— William Shakespeare, Pericles, act 2, scene 1
Many invertebrates do not recognize individuals of their own kind as anything more than food, and so a significant amount of cannibalism takes place within invertebrate groups like mollusks (clams and their pals), insects, and arachnids (spiders and scorpions). Thousands of aquatic invertebrates like clams and corals have tiny, planktonic eggs and larvae, and these are often a major food source for the filter-feeding adults. Since the planktonic forms often belong to the same species as the adults feeding on them, by definition this makes filter-feeding a form of indiscriminate cannibalism.
Although both fertilized and unfertilized eggs are eaten by thousands of species, the practice of consuming conspecific eggs appears to have led to the evolution of an interesting take on the concept of the “kids’ meal.” As the name implies, trophic eggs, produced by some species of spiders, lady beetles, and snails, function solely as food. These prepackaged meals often outnumber the fertilized eggs in a given clutch—a fact exemplified by the results of an observational study on the rock snail (Thais emarginata). This species commonly lays a clutch of around 500 eggs but averages only 16 egg-munching hatchlings.
The black lace-weaver spider (Amaurobius ferox) behaves similarly: one day after their spiderlings hatch, new mothers lay a clutch of trophic eggs, which are doled out to their hungry babies. The trophic eggs last for three days, after which the spiderlings are ready for their next stage of development, but in this case, the “smaller gets eaten” rule gets turned on its head.
Arthropods like spiders, insects, and crabs are characterized by having their skeletons on the outside of their bodies. To grow in size, they undergo a regular series of molts, during which their jointed cuticle or exoskeleton is shed and replaced by a new skeleton arising from beneath the old. After their first molt and after the trophic eggs have been consumed, black lace-weaver spiderlings are too large for their mother to care for, though they are in dire need of additional food. In an extreme act of parental care, she calls the babies to her by drumming on their web and presses her body down into the gathering crowd. The ravenous spiderlings swarm over their mother’s body. Then they eat her alive, draining her bodily fluids and leaving behind a husklike corpse.
Insects undergoing pupation, the quiescent stage of metamorphosis associated with the production of a chrysalis or cocoon, are also vulnerable to attack from younger conspecifics. The ravenous larva of the elephant mosquito (Toxorhynchites) not only consumes conspecific pupae, but also embarks on a killing frenzy, slaying but not eating anything unlucky enough to cross its path. The reason for this butchery appears to be the elimination of any and all potential predators before the larva enters the helpless pupal stage itself.
In some snail species, cannibalistic young transform into vegetarian adults. In one food preference test, hatchlings from an herbivorous snail fed on conspecific eggs exclusively (even when offered lettuce); four-day-old individuals ate equal amounts of eggs and lettuce; and 16-day-old individuals preferred the veggies. When snails older than four weeks of age were denied the lettuce option, they starved to death, even in the presence of eggs. The reason for this gradual transition in feeding preference appears to be that these snails, like other herbivores (from termites to cows), require a gut full of symbiotic bacteria before they can digest plant material. Since newly hatched snails have no gut bacteria, they’re compelled to consume material that can be digested, even if this turns out to be their own unhatched siblings.