The Growth, Reproduction and Body Color Pattern of Cleantiella Isopus (Isopoda: Valvifera) in Hakodate Bay, Japan
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CRUSTACEAN RESEARCH, NO. 41: 1–10, 2012 The growth, reproduction and body color pattern of Cleantiella isopus (Isopoda: Valvifera) in Hakodate Bay, Japan Tomohiro Takahashi and Seiji Goshima A b s t r a c t . — We studied the growth, cycling in the intertidal and subtidal areas. reproduction and body color pattern of the Most isopod studies focus on the life marine isopod Cleantiella isopus in Hakodate Bay, cycle, feeding habits and mate choice; Japan from May 2009 to July 2010. Individuals almost all are exclusively on European were collected every month and the sex, body species of the genus Idotea (Naylor, 1955a, length, color pattern, number of eggs per clutch b; Jormalainen et al., 1992). The breeding and developmental stage of embryos for ovigerous period and its length differ among species females were recorded. Five body color patterns or conspecific populations that occupy were identified in C. isopus at Hakodate Bay, and different habitats (Naylor, 1955b; Sheader, their composition was maintained throughout 1977; Salemaa, 1979; Healy & O’Neill, the year. Breeding females (guarded by a male 1984). Although about 20 species have been or carrying eggs) were observed in the field from reported in Japan, as far as we know, there February to August. Newly recruited individuals is only one study about the feeding habit of were first observed in July and grew to mature size by the next breeding season, indicating a Idotea ochotensis (Suzuki et al., 2002). Isopod lifespan of 13–15 months. Precopulatory mate species are common in northern Japan and guarding in which a male holds a female using his play an important role in the community of pereopods was observed. There was no correlation the littoral zone, but their basic biology such between body lengths of guarding males and as life cycle, growth, and reproduction are guarded females. The female reproductive cycle almost unknown. Cleantiella isopus, belonging was synchronous, and two defined reproductive to the family Idoteidae, is distributed from events were detected during the breeding periods, Hokkaido to Kyushu in Japan and inhabits indicating that females bred twice or more during spaces under stones or algae in the intertidal- a lifetime. subtidal zone (Nunomura, 1995, 2011). Body shape is rectangular, and the lateral margins are parallel in the male, while in the female Introduction they are slightly curved. There are several Isopods are an important component of body color patterns (e.g. green, white, yellow coastal ecosystems and fish diets (Sywula, and brown). Antennae consist of five scapes 1964; Strong & Daborn, 1979). Four families with a flagellum, and often the tip is white and about 550 species are known in the in color. The posterior edge of the telson world, and three families and 20 species is slightly rounded and triangular in shape have been reported in Japan (Nunomura, (Nunomura, 1995). 1995). Their feeding habits vary among The aim of this study was to clarify the species, and there are species which forage basic biology of growth and reproduction of algae, seagrass, and animal or animal debris C. isopus and compare it with related species (Kjennerud, 1950; Naylor, 1955a). The living in European regions. In addition, feeding behavior of herbivorous species we categorized the body color pattern of promotes fracture and decomposition of dead C. isopus and considered the relationship plants (Robertson & Mann, 1980). Therefore between body color and habitat. they may play an important role in material LIFE HISTORY OF ISOPOD CLEANTIELLA ISOPUS 3 2 T. TAKAHASHI & S. GOSHIMA 2 animals to the laboratory and measured body Materials and Methods length using a caliper and then proceeded Sampling was carried out every month to determine the sex, number of eggs per from May 2009 to July 2010 in Kattoshi (41° clutch and developmental stages of embryos 44’ N, 140° 36’ E), located on the west side for ovigerous females. Body length was of Hakodate Bay, southwestern Hokkaido. measured as the distance from the anterior The sampling site was at the exposed rocky border of the cephalon to the posterior shore during low tide. The bottom of the border of the telson. All animals over 10 tide pools was covered with stones of mm were sexed by presence or absence of 20–50 cm in diameter. Brown algae such as penis because only after a length of about Undaria pinnatifida, Saccharina japonica and 10 mm was the character recognizable. The Sargassum fulvellum and seagrass such as size frequency distribution of individuals Phyllospadix iwatensis lived on the bedrock was observed and cohorts (age-class) were area without stones. The maximum water identified by approximate body length in temperature recorded was 21°C in July 2010, order to follow changes in mean size of each while the minimum water temperature was cohort. 