Moulting of the Semi-Terrestrial Crab Chiromantes Haematocheir (De Haan, 1833) (Decapoda, Sesarmidae) in Taiwan
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Crustaceana 90 (14) 1731-1745 MOULTING OF THE SEMI-TERRESTRIAL CRAB CHIROMANTES HAEMATOCHEIR (DE HAAN, 1833) (DECAPODA, SESARMIDAE) IN TAIWAN BY HUNG-CHANG LIU1), MING-SHIOU JENG2) and RICHARD G. HARTNOLL3,4) 1) No. 53 Chenggong 11th St., Jhubei City, Hsinchu County, Taiwan 302 2) Research Centre for Biodiversity, Academia Sinica, Nankang, Taipei 115, Taiwan 3) School of Environmental Sciences, University of Liverpool, Liverpool L69 3BX, U.K. ABSTRACT Population structure and moulting of the semi-terrestrial crab Chiromantes haematocheir (De Haan, 1833) were studied in Taiwan. The crab moults nocturnally in small freshwater pools, and newly moulted crabs and cast integuments were used to assess moult increment. Males reached a larger size (max. CW 36 mm, n = 272) than females (max. CW 33 mm, n = 164): from 22 mm CW males increasingly dominated the population. Size at maturity was estimated at 17.5 mm CW. The percentage moult increment averaged 11.5% in males (5.5-19%, n = 153) and 13.9% in females (7-23%, n = 72). Female increment exceeded male increment for all overlapping size classes. The larger size of mature males, despite a smaller percentage increment, is explained by a higher post-puberty moult frequency. Of the moulting crabs, 25% of males and 38% of females had one or more missing or regenerating peraeopods. In both sexes this reduced the percentage increment, more so the larger number of limbs affected. The moulting conditions for C. haematocheir are not ideal, with constraints in relation to calcium supplies, and shelter. So the moult increments are unsurprisingly less than those of shallow water marine crabs moulting in an optimal environment, but larger than those of land crabs moulting without access to standing water. RÉSUMÉ La structure de la population et la mue du crabe semi-terrestre Chiromantes haematocheir (De Haan, 1833) ont été étudiées à Taiwan. Le crabe mue la nuit dans des petites mares d’eau douce, et les crabes venant de muer et les téguments rejetés ont été utilisés pour évaluer le taux de croissance à la mue. Les mâles atteignent une taille plus grande (max. CW 36 mm, n = 272) que les femelles (max. CW 33 mm, n = 164): à partir de 22 mm de CW (largeur céphalothoracique) les mâles dominent de plus en plus dans la population. La taille à la maturité a été estimée à 17,5 mm CW. La valeur moyenne de croissance à la mue a été de 11,5% chez les mâles (5,5-19%, n = 153) et 13,9% chez les femelles (7-23%, n = 72). Le taux de croissance femelle a excédé celui des mâles pour toutes les classes de taille se chevauchant. La plus grande taille des mâles matures, en dépit de leur taux de 4) Corresponding author; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2017Downloaded DOI 10.1163/15685403-00003711 from Brill.com10/02/2021 03:21:35AM via free access 1732 HUNG-CHANG LIU, MING-SHIOU JENG & RICHARD G. HARTNOLL croissance plus faible, est expliquée par une fréquence des mues après la puberté plus élevée. Parmi les crabes en mue, 25% des mâles et 38% des femelles avaient une ou plusieurs pattes manquantes ou en régénération. Dans les deux sexes, cela réduit d’autant plus le taux de croissance que le nombre de pattes affectées est élevé. Les conditions de la mue pour C. haematocheir ne sont pas idéales, avec des contraintes pour la fourniture du calcium, et les abris. Aussi les taux de croissance sont, sans surprise, moindres que ceux des crabes marins d’eau peu profonde muant dans un environnement optimal, mais plus élevés que ceux des crabes terrestres muant sans accès à une eau stagnante. INTRODUCTION Two components determine the growth rate of crabs: the increase in size at moulting (the moult increment), and the interval between successive moults (the intermoult period). This study will focus on the former, though the latter will be briefly considered. The moult increment can be expressed as an absolute measure, but this renders comparison between different sized crabs difficult. It is preferable to use the “percentage moult increment”, expressed as the percentage increment on moulting of the pre-moult size. Unless otherwise stated, all references to “increment” relate to this factor. Measuring the increment is not straightforward. It can sometimes be determined from the successive peaks in size-frequency distributions, though discrimination is generally limited to the first five or six postlarval instars (Hartnoll, 1978). There may be morphological differences between instars, and on this basis plus bivariate analysis Sasaki & Kuwahara (1999) discriminated 17 instars in Erimacrus isenbeckii (Brandt, 1848): this complex process has not been repeated for other species. The observation of moults of captive specimens is the commonest method, as in Carcinus maenas (Linnaeus, 1758) (cf. Mohamedeen & Hartnoll, 1989): however, there is uncertainty as to