For many ocean invertebrates, the first stage of life occurs as tiny larvae in the plankton. The toughness of the planktonic larval life has caused many scientists to wax poetically, as we tend to do on subjects of invertebrates. As noted by Emery in 1973, these larvae face a “wall of mouths” ready to consume them. More recently Miller and Morgan (2014) state the larvae navigate a “gauntlet of planktonic predators.” A host of adaptations by both the larvae and adults try to change the rules of this predation game. Some larvae have chemicals that make them unpalatable. Others possess spines that ward off predators. Some larvae are able to sink or flee in the presence of a hungry mouth. Adults do their part by synchronizing larval release during times of less predation or by moving to new areas with fewer predators to release larvae.
But sometimes the hungry mouths that larvae need to avoid are their own parents and relatives.
Many marine invertebrates suspension feed, basically straining plankton snacks out of the ocean. But what is a good parent to do when their own young gets mixed into the food bowl? Yellow shore crabs and European green crabs reduced their eating when they recently released young. Note I say reduced and not just stop. A hungry parent still needs to eat. Female crabs that recently carried eggs, i.e. ovigerous, ate only 25-30% of their own larvae. Non-ovigerous…well they got their ‘eat on’ consuming about 90% of larvae. However, a year later in 2007 there was little difference between ovigerous and non-ovigerous females in cannibalism of their own larvae. The authors hypothesized and then tested the idea that hungrier females would consume more of their own larvae. Twenty-two days of starvation increased the cannibalism of larvae by 30%!
So to sum up, larvae are fine just along as the parents aren’t too hungry. Miller, S., & Morgan, S. (2014). Temporal variation in cannibalistic infanticide by the shore crab: implications for reproductive success Marine Ecology DOI: 10.1111/maec.12172
Emery A.R. (1973) Comparative ecology and functional osteology of fourteen species of damselfish (Pisces: Pomacentridae) at Alligator Reef, Florida Keys. Bulletin of Marine Science, 23, 649–770.