The Price of Motherhood: Deepwater Fish Regenerate Their Tails to Increase Reproductive Fitness

Article: Triay-Portella, R., González, J. A., & Pajuelo, J. G. (2019). Caudal region regeneration in a natural population of the morid fish Physiculus cyanostrophus in the tropical eastern Atlantic ocean. Deep Sea Research Part I: Oceanographic Research Papers.

What happens if a fish gets its tail bitten off? Well, the attacker enjoys a snack with the promise of an entire meal. But even if the now-tailless fish manages to get away and not become dinner, it must survive its grievous injury.

Such sublethal attacks can claim up to 45 percent of the body length. But many fish species actually stand a chance of recovering, as they can regenerate large portions of their bodies, and even internal organs. This amazing ability is particularly useful for the prey species, who often get nibbled on by predators.

Second chance at growing a tail

Physiculus cyanostrophus counts itself among the fortunate regenerative species. This deep-sea fish lives in tropical and subtropical waters, feeding on the sea bottom. The actual regenerative process in this species is imperfect, as the fish never recover the lost spinal cord, regenerating tail tissue called “pseudo-hypural plate” instead.

Luckily, Physiculus cyanostrophus can survive a loss of up to 28 vertebrae and regenerate missing tails, even though each fish is only 2-3 centimeters long.  This impressive recovery rate gives the fish an upper hand in living a long life (up to 20 years!).

Living longer also may mean leaving more offspring. On the other hand, a fish that has to recreate one-third of its body might let egg-laying slide down on its list of priorities.

Physiculus is a genus of morid cods. Depicted is Physiculus fulvus, a species related to Physiculus cyanotrophus. Photo: Wikimedia Commons Open Access

Scientists from the University de Las Palmas de Gros Canaria, Spain, set out to investigate how regenerating the back, or caudal, body impacts reproductive fitness of this deep-sea fish species.

To collect the fish, the scientists traveled to the islands of Boa Vista of the Cape Verde archipelago off the West Coast of Africa. They set bottom traps and semi-floating traps between 66 and 458 meters deep in the water, trapping 917 fish.

The researchers counted vertebrae in the fish tails and found that 27 percent of all trapped fish had previously regenerated their tails, as indicated by missing vertebrae. Among the oldest fish, who had probably fled from lots of predators in their lifetimes, 67.5 percent had regenerated tails.

Making fish babies

When it comes to procreation, some species, like humans, favor offspring fitness by having only one, but a very large offspring at a time. Other species, including Physiculus cyanostrophus, produce lots of small offspring. These species favor maternal fitness, as mothers’ survival and well-being comes first.

Small fish are always in danger of having their tails bitten off by a predator, such as by this hungry eel. Photo by David Clode on Unsplash

Fish reproductive abilities are described by total fecundity, or