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Excirolαnα Chiltoni in Sendai CRUSTACEAN RESEARCH，NO .34 ・27-39，2005 Life history and utilization ofthe beach face slope by the sandy beach isopod Excirolαn α chiltoni in Sendai Bay，northern Japan Kenji Kaneko &Michio Omori Abstract.一一 The life history of a molting and reproductive cycle followed a cirolanid isopod，Excirol αn α chiltoni fortnightly pattern. Shimoizumi (1984) Richardson，1905 ，was studied on a reportedthe distribution migrated up and sandy beach in Sendai Bay，northern down in relation to the level of the high-tide Japan，during the period October 1996 wash-line and the population structure (as E. to March 1998. Cohort analyses indicat- japonica). Jones & Hobbins (1985) suggesι ed two or three age groups in each sam- ed that the specie's endogenous rhythmicity ple with'a life span estimated at 2 was due to spatial zonation in the distribu- years. The growth rate was high in the tion of E. chiltoni. warm season and low in winter. The For cirolanid isopods dwelling in the breeding season lasted from early April intertidal zone of sandy beaches，it has been through to late September. Two-year- reported that the distribution pa仕erns were old females began to carry embryos differ ent according to body-size and develop- earlier，followed by the smaller 1-year- mental stage of E .braziliensis (Glynn et al. ， olds. Excirolαn α chiltoni showed spa- 1975; D exter，1977) and E .armata (de Alava tial segregation during the breeding & Defeo，1991). However，few papers report season. Namely，ovigerous females changes in distribution along the beach face occurred on the high shore and juve- slope on asandy beach throughout their life niles were closest to sea. Thus，E. history in detai l. Theaim of this paper is to chiltoni utilize environmental gradi- examine the growth，reproduction and distri- ents along the beach face slope on bution of E. chiltoni;and to understand ，how sandy beaches throughout their life his- this organism utilizes space along abeach tory. face throughout its life history. Introduction Materials and Methods Thecirolanid isopod ，Excirolana chiltoni This study was carried out at Shobuta Ri chardson，1905 ，occurs in the intertidal Beach in Sendai Bay in northern Japan zone of sandy beaches in the North Pacific (38 0 17' N ，141 0 04'E). The beach is mainly (Bruce & Jones，1981). Excirolana chiltoni is composed of fine sand and categorized as an the predominant species in Sendai Bay， int ermediate beach (reflective-dissipative northern Japan，and it appears to play an categories，see Wright & Short，1984). A important role as apredator of other crus- detailed description of the study site is given taceans，and/or as ascavenger in the sandy in Kaneko & Omori (2003). Excirolana beach community (Kaneko & Omori，2003). chiltoni was collectedduring th eday ，onc e Several studies haveb eenmade of the life or twicea month from October 1996 to history of E. chiltoni. For example，Kl apow March 1998. Sampling was carried out with- (1972) reported that the vertical distribution ， out consideration of the tide from October 28 K. KANEKO &M. OMORI 1996 to March 1997; and during low tide niles collected in the field. Moreover，we from April 1997 to March 1998. A transect determined whether females had released was set perpendicular to the shoreline and juveniles by the presence of abreak mark on sand within an area of 25 x25 cm was taken the ventral thoracic wal l. to a5 cm depth every 2m from the drift-line Weexamined gut contents of all individu- to the lower limit of the swash zone along als by developmental stages collected on 9 the transec t. Animals，sorted with a0.5 mm June 1997 and in all individuals collected 8m mesh sieve ，were immediately fixed in 10% seaward from the drift-line (where most indi - formalin. The drift-line was estimated from viduals were collected) on 22 luly 1997. The the existence of stranded ma仕er lying paral- gut contents were removed and identified. lel to the shoreline. Upper and lower limits Feeding index (the percentage of individuals of the swash zone were determined by three with food in the gut) of E. chiltoni was calcu- minutes of observation of water movemen t. lated. Seawater temperature was measured in the Population abundance was estimated by sublittoral area every sampling day. Sand the number of individuals per strip transect temperature at adepth of 3cm was mea- (1 ST:ind ./ m shorelin e; Brazeiro & Defeo， sured at 4m intervals from the drift-line to 1996). 