Complex vertical migration of larvae of the ghost shrimp, Nihonotrypaea harmandi, in inner shelf waters of western Kyushu,Japan |
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| Authors: | Akio Tamaki Sumit Mandal Yoshihiro Agata Ikumi Aoki Toshikazu Suzuki Hisao Kanehara Takashi Aoshima Yasushi Fukuda Hideshi Tsukamoto Tetsuo Yanagi |
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| Institution: | 1. Faculty of Fisheries, Nagasaki University, Bunkyo-machi 1-14, Nagasaki 852-8521, Japan;2. Graduate School of Science and Technology, Nagasaki University, Nagasaki 852-8521, Japan;3. Kyushu Lutheran College, Kumamoto 860-8520, Japan;4. Yuge National College of Maritime Technology, Yuge, Kamijima 794-2593, Japan;5. Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan |
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| Abstract: | The position of meroplanktonic larvae in the water column with depth-dependent current velocities determines horizontal transport trajectories. For those larvae occurring in inner shelf waters, little is known about how combined diel and tidally-synchronized vertical migration patterns shift ontogenetically. The vertical migration of larvae of Nihonotrypaea harmandi (Decapoda: Thalassinidea: Callianassidae) was investigated in mesotidal, inner shelf waters of western Kyushu, Japan in July–August 2006. The larval sampling at seven depth layers down to 60 m was conducted every 3 h for 36 h in a 68.5-m deep area 10 km off a major coastal adult habitat. Within a 61–65-m deep area 5–7.5 km off the adult habitat, water temperature, salinity, chlorophyll a concentration, and photon flux density were measured, and water currents there were characterized from harmonic analysis of current meter data collected in 2008. The water column was stratified, with pycnocline, chlorophyll a concentration maximum, and 2% of photon flux density at 2 m, recorded at around 22–24 m. The stratified residual currents were detected in their north component, directed offshore and onshore in the upper and lower mixed layers, respectively. More than 87% of larvae occurred between 20 m and 60 m, producing a net onshore transport of approximately 1.3 km d−1. At the sunset flooding tide, all zoeal-stage larvae ascended, which could further promote retention (1.4-km potential onshore transport in 3 h). The actual onshore transport of larvae was detected by observing their occurrence pattern in a shallow embayment area with the adult habitat for 24 h in October 1994. However, ontogenetic differences in the vertical migration pattern in inner shelf waters were also apparent, with the maximum mean positions of zoeae deepening with increasing stages. Zoeae I and II performed a reverse diel migration, with their minimum and maximum depths being reached around noon and midnight, respectively. Zoeae IV and V descended continuously. Zoeae III had behaviors that were intermediate to those of the earlier- and later-stage zoeae. Postlarvae underwent a normal diel migration (nocturnal ascent) regardless of tides, with the deepest position (below 60 m and/or on the bottom) during the day. These findings give a new perspective towards how complex vertical migration patterns in meroplanktonic larvae enable their retention in inner shelf waters before the final entry of postlarvae into their natal populations. |
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| Keywords: | Japan Kyushu Tachibana Bay |
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