Puneeta Pandey, Vijai Dharmamony, Yamamoto Jun, Adachi Kohsuke, Kato Yoshiki, Sakurai Yasunori
Department of Marine Bioresources and Ecology, School of Fisheries Sciences, Hokkaido University, Hakodate, Japan.
Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Japan.
PLoS One. 2017 Aug 2;12(8):e0182261. doi: 10.1371/journal.pone.0182261. eCollection 2017.
The Japanese flying squid, Todarodes pacificus, is thought to spawn neutrally buoyant egg masses that retain a specific location in the water column by floating at the interface between water layers of slightly different densities. It is important to understand the physical process that determines the vertical distribution of the egg masses to predict their horizontal drift in relation to embryo survival and subsequent recruitment. Here, mesocosm experiments were conducted in a 300 m3 tank by creating a thermally stratified (17-22°C) water column to obtain egg masses. A cage net methodology was developed to sustain egg masses for detailed observation. We measured the density of the egg masses of T. pacificus, and used this information to infer the vertical distribution patterns of the egg masses at the spawning grounds (Tsushima Strait, Japan). When measured separately, the density of the outer jelly of each egg mass was 2.7 σ units higher than that of the surrounding water. The outer jelly and the specific gravity of embedded individual eggs (~1.10) cause the egg masses to have very slight negative buoyancy relative to the water in which they are formed. Analysis of the vertical profile of the spawning ground showed that water density (σθ) increased sharply at ~30 m depth; thus, egg masses might settle above the pycnocline layer. In conclusion, we suggest that T. pacificus egg masses might retain their location in the water column by floating at the interface between water layers of slightly different densities, which happen to be above the pycnocline layer (actual depth varies seasonally/annually) in the Tsushima Strait between Korea and Japan.
太平洋褶柔鱼（Todarodes pacificus）被认为会产出中性浮力的卵团，这些卵团通过漂浮在密度略有不同的水层之间的界面处，从而在水柱中保持特定位置。了解决定卵团垂直分布的物理过程对于预测其水平漂移与胚胎存活及后续补充的关系至关重要。在此，我们在一个300立方米的水箱中进行了中宇宙实验，通过创造一个热分层（17 - 22°C）的水柱来获取卵团。开发了一种网箱方法来维持卵团以便进行详细观察。我们测量了太平洋褶柔鱼卵团的密度，并利用这些信息推断产卵场（日本对马海峡）卵团的垂直分布模式。单独测量时，每个卵团外层凝胶的密度比周围海水高2.7 σ单位。外层凝胶和其中单个卵的比重（约1.10）使得卵团相对于其形成时所处的水具有非常轻微的负浮力。对产卵场垂直剖面的分析表明，水密度（σθ）在约30米深度处急剧增加；因此，卵团可能会沉降在温跃层之上。总之，我们认为太平洋褶柔鱼的卵团可能通过漂浮在密度略有不同的水层之间的界面处来保持其在水柱中的位置，而这个界面恰好位于韩国和日本之间对马海峡的温跃层之上（实际深度随季节/年份变化）。