3°C in February 2010 (Fig. 1). We found C. We defined a functional mature male isopus almost exclusively at our sampling as one that guards a female and functional site, however C. strasseni was also found, mature female as the individual that is but only three times throughout the present guarded by the male or the individual having study. Idotea ochotensis, which is distributed a brood pouch. in various regions of Hokkaido (Nunomura, The developmental stages of embryos Fig. 2. Body color patterns of Cleantiella isopus. A: brown, B: brown-white line, C: brown-white spots, D: light 2011), was never found at the present within the brood pouch were defined as brown, and E: green type. sampling site. follows. Stage I: spherical-ovoid egg of Sampling was conducted during every 0.9 mm in diameter surrounded by two spring low tide for two hours. Isopods were membranes called “chorion” and “vitelline found frequently attached to the bottom of membrane”, stage II: elongate embryo of penis appeared then as well. Males reached stones in tide pools. Sampling was conducted 1.2–2.0 mm in length which has molted 14.3 mm in body length, while females during daytime from May 7 to October 1 in from these two membranes and surrounded Results attained 12.8 mm by August; 16.7 mm in 2009 and from March 23 to July 27 in 2010, by a single membrane called “embryonic Growth males and 14.8 mm in females by September and during nighttime from October 20 in membrane”, and stage III: the embryo of 2.5– The size composition of the sample is and finally 21.5 mm in both sexes by 2009 to February 27 in 2010. We brought 4.0 mm in length which has moulted from illustrated in Fig. 3. In both 2009 and 2010, December (Fig. 4). Growth was suspended in embryonic membrane. Stage III embryos the range of male body length was greater February when water temperature dropped to were not included in the number of eggs than that of females throughout the year 3ºC (Fig. 1). because of the possibility that some embryos (Fig. 3). The population was composed of Breeding females appeared in mid- already escaped from the brood pouch. only large males and females in May 2009 February when minimum water temperature Cleantiella isopus showed several body when we started the study. Newly recruited (3ºC) was recorded. Both sexes reached 25 color patterns. These patterns were easily juveniles were first confirmed in July 7 mm in average length, and most females distinguishable and we divided them into five 2009 (9.6 mm in average length), and two carried eggs by mid-March. Release of categories (Fig. 2). “Brown” is an individual distinct cohorts were detected during July juveniles from the brood pouch occurred that has dark brown body color and often to August 2009 and also in July 2010. The from May to July when they were around has a small white spot on the third thoracic sex of these newly recruited individuals was 5 mm in body length. Large individuals segment. “Brown-white line” is a “brown” unknown because sexual organs (appendages) decreased from July and disappeared by individual whose entire third thoracic were not visible. Both larger males and October in 2009. The results obtained segment is white color. “Brown-white spots” females decreased in number from July and indicate that C. isopus has a lifespan of 13–15 is an individual that has irregular white spots disappeared by October. The sex ratio (male months. over a brown background throughout the /female) was greatly biased toward female Reproduction (0.3–0.6, mean = 0.5) from May to August. Fig. 1. Monthly mean surface water temperature at whole body. “Light brown” is an individual Precopulatory mate guarding was Kattoshi area (open squares) and those in the tide pool that has light brown body color. “Green” is The newly recruited juveniles were first observed, in which a male mounts and (filled squares). an individual that has whitish-green body confirmed and grew to more than 10 mm in restrains a female; grasping her body by color and a black head. average length in July 2009 and 2010 (Figs. using his pereopods (Fig. 5A). Females shed 3, 4) and individuals which had a developed the posterior half of the exoskeleton and then LIFE HISTORY OF ISOPOD CLEANTIELLA ISOPUS 3 2 T. TAKAHASHI & S. GOSHIMA 3 animals to the laboratory and measured body Materials and Methods length using a caliper and then proceeded Sampling was carried out every month to determine the sex, number of eggs per from May 2009 to July 2010 in Kattoshi (41° clutch and developmental stages of embryos 44’ N, 140° 36’ E), located on the west side for ovigerous females. Body length was of Hakodate Bay, southwestern Hokkaido. measured as the distance from the anterior The sampling site was at the exposed rocky border of the cephalon to the posterior shore during low tide.