Cannibalism: A Perfectly Natural History(10)

At its most extreme, reproductive success in many fish species depends on a romantic-sounding technique known as broadcast spawning, during which females can release millions of eggs, while males simultaneously release clouds of sperm (milt). The end result is that some of the eggs get fertilized. Conceptually, given our own reproductive behavior, one might be misled into thinking that broadcast spawning is an inefficient mating technique. The bottom line, though, is that it works, as do similar variations on this theme employed by many amphibians and invertebrate species alike. Such reproductive strategies are successful because the vast number of eggs released offsets the low probability that any single egg will develop into a mature individual. Along those lines, scientists estimate that for every million eggs produced by an Atlantic cod (Gadus morhua), approximately one egg will result in an adult fish. Partially compensating for these lottery-like odds is the fact that each female produces between four and ten million eggs in a single spawning. On a related note, while this is a remarkable number of potential offspring for most vertebrates, it’s something akin to sexual dysfunction in the ocean sunfish (Mola mola), a strange-looking, open-ocean species that can broadcast 300 million eggs in a single spawning—a vertebrate record.

But it’s not just the abundance of eggs and young that makes fish such a popular menu item for members of their own species. Many terrestrial vertebrates produce few or even a single offspring, and most of these newly born or newly hatched individuals already exhibit considerable body size. In many fish species, the extraordinary number of eggs produced imposes a limit on their size, and so a full-grown cod might be a million times (or six orders of magnitude) larger than its own eggs. This fact goes a long way to explain why the majority of them exhibit about as much individual recognition of their offspring as humans do for a handful of raisins. Fish eggs, larvae, and fry (i.e., young fish) are vast in number, minute in size, and high in nutritional value. This makes them an abundant, nonthreatening, and easily collected food source. It’s also why ichthyologists consider the absence of cannibalism in fishes, rather than its presence, to be the exceptional case.

Although not quite as infrequently practiced as it is among the invertebrates, parental care occurs in only around 20 percent of the 420 families of bony fishes (a group composed of nearly all living species except sharks and their flattened relatives, the skates and rays). The primary reason for this trend can be explained by the fact that the natural world is full of tradeoffs. Here the tradeoff works like this: Since females expend a tremendous amount of energy producing eggs (sometimes millions of them), they can’t afford to expend much energy caring for them or their young when they hatch. For this reason, the eggs and fry of most fish species exist in dangerous environments inhabited by a long list of potential predators, including conspecifics. But even in the 90 or so piscine families where parental care does occur, filial cannibalism is an extremely common practice, and here the primary reason has to do with who is doing the babysitting.

Among the land-dwelling vertebrates, females are the principal caregivers, while males take on support roles or simply make themselves scarce. In bony fishes that guard their own eggs, though, it’s usually the males who are involved, undertaking these chores at nests otherwise known as oviposition sites. These can range from slight depressions in the substrate, to rocks, plants, and other materials to which the sticky eggs (generally numbering in the hundreds) adhere in discrete clumps. The male guardians often wind up consuming some of the eggs (partial filial cannibalism), and sometimes all of them (total filial cannibalism).

One reason that male fish engage in this seemingly counterproductive behavior is that generally, they have much less invested in the brood than do females. It is less costly to produce a cloud of sperm than it is to produce, carry around, and distribute an abdomen full of eggs. Furthermore, with their ability to search for food seriously constrained by caregiving duties, males are forced to undertake at least some degree of fasting. This practice decreases their overall physical condition and thus the likelihood of future reproductive success. By consuming a portion of their own brood, males can increase the chances that they’ll survive and produce additional offspring. New eggs are consumed more often than older eggs because there has been less parental investment in maintaining them.

In some examples, though, the loss of eggs from an oviposition site is not the fault of a hungry male guardian. Unrelated conspecific males regularly raid nests in order to consume or steal eggs. Egg theft can be explained by the preference for some females to spawn at sites already containing eggs, even if they’re not hers. In these instances, once a female deposits her own clutch, the male will selectively eat the eggs he previously stole and deposited there.

While we’re on the topic of parental care in fishes, mouthbrooding cichlids deserve a brief mention, if only because they serve to strengthen the often tenuous link between a mouthful of kids and lunch. Mouthbrooding occurs in at least nine piscine families, most famously in the freshwater Cichlidae. Cichlids, especially the African varieties, are extremely popular with aquarium keepers, as well as connoisseurs of tilapia—the Spam of gourmet fish. With more than 1,300 species, cichlids have evolved extremely specialized lifestyles that serve to reduce competition with related species living in the same area.

Mouthbrooding is a common form of behavior in cichlids. Typically, it refers to post-spawning behavior in which parents (usually females) hold their brood of fertilized eggs inside their mouths until they hatch and sometimes even after that. This provides the eggs and fry with a haven from predators, a point commonly portrayed in crowd-pleasing nature videos that depict young fish darting back into their parent’s mouth at the first sign of danger. Conspicuously missing from these lighthearted reports is the fact that parents holding a mouthful of eggs usually eat a considerable portion of them, and sometimes the entire brood. Also destined for the digital equivalent of the cutting room floor are shots showing male cichlids fertilizing the eggs in the females’ mouths, always a difficult topic to explain during family TV time.