Abstract
The horseshoe crab Tachypleus tridentatus is critically endangered in Japan due to rapidly decreasing numbers resulting from the loss of tidal flats and sandy beaches, and the deterioration of coastal environments. We monitored the year-round migratory patterns and residency of this species in a coastal embayment at Tsuyazaki, Japan, using acoustic telemetry. Total 20 adult crabs (15 males and 5 females) were tagged with ultrasonic transmitters and tracked during two periods (2006–2008; n = 10 and 2007–2009; n = 10). Adult crabs were more active during periods of higher water temperatures and their activity peaked in July, during the spawning period. Water temperature appeared to be one of the key factors influencing the movement patterns for the species. Moreover, the crabs tended to be more active at night than in the day. The nocturnal activity pattern was clearly evident before and during the reproductive period (May–August). Tracking data also showed that one pair-bond was maintained for a maximum of 17 days after the pair-bonded female had spawned. Overall, 11 males (73% of 15 individuals) remained in the bay area over winter, whereas three females (60% of 5 individuals) overwintered outside of the bay. Telemetry data showed that over 60% (13 of 20) of tagged crabs overwintered within the bay where there are sandy beaches, mudflats, and scattered seagrass beds. This year-round residence by adult T. tridentatus in the bay area identifies it as a critical habitat for the management of this species, regardless of life-stage. Not only is it a comprehensive management strategy that effectively reflects this species’ habitat use patterns but also its implementation, such as the establishment of a protected area, would contribute to its conservation.
Highlights
The horseshoe crab Tachypleus tridentatus is distributed throughout coastal waters from Southeast Asia to the western part of Japan [1]
Acoustic telemetry data made it possible to monitor the locations and movements of adult horseshoe crabs T. tridentatus in a coastal embayment; the data demonstrated that the majority of crabs inhabited the bay throughout the year
A previous study reported that the crabs used deeper offshore habitats (>20m depth) during winter [1], but little is known about their migration and habitat use except for along the shoreline during the reproductive period
Summary
The horseshoe crab Tachypleus tridentatus is distributed throughout coastal waters from Southeast Asia to the western part of Japan [1]. This species has been decreasing in numbers because of the loss of tidal flats and sandy beaches, and the deterioration of coastal environments in Japan, Taiwan, and Hong Kong [2,3,4]. The Japanese population is included on the Red List and in the Red Data Book of Japan as critically endangered (CR + EN) [5, 6]. During several months including the spawning period, adult and juvenile crabs are often observed in the shallows and intertidal areas. An understanding of their movement patterns and habitat use during the remainder of the year is largely lacking because of the difficulty in defining fine-scale and long-term movement patterns of individuals underwater
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