EVERY March, when the moon is full, the beaches along the Pacific coast of southern California are silvered by the shiny bodies of grunions. These small fish strand themselves in a ribbon stretching along the water’s edge as far as the eyes can see and, as each wave breaks over the sand, it brings in a fresh cargo to lie writhing on the shore.
The females wriggle themselves swiftly into the sand by thrashing their tails until they are wedged upright with only their heads sticking out. Each male grunion selects a partner and bends himself on her deposit his sperms, or milt, as she lays her eggs. The next wave washes the pair back into the sea.
If the grunions get their timings right and the females lay their eggs at the right spot, the fertilised eggs remain undisturbed in the damp sand during the ensuing two weeks, out of reach of any sea predators, until the gravitational pull of moon and sun combine to raise the highest tide of the month. As the waves thunder across the sands of the spawning ground, their vibrations trigger the hatching of the eggs.
The grunions breeding method seems hazardous and inevitably many are left stranded above the waterline to provide an easy meal for gulls — a fate that also awaits many of the eggs and hatchlings. The present-day grunions have inherited this dangerous breeding practice from a myriad of earlier generations, but why should they have adopted it in the first place remains a mystery. Perhaps long ago there were fewer predators on the shoreline, so the eggs were safer on the beach than at sea. If so, then at that time, the risk was worth taking.
Among the blueheaded wrasses living on the western Atlantic coral reefs, there are two types of males. Large and brightly coloured males that are about 6 inches long and compete for mating sites with each other and pair up with a succession of females are in a smaller proportion to the smaller, yellow males that spawn in groups of females.
Blueheads are a wrasse species that feeds on small marine animals. The large, blueheaded breeding males actually start life as diminutive, dull-coloured females. A large male’s presence suppresses the sex change of other females, but when such a male dies, the largest of the females changes both her sex and colour, developing a bright blue head and a black-and-white collar. Females may also be stimulated to change into males if the number of females in a particular community increases greatly.
On Australia’s Great Barrier Reef, thumb-length anemone clown-fish have a different strategy. They start life as males, but change to females when they are fully grown. These fish live among the sea anemones and feed on planktons — minute plants and animals.
A family group is dominated by a large female who mates with the males of her group. Her offsprings all hatch as males, but some later develop into females. Individual offspring may split away from the group to join other shoals or to become rovers. When she dies, the largest male of her group changes sex and takes over.
As the name indicates anemone clown-fish live among the poisonous tendrills of a sea anemone which provides them protection, as well as their eggs. These fish are immune to the anemone’s stings.