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Spawning and egg/nauplius development |
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The act of spawning or egg expulsion occurs in one to several minutes and usually occurs between 22.30 and 00.30 hours in both open and closed thelycum species. A female initially is resting on the bottom of a spawning tank. She begins to swim upward just before spawning, sometimes with rapid flexing of the abdomen. Her swimming slows, and eggs are expelled freely from the oviducts into the water. Pleopods are beating rapidly. Spawning is frequently observed at the water surface. If a female is interrupted during spawning, sometimes spawning will occur without appropriate movement of the female, leading to a pile of eggs being deposited on the bottom. Such eggs adhere together and never hatch (Bray & Lawrence, 1992). Fertilization of spawned eggs apparently occurs as the eggs are expelled. In open thelycum species, the sperm mass is external, leading to the conclusion that eggs would be exposed to sperm as the eggs brush past the sperm mass, or else the sperm matrix and sperm are dispersed into the water column at the time of spawning, requiring a mechanism for chemical attraction and fertilization to occur freely in the water column (Bray & Lawrence, 1992). Spawned eggs go through a striking process of changes immediately after extrusion, and accompanying the fertilization process. Egg diameter reported for various species is fairly consistent (e.g., 260 to 280 micron, P. japonicus, 270 to 310 micron, P. monodon). An embryonic membrane surrounds the developing embryo about 2 to 2.5 hours after spawning. Hatching occurs about 12 to 14 hours after spawning. Eggs are slightly negatively buoyant, and will stay suspended in the water column with little circulation. The first stage larva, or Nauplius I, convulses free from the egg, remains in a resting phase for about 3 to 4 minutes, and then begins sporadic locomotion. Locomotion becomes brisk after about 20 minutes, and the nauplius already displays positive phototaxis (Bray & Lawrence, 1992). | |