Decorating Behavior of the Majid Crab

CRUSTACEAN RESEARCH，NO. 35: 79-91，2006 79 Decorating behaviorof the majid crab H y astenus diacαnthus Takao Yamaguchi，Hiroshi Honda，Hisamichi Aratake and Yasuhisa Henmi Abstract. - The decorating behavior Halimus dicanthus (De Haao) (=均1α stenus of 17 newly molted individuals of the diacanthus) had sponges，barnacles and m 吋id crab Hyαstenus diαc α nthus (De sometimes bryozoao 00 its carapace. The Haan，1837) was recorded on VTR. The crab attached hydroids，spooges ，brγozoao s， crabs attached various living materials compouod ascidians ，aod piecesof algae to hooked setae on the carapace and ooto its carapace and walkiog legs ，except walking legs after careful selection by for the tips of the chela of the cheliped the chelae and processing with the (Yamaguchi et al. ，uopublished). It was ve庁 mouthparts. The crabs used chelipeds di 血cult to fiod well-decorated crabs io oa胞子 in a fixed way in the attachment al habitats.The compositioo of attached process. Except for the horn and spots materials depeoded 00 habitat cooditioos. on the carapace，the crabs attached 羽市 ere sponges were abuodaot，the crabs materials using the chelipeds of the covered their bodies with sponges，aod in opposite side. Newly molted crabs had locatiooswhere compouod ascidiaos were almost finished attachment within 2 domioaot，the crabs attached them to their hours，but the crabs continued to work bodies. Although H. di αcanthus rarely deco- sporadically for another 1or 2hours. rated themselves with algae，the crabs used The average frequency of attachments pieces of algae io habitats where inverte- was 91 (N =7). The hooked setae cover brate materials were scarce，but they were only aportion of the body，and other usually covered with spooges，feathery areas，except for the tips of the chelae， hydroids，or compound ascidiaos. are covered with barbed setae. The Wicksteo (1975，1979 ，1980 ，1993) coo- barbed setae hold the materials，which ducted detail edobservatioos 00 the decorat- grow outside of the hooked setae sites. iog behavior of majid crabs，and showed the The crabs were indifferent to the set- sequence of decoratioo. The crabs attach a tlement of larvae of sessile animals， piece of material to the carapace or walkiog such as barnacles，and did not remove legs .Rows of hooked setae occur 00 the them. Consequently，portions of the carapace aod walkiog legs and the crabs rub crabs were covered with sessile ani- the piece agaiost the rows usiog its chelae. mals. The slender setae on the dactyl of めlast enus diacanthus occurs io the lower walking legs and the short setae in the iotertidal shores aod shallow sea bottom of pits of chelae are likely sensory organs ]apao aod the adjaceot seas toward Australia to detect suitable materials for attach- (Miyake，1983). Our objective is to deter- ment. mine how H. di αcanthus decorated themselves ，bas ed 00 the video recordiog of th e decoratiog behavior of oewly molted crabs lntroduction and observatioo of setae 00 carapaceaod Sakai (1965) mentiooed that the majid walkiog legs by scanoiog electroo crab 均lastenus diacanthus planted hydroids mlcroscopy. 00 its body. Dudgeon (1980) reported 80 T. YAMAGUCHI ETAL Materials and Methods Results The crabs used in the video recording Distributio 汎 ofsetαe were obtained by dredging ashallow subti- Rows of hooked setae occur on the divid- dal area with depths of 5to 15 m in the vicin- ed large rostrum (Fig. 1). A long row of ity of the Aitsu Marine Station ，Kumamoto hooked setae is present on the dorsal sur- University in the Ariake Sea (32 0 31'N， face of the three segments of walking legs ， 130 0 19'E) from April to August 1992. The i.e. ，the manus，carpus ，and merus. In addi- crabs without injuries were put in aquaria tion to the rostrum，portions of the carapace and were given pieces of sliced fresh sardine are covered with hooked setae. Thehooked as food. Thenewly molted crabs were trans- setae on the walking legs are in asingle row ferred to another aquarium and kept until as shown in Fig. 2A. Thesetae have afish- their bodies became hard and they were hook shape and are serrated but the se 汀 a- ready to decorate. Video recording was car- tion is shallow and not so conspicuous (Fig. ried out on the newly moulted crab kept in a 2B). On the lateral sides ，the hooked seta small aquarium placed in adark room under has agroove as reported for L. crispatus constant il1 umination. A video camera (Wicksten ，1978). Hooked setae are most attached on atripod was placed in front of well developed at the rostrum. Each of the the aquarium aiming to record not only the horns is covered with setae ，which form two decorating behavior but also the manipula- double 司 rows as shown in Fig. 2C. Hooked