Onset of breeding in most animals depends on: (a)the time needed to grow to a
certain size and for reproductive organs to mature (see Todd, 1978, for the opisthobranch
Onchidoris); and then (b) environmental cues such as changes in temperature,
daylength and, in the sea, aspects of lunar-tidal regimes which trigger maturation of
gametes, mating and spawning (see Giese & Kanatani, 1987, for invertebrates generally;
Hadfield & Switzer-Dunlap, 1984, for opistlaobranchs). The annual predatory
snail Retusa obtusa (Montagu) in the Forth estuary hatch at