Observations on the spawning behavior, egg masses and paralarval development of the ommastrephid squid Todarodes pacificus in a laboratory mesocosm

Abstract

The spawning behavior of ommastrephid squids has never been observed under natural conditions. Previous laboratory observations of Japanese flying squid (Todarodes pacificus) suggest that prespawning females might rest on the continental shelf or slope before they ascend above the pycnocline to spawn, and that the egg masses might settle in the pycnocline. Here, two mesocosm experiments were conducted in a 300 m 3 tank that was 6 m deep to investigate this hypothesis. In the first experiment, a thermocline (2.5-3.5 m) was established in the tank by creating a thermally stratified (17-22°C) water column. In the second experiment, the temperature was uniform (22°C) at all depths. Prior to spawning, females did not rest on the tank floor. In the stratified water column, egg masses remained suspended in the thermocline, but in an unstratified water column, they settled on the tank bottom, collapsed and were infested by microbes, resulting in abnormal or nonviable embryos. Eleven females spawned a total of 18 egg masses (17-80 cm in diameter), indicating that females can spawn more than once when under stress. Paralarvae hatched at stage 30/31 and survived for up to 10 days, allowing us to observe the most advanced stage of paralarvae in captivity. Paralarvae survived after consumption of the inner yolk, suggesting they might have fed in the tank.

Seasonal Habitat Patterns of Japanese Common Squid (Todarodes Pacificus) Inferred from Satellite-Based Species Distribution Models

“…The pattern of utilization could be potentially linked to the preferences of the squid in searching for the optimal spawning and foraging grounds. Following the reproduction hypothesis for T. pacificus, adult squid generally moved to the shallow continental shelves and slopes (100-500 m) for spawning [1,43]. However, during summer feeding migration (June-August), the squid undergoes a northward movement into deeper waters of the Sea of Japan.…”

“…Off the southern part of the Sea of Japan, extending towards the East China Sea in winter, SST values in this area ranged from 12.7 • C -23.2 • C. Within this observed SST range, SST from 15 • C-18 • C were found favorable for spawning [41]. Moreover, results from laboratory studies of reared individuals showed that normal embryonic development occurred in temperatures between 14 • C and 26 • C, with the highest survival rates from 14.7 • C and 22.2 • C [42,43]. Model-derived SST ranges inferred from our study ( Figure 3A) were comparable with these reported values, albeit our results covered a broader SST limit than the latter.…”

Structure and properties of the egg mass of the ommastrephid squid Todarodes pacificus

“…The water-soluble mucin-type glycoprotein component of the mucosubstance is thought to generate the density and viscosity of the outer jelly of the egg mass [ 18 – 21 ]. The jelly also acts as a physical barrier between the eggs and the surrounding water, protecting the eggs from infestation [ 13 , 14 , 18 ].…”

“…The interior portion of the egg mass where eggs are embedded homogenously is a mucous matrix that is predominantly formed from water and an oviducal gland secretion [ 13 , 14 , 20 ]. The salt-soluble components in the oviducal gland secretion induce chorion expansion and perivitelline space formation, which are necessary for the normal development of the squid embryo [ 22 – 24 ].…”

“…Todarodes pacificus is a commercially and ecologically important ommastrephid species distributed around Japan [ 25 , 26 ]. Their nerito-oceanic distribution, and the relatively low effort to collect and maintain them live in land-based laboratories, made them an excellent model animal in several experiments focused on egg masses [ 4 , 13 , 14 , 20 , 27 ]. Following the generalized pattern for any ommastrephid squid, the “reproductive hypothesis” proposed for T .…”