1ST is estimated by averaging the the upper limit of the swash zone during the density qat each sampling station iof all n day on 8July 1997 (summer)，16 February samples and multiplying by the correspond- 1998 (winter) ，and 18 May 1998 (spring) ， ing entire area of the transect (ω) with the and at night on 16 February 1998 (winter). following equation (Brazeiro & Defeo，1996) ・ Body width (6th thoracic segment width) was measured to the nearest 0.05 mmusing Lqi amicroscope with amicrometer and was 1ST =_ '-_'- w converted to the total length (TL: from the n tip of the cephalon to the end of the telson) 1ST is used to avoid biased results from using the relationship between body width using mean density per quadrat as and T L. τb e animals collected were sorted an abundance，because the variable environ- into ovigerous females，non -o vigerous mental conditions can result in contraction females，males or juveniles .Sexes were dis- or expansion of the distribution of macroin- tinguished by the presence or absence of a fauna along the beach face slope (Defeo & penis. Thesmallest individuals possessing a Rueda，2002). penis were about 5mm T L. Accordingly， Growth curves were estimated for both individuals smaller than 5mm TL were cate- sexes，assuming that sex ration of unsexed gorized as juveniles. The number of eggs or juveniles (く 5 mmTL) is even. Each frequen- embryos in the marsupium of ovigerous cy distribution is separated into amultiple females was counted. The developmental normal distribution by using Cassie's stages of eggs or embryos were categorized method (1954). Growth curves were into three stages according to Klapow described by applying the normal distribu- (1970); stage 1: Klapow's stages aand b， tion. recently fertilized eggs or considerably more elongate eggs; stage II: Klapow's stage c，d 江- ferentiation of the body has progressed to Results the extent that th ehead ，thoracic ，and Enviγon悦側talfactors abdominal regions are distinct; stage III: Water temperatures ranged from 6.7 to Klapow's stages dand e: the embryonic 26.6 0 C(Fig. 1). The water temperature was membrane has ruptured and the embrγo is >20 0 Cfrom late June to early October. The 0 capable of f1 exing its body or the shape is water temperature was く 10 C from January indistinguishable from the free living juve- to March. LIFE HIS1ひ RYOFEXCIROLANA CHILTONI 29 ( The sand temperature gradually ハ 今 ゲ 3un decreased from the drift-line to the swash- ) 25 line during the day (Figs. 2a-c). It was high- est at 4m seaward of the drift-line. Thepeak 520 temperatures were about 60 C higher than 邑 15 the water temperature in spring and sum- E 10 mer and 3.6 0 C higher in winter. On the S5 司 other hand，the sand was colder than the 注 O water at low tide during night in winter (Fig. ONDJFMAMJJASONDJFM 2d) ，being -1. 20 C at the drift-line ，11.8 0 C 1996 1997 1998 0 lower than in the daytime and 6.2 C lower Month than the water temperature. Fig.1. Seasonal changes in water temperature at Population abundance the study site in Sendai Bay，Japan. Population abundance of E. chiltoni ranged from 1088 (2 Feb. 1998) to 26784 (4 Aug. 1997) ind./m shoreline and the annual 22 (a) mean abundance was 7992 (SD:t 5620) 20 ind./m shoreline (Fig. 3). The abundances 18 were slightly lower in winter (from 16 December to February). 14 12 Growth 30 The frequency distributions were sepa- 28 rated into two or three normal distributions P26 (Figs. 4，5). Three cohorts，regarded as 副』 24 being hatched over the period 1995-1997， 92d were recognized. Estimated growth curves 豆白 20 of each cohort are shown in Fig. 6. c.. Recruitment of juveniles (mean size is about 16 14 3.5 mmTL) began in June and the size "0 包 12 (の increased little until October because of con- 帽 ∞ 10 tinued recruitment. Both sexes grew 8 remarkably during two periods: from late 6 September to mid-December in the year they were recruited; and from mid-May to 6l mid-November in the first year after their 41 release. The mean growth rates in both 21 -'畠ー---- these periods were 0.014-0.017 mmT L/day (d) -2 i and 0.026-0.032 mmT L/day，respectively. -4 ; On the other hand，the mean growth rate 。 5 10 15 20 25 was low (-0.010-0.008 mmTL/day) during the cool-water season (from January to Seaward distance from the drift line (m) March). One-year-old females grew rapidly at amean growth rate of 0.044 mmTL / day Fig. 2. Sand temperature along the beach slop e. from mid-May to lat eJun e，but grew slowly (a): spring (18 May 1998); (b): summer (8 July at 0.010 mmTL /day from lat eJun eto mid- 1997); (c) :winter day (16 February 1998); (d): winter night (16 February 1998). Broken lines Novembe One-year-old males grew gradu- r. denote water temperature. ally at arate of 0.018 mmTL /day from mid- 30 K.
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