.80 ± 0.28 (2) T. ferruginia low-tide 0.90 - (I) 0.55 ± 1.91 (2) subtidal 1.80 ± 0.90 (8) ** 2.70 ± 1.52 (7) ** T. formosa low-tide 0.71 ±0.66(21) ** 0.40 ± 0.61 (30) ** subtidal 0.45 ± 0.97 (4) T. garthi low-tide 0.67 ± 0.43 (14) ** 0.53 ± 0.59 (12) ** T. septata low-tide 0.54 ± 0.62 (5) 0.44 ± 0.70 (9) subtidal 0.54 ± 0.85 (5) 0.84 ± 1.01 (7)

• Significantly dilfcrcnt from zero (Hcst, P < 0.05) . •• Significantly dilferent from zero (t-test, P < 0.01). areas (Edwards and Emberton, 1980; Gotelli et a1., 1985). The abundance of host corals and/or the scarceness of predatory fishes have been suggested to be the factors contributing to the greater abundance of Trapezia in shallow locations (Gotelli et a1., 1985). The body sizes of Trapezia spp. are larger in deeper locations than in shallow locations. This might be related to the increase of available living-space of host corals from shallow to deeper locations (Edwards and Emberton, 1980; Chen, 1983). Pairing. -Crabs of the genus Trapezia tend to form heterosexual pairs (Preston, 1973; Patton, 1974; Castro, 1976; Huber, 1985; present study) and the carapace width of females larger than that of paired males (Preston, 1973; present study; Adams et a1., 1985). Patton (1974) proposed that the mechanism of heterosexual pairing in the crabs of the genus Trapezia seems to be of an early-long-lasting type. However, Huber (1985) has showed in his experiments that both male and female crabs of the genus Trapezia pair with the larger prospective mates signif- icantly more often than expected on the basis of random movement. This seems to suggest that the pairing can be formed by changing-larger-mate pairing especially when a partner left. However, Huber's experiments did not explain why the females are consistently larger than the males in pairing. Finney and Abele (1981) proved that the morphological variation in male T. ferruginea is related primarily to size, while in females it related to both size and maturity. The lateral expansion of the carapace of the females is probably to accommodate the reproductive organs. This might explain partially the difference of body size between paired female and male. In contrast, Adams et a1., (1985) demonstrated that the mating of T. ferruginea is size assortative and suggested that larger male crabs actively select appropriate sized coral colonies and then females subsequently select their mates on some other basis. Space Competition. - The crabs of the genus Trapezia are obligately commensal to the Pocilloporidae corals and compete interferently for space with each other (Preston, 1973; Lassig, 1977; Castro, 1978). Present study reveals that a single CHANG ET AL.: XANTHID CRABS IN POCILLOPORID CORALS OF TAIWAN 219 adult or a single adult pair of Trapezia existing in a coral head occurs high- frequently in subtidal Pocil!opora, but all adult Trapezia crabs coexist with other congeneric species in low-tide pocilloporid corals, suggesting that the interference competition among species of Trapezia is stronger in the subtidal corals than in the low-tide corals. The body size of adult Trapezia, larger in the subtidal corals than that in the low-tide corals, might contribute to this stronger competition. Moreover, the density and host-size requirement of crabs, and the density and suitability of the corals also determine the outcome of the interference competition (Preston, 1973). The heterogeneities of physical environment might also play an important role in it. Further studies are needed to know the mechanisms which cause the different results of the crabs of Trapezia coexisting in these two depths of southern Taiwan.

ACKNOWLEDGMENTS

We thank Drs. J. S. Garthi, Allan Hancock Museum and 8. Galil, Department of Zoology, Tel Aviv University, Tel Aviv, for help in identifying the xanthid crabs.

LITERATURE CITED

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DATEACCEPTED: October 8,1986.

ADDRESS: Institute of Zoology, Academia Sinica, Nankang, Taipei 11529, Taiwan, Republic of China.