Abstract
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.
Highlights
Following egg maturation, the eggs of female squids belonging to the family Ommastrephidae accumulate in paired oviducts [1]
Egg masses were obtained from captive mature T. pacificus females that had been maintained in a large experimental tank (10 m [length] × 5 m [width] × 6 m [height]; volume = 300 m3) located at Hakodate Research Center for Fisheries and Oceans (HRCFO), Japan, from September to October 2015
The jelly substance that was collected from the broken egg masses was devoid of eggs, indicating that, once broken, the eggs leaked out into the surrounding water
Summary
The eggs of female squids belonging to the family Ommastrephidae accumulate in paired oviducts [1]. Captive experiments [4, 7,8,9,10,11,12,13,14] and observations under natural conditions [12, 15,16,17] have substantially advanced our knowledge on egg mass characteristics. The researchers conducted captive experiments on the egg masses of the ommastrephid Illex illecebrosus and reported that the initial density of the egg mass was 0.03 σt units higher than the water from which it was collected. This difference made the egg mass slightly negatively buoyant relative to the water in which it was spawned [9]. Since density depends on temperature and salinity, the subsequent sinking and settling depth (isopycnic level) of the egg mass should be determined by the rate of temperature and ionic equilibrium [9]
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