how such data relate to performance in the wild (see discussion in Hartnoll, 1982). A marking programme using tags retained at moulting can provide comprehensive data, but is logistically complex and expensive and has been limited to larger species of commercial and/or conservation importance: e.g., Cancer pagurus Linnaeus, 1758 (cf. Edwards, 1965), Callinectes sapidus Rathbun, 1896 (cf. Fitz & Wiegert, 1991), and Birgus latro Linnaeus, 1767 (cf. Drew et al., 2012). For terrestrial crabs, there are limited data on moult increments and in most cases these have been obtained from captive specimens (Hartnoll, 1988b). Moult- ing in terrestrial crabs, such as Birgus latro (cf. Fletcher et al., 1990), Cardisoma guanhumi Latreille, 1828 (cf. Gifford, 1962), and Gecarcinus lateralis (Guérin, 1832) (cf. Wolcott, 1988) usually occurs within their burrows, hence it is difficult to observe moulting behaviour and obtain natural moult increment data. There is, however, one simple method to determine the increment under natural conditions. This is where newly moulted crabs can be collected together with their Downloaded from Brill.com10/02/2021 03:21:35AM via free access MOULTING OF CHIROMANTES HAEMATOCHEIR 1733 cast integuments, and/or imminently moulting crabs can be collected to then moult very soon in captivity: such studies have previously been in intertidal habitats: e.g., Carcinus maenas (cf. Hogarth, 1975), Hemigrapsus sanguineus (De Haan, 1835) (cf. Kurata, 1962) and Macrophthalmus boteltobagoe Sakai, 1939 (cf. Kosuge, 1993). These cases are few and have generally involved small samples sizes. However, Chiromantes haematocheir (De Haan, 1833) is a common semi- terrestrial crab in Taiwan, and inhabits areas close to swamps and muddy banks of freshwater streams not far from the sea. For this species, individuals that are at the onset of ecdysis, together with recently moulted crabs along with their cast integuments, can all be readily found in the field; this provides an unusual opportunity to study moulting behaviour and gather moult increment data for a terrestrial crab. In this paper, the moulting behaviour and the natural moult increment of C. haematocheir are examined on the basis of substantial samples. The effect of limb regeneration on growth increment is also investigated. MATERIAL AND METHODS The study area was near the mouth of the Meilun River (23°5837N 121°3618E), Hualien City, Hualien County, eastern Taiwan. In the area there are small springs which form freshwater pools throughout the year, although the amount of water in them changes seasonally. A belt transect (0.5 m × 100 m) was established close to the estuary at the foot of Meilun Hill and monitored for 11 months, from September 2000 to July 2001, so including both the wet (May to September) and dry (October to April) seasons. Chiromantes haematocheir is pre- dominantly nocturnal, so surveys were conducted at night: crabs were hand caught using lamps. To obtain population data, every specimen found outside their burrows within the transect was sampled monthly from March to July 2001. The sex, carapace width (CW), and whether females were ovigerous, were recorded, after which the crabs were released in their original habitat. During the monitoring period, 115 field surveys were carried out to collect data on moulting. Moulting behaviour was recorded. To investigate increment, recently moulted crabs (plus their cast shell), and also those close to moulting, were collected: crabs close to moulting were readily identified by a fragile carapace, an empty intestine and feeble chelae. Collected crabs were kept singly in plastic aquaria (0.3 m L × 0.2 m W × 0.15 m H). The aquaria contained fallen leaves, mud and water taken from the study area to simulate the natural habitat, providing both a dry and a wet area for the crabs: the deepest water allowed complete immersion. The sex and number of Downloaded from Brill.com10/02/2021 03:21:35AM via free access 1734 HUNG-CHANG LIU, MING-SHIOU JENG & RICHARD G. HARTNOLL missing/regenerating limbs were recorded. Once sufficiently hardened, the CW of the newly moulted crabs was measured, and the carapace marked with an indelible pen. They were then released in their original habitats, and any crabs subsequently recaptured undergoing a further moult provided data on the intermoult period. Crabs were also released if they had failed to moult within three days of collection. Percentage moult increment (% MI) was calculated as %MI = (CW2 − CW1)/CW1 × 100 where CW1 and CW2 are premoult and postmoult carapace widths, respectively. In an attempt to determine the size of morphological maturity, measurements were made on the height of the chelar propodus in males, and the width of the pleon in females. These data were not conclusive, and are not reported further. Data on the precipitation and air temperature were obtained from the meteoro- logical station at Hualien City (24°09N 120°41E; 19 m above sea level), a few hundred meters from the study area.