Crustacean Research 2019 Vol.48: 67–80 ©Carcinological Society of Japan. doi: 10.18353/crustacea.48.0_67 Seaward migration and larval release of the land hermit crab Coenobita brevimanus Dana, 1852 (Anomura: Coenobitidae) on Iriomote Island, Japan Taketo Nio, Wataru Doi, Akira Mizutani, Hiroyoshi Kohno Abstract.̶ The seaward migration and larval release of Coenobita brevimanus Dana, 1852 on a sandy beach of Iriomote Island, Japan, was studied between May and November 2009 and between May and December 2010. Seaward migration and larval release were mainly observed for several days before and after new moons during the period from June to November, and the migration was mainly focused on 0.82 days after the new moon. Around full moons, some crabs were also found, with the highest numbers at 1.34 days after spring tides (semi-lunar rhythms). Almost half of the crabs that appeared on the beach entered seawater, and almost half of the immersed crabs released zoeas or exhibited swinging behavior. Appearance of crabs on the beach and entering seawater occurred only after sunset between 19:00 to 23:10. The peak time of the appearance on the beach was within a certain range each month. Crabs entered seawater at a mean of 2 h after the nighttime high tide. Seaward migration of the crabs showed clear seasonality, and many crabs migrated and entered seawater during the period from July to September, the months with higher temperature (＞25°C). This species shows a phenology and rhythm of spawning common to coenobitids on the Ryukyu Islands. Key words: hatch, reproduction, Ryukyu Islands, spawning, Yaeyama Islands ■ Introduction swash zone, which allows us to directly ob- serve those behaviors, unlike aquatic species, As a taxon, the family Coenobitidae inhabits which release larvae underwater. In addition, the largest terrestrial area of the anomuran ovigerous females of coenobitids migrate at a decapods and has worldwide subtropical and slower walking speed on beaches with poor tropical distribution. They are found on shores, vegetation cover. Therefore, members of this inland areas, coastal forests, rainforests and family are easier to observe than intertidal and arid scrublands of small islands or narrow supratidal brachyurans which can move more coastal strips (Hartnoll, 1988; Greenaway, quickly. 2003). Because their aquatic larvae need saline The general biology and chronobiology of water to hatch (Hamasaki et al., 2018a), ovig- spawning have been well documented in field erous females of coenobitids move to the wa- studies of brachyurans (e.g., Forward, 1987; ter’s edge and release larvae into seawater (Ya- Morgan, 1995; Christy, 2011). In the case of maguchi, 1938; de Wilde, 1973; Imafuku, littoral and supralittoral brachyurans, larval re- 2001; Nakasone, 2001; Nieves-Rivera & Wil- lease generally occurs at night during high tide liams, 2003; Doi et al., 2016, 2018a). Larval on larger amplitude tides on a bi-weekly or release by coenobitid crabs takes place in the monthly cycle of the tidal amplitude (Christy, Received: 1 Apr 2019. Accepted: 9 July 2019. Published online: 23 Aug 2019. 67 TAKETO NIO ET AL. 2011). The larval release by coenobitids has brevimanus is scarce in the northern part, but been documented with respect to the lunar cy- some local populations of this species are cle but not the tidal amplitude cycle (de Wilde, found in the southern part, the Yaeyama Islands 1973; Nakasone, 2001; Nieve-Rivera & Wil- (Okinawa Prefectural Board of Education, liams, 2003; Sato & Yoseda, 2009; Doi et al., 1987; 2006). In this area, C. brevimanus is the 2016). Spatiotemporal patterns of larval release most terrestrially adapted Coenobita and in- are important parameters that determine the habits inland and coastal forests in lower den- survival and transport of early-stage larvae by sity than its sympatric congener C. cavipes currents (Christy, 2003, Murakami et al., 2014; (Kosuge & Kohno, 2010; Hamasaki et al., Doi et al., 2018b). Moreover, expansion of the 2017). The abundance of C. brevimanus is geographical distribution and metapopulation higher in the area with human settlement network of benthic populations of terrestrial (maintained and abandoned villages) and the decapods including coenobitids is determined adjacent beach (Kosuge & Kohno, 2010; by the transport of aquatic larvae (Anger, Mizutani et al., 2012) than in natural habitats 2006). Therefore, characterizing the dynamics such as on sandy beaches and mangrove estu- of larval release by coenobitids is indispens- aries (Kohno et al., 2012; Fujikawa et al., able for understanding the population structure. 