Spawning and Development of T. Robustum

In the Lyall Bay and Island Bay areas T. robustum spawns from early July to late October. Egg masses are found throughout the tidal zone beneath stones, broken clay pipes, sheets of asbestos and Haliotis shells. Egg clusters appear most abundant about the upper mid-tide level although many are found further up the tide, sometimes completely out of the water for several hours during low tides. Several egg clusters were found in oyster shells located in spat collecting trays that were suspended from rafts 20 feet above the nearest portion of rocky seabed. page 5The size of the masses range from 5cm in diameter, with 1-2 hundred eggs, to about 12cm in diameter with several thousand eggs. The eggs are laid close together forming flat irregularly arranged clusters.

The large egg masses contain groups of eggs at different stages of development, suggesting that like T. capito, these clusters are the result of several different spawnings. The eggs are essentially colourless although collectively they appear pale green. The colour of the yolk (pale yellow) changes very little as development proceeds. Five separate egg masses were laid in the asbestos aquarium and in each case the male parent attended the eggs. The male T. robustum, like the parent T. capito, lay close to the eggs and created a current of water by movement of its anal and pectoral fins. Again this ensured a supply of oxygenated water and the removal of detritus. Egg masses kept without a male parent or adequate aeration suffered up to 90% mortality. These eggs also accumulated masses of minute debris which covered the entire chorion and obscured the embryo.