tion at the mouthparts. The video camera setae occur along the anterolateral carapace was connected to adouble-cassette video margins as shown by RC and LC in Fig. 1. In recorder and amonitor television placed out- addition to these locations ，the hooked setae side of the dark room. The recording of a occur at six spots on the carapace as indicat- single crab was carried out for two to five ed by RA，LA ，RB ，LB ，RD ，and LD. Other hours. surfaces of the carapace and walking legs To determine the percentage of crabs are covered with short barbed setae. An with sessile animals such as barnacles，ca Ic i- electron micrograph of the short barbed fied annelid tubes，or tunicates on their bod- setae is shown in Fig. 2D. The setae are ies ，specimens deposited in the Aitsu Marine thick but pointed. About one-third of their Station were examined.τbey were obtained surface is covered with short spines. The without selection in the vicinity of the station barbed setae are found over large areas during the years from 1974 to 1990 and were except for the sites covered with hooked preserved in 75 %ethano l. setae and the chelae of adult males. The Scanning electron microscopy of the abdominal surface of the crab ，where no setaeon the carapace and walking legs was hooked setae occur，is also covered with carried out in 2006. The materials were barbed setae. dehydrated after sonication for 5to 15 sec- onds in 75 %ethanol in agraded ethanol-t- Attaching behavior butanol series and were then freeze-dried. Weobserved the behavior of 17 newly Thedried samples were attached with adhe- molted crabs (8 females and 9males). They sive tape onto ametal stub. After sputter were given stems of featherγhydroids and coating with gold-pal1 adium，they were pieces of sponges. They quickly started examined in a]EOL 5400-LV scanning elec- attaching the given materials and continued tron microscope with an accelerating voltage for 120 to 270 minutes. Crabs had almost fin- of5 KV. ished attachment within 2hours ，but continued to work sporadically for another 1or 2 hours. Some crabs sometimes took apos- ture or aposition di 血cult to know how they DECORATING BYHYASTENUS D lACANTHUS 81 R-l st Fig. 1. Thedistribution of hooked setae in adult male 均 astenus diacanthus. Thesites are shaded and each is given an abbreviated name.R indicates the right side and Lthe left sid e. were manipulating or attaching the decorat- apparent in the length of walking legs and ing materia 1. Weexduded such crabs and the size of chelae of the cheliped (Fig. 4). the results of seven crabs (4 females and 3 Large males develop wider and longer，mas- males) are presented in Fig. 3.Th eaverage sive chelipeds and longer walking legs and frequency of attachment was 91. 4. However， they usually move slowly. However，we considerable differences were observed could not discern any dear differences in among individuals. Crab F-2 (female) the treatment and attachment of decorating attached 117 times in 186 minutes but M-2 material between large and small crabs. (male) attached only 46 times in 185 min- Crabs attached materials most frequently utes. M-2 was alarge crab，59.7 mmCL to the walking legs. They tended to attach (carapace length including rostrum) ，and materials from the anterior half of the body. had well developed large chelae and moved Seven of the observed 17 crabs started slowly. In larger crabs，sexual dimorphism is attachment at the rostrum. Five others first 82 T 目 YAMAGUCHI ETAL. Fig. 2. Two kinds of setae occur on the surface of Hyastenus diacanthus. (A): Arow of hooked setae occur on the merus of the right 4th walking Ieg of a44.7-mm CL male.(B): Ahook edseta occurs on the right horn of the rostrum of a36 ふmmCL male. (C): Aphotograph of the right horn of the rostrum of a 38. O- mmCL newly molted female. On the horn，two doublerows of hooked setae occur. This is not an electoronmicrograph but aphotogaph by an ordinarily 35 mmcamera attached with aclose up lens . (D): Barbed setae occur at the posterior fringe of the carapace of a44 .7-mm CL male. attached material onto the 1st walking leg of the mouthparts，the crab rubbed or pushed either side. None of the crabs first attached it onto its body using the chela of either material onto the 2nd to 4th walking legs. side. Usually the attachment was finished in Crabs touched the material using the chela less than half aminute. of the cheliped or the dactyl of walking legs， No specific order was observed in the especially those of the 1st walking legs of attachmen t. Crab F-1 (female) in Fig. 3con- both sides，caught or grasped it using one ducted the first seven attachments in the fol- chela，and carried it to the mouthparts.

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