2017). The land hermit crab Coenobita brevimanus The population of C. brevimanus in Amitori Dana, 1852 (Anomura: Coenobitidae) is widely Bay of Iriomote Island in the Yaeyama Islands distributed and a common species throughout (24°24′N, 123°46′E) (Fig. 1) has many crabs of the Indo-Pacific area (Hartnoll, 1988; Vannini large size (Mizutani & Kohno, 2012; Doi et al., & Ferretti, 1997; McLaughlin et al., 2007; 2019). It could serve as a population source to Hamasaki et al., 2015a; Hsu et al., 2018). The other areas in the northern marginal part of the southern islands of the Japanese archipelago range through dispersal during pelagic larval are the extreme northern limit of the biological stages, because larger females can be expected range of this species. In the Ryukyu Islands, C. to have higher fecundity and, thus, release Fig. 1. Location of the study area. A, location of Iriomote Island and Amitori (inset, arrow); B, aerial photograph of the study site near the buildings of the Okinawa Regional Research Center, Tokai University (ORRC, the abandoned village of Amitori) (Image source: Geospatial Information Authority of Japan). Areas enclosed by solid lines and covered by crosshatching indicate sampling sites on the beach during 2009 and 2010, respectively. 68 Crustacean Research 48 SEAWARD MIGRATION AND LARVAL RELEASE OF COENOBITA BREVIMANUS more larvae. To consider conservation of the near-threatened C. brevimanus in this area (Ministry of the Environment, Japan, 2019) with fragmented distribution, it is necessary to elucidate the dynamics of recruitment and the transition from sea to land of juveniles through pelagic larval stages. The aim of this investigation was to analyze the temporal changes in seaward migration and spawning in an effort to understand the repro- ductive biology of C. brevimanus and to deter- mine the role of the local Amitori population as a source of the larvae. Field surveys were con- ducted at different times under different tidal conditions on a sandy beach of Iriomote Island. Fig. 2. Monthly changes in mean±standard deviation (–) of air temperature recorded near the study site at Amitori Branch, Okinawa Regional Research Center, Tokai University on ■ Materials and Methods Iriomote Island during 2009 and 2010. Study site The study area, Amitori Bay, an inlet measur- ing 2.3 km wide and 3.5 km long, is on the northwestern coast of Iriomote Island (24°24′ N, 123°46′E) in the Yeayama Islands (Fig. 1). On the western coast of this bay, Amitori Vil- lage, which was inhabited by a maximum of 200 people between the 17th century and 1971, is situated. In the areas around this village, many large-sized C. brevimanus greater than 20 mm in palm width that that were utilize the shells of edible gastropods such as the silver- mouthed turban Turbo argyrostomus that were discarded by former residents (Doi et al., 2019). Some individuals of C. brevimanus have been documented to migrate to the beach in front of the village and release larvae, but Fig. 3. Temporal changes in the monthly precipitation descriptions of the larval release are brief and (mm) recorded at a meteorological station (24°25.6′N, 123°45.9′E) on Iriomote Island during 2009 (black bars) cover limited periods of observation (Mizutani and 2010 (gray bars). Data were obtained from the Japan & Kohno, 2012; Murakami et al., 2014; Nio et Meteorological Agency website (https://www.data.jma.go.jp/ al., 2014). gmd/risk/obsdl/index.php). Sampling was conducted on an area of sandy beach near Amitori (hereinafter referred to as intervals using Thermochron SL (KN Labora- Amitori Beach) with a length of 300 m (Fig. tories, Inc.) (Fig. 2). Monthly precipitation on 1B). The monthly mean air temperature at Iriomote Island ranged from 71 to 419 mm in Amitori ranged from 18.2 to 28.8°C in 2009 2009 and 40 to 500 mm in 2010 (Fig. 3), as re- and 18.9 to 29.1°C in 2010, recorded at 10-min corded on a meteorological station (24°25.6′N, Crustacean Research 48 69 TAKETO NIO ET AL. 123°45.9′E) on Iriomote Island and available Statistical analysis on the Japan Meteorological Agency (JMA) To determine whether intervals of seaward- website (https://www.data.jma.go.jp/gmd/risk/ migration/larval release for C. brevimanus are obsdl/index.php). Astronomical tide level data correlated with lunar (29.5 days) or semi-lunar for the study site was obtained from the JMA (14.8 days) cycles or with the size of the spring website (http://www.data.jma.go.jp/kaiyou/db/ tides, circular plots of the timing of reproduc- tide/suisan/). tion with phases of the moon and with the spring-neap tidal cycles (14.8-day cycles of the Field observations difference in height between high and low tides From May to November 2009, a single re- taken over a day) were used (Collin et al. searcher walked and searched for crabs along a 2017). The number of crawling/releasing crabs 300 m long stretch of shoreline with sandy encountered on each date was plotted against beach in Amitori (Fig. 1B) every 2 h from sun- dates (degrees) with the new moon or the set to 6 am the next morning on a couple of spring tides as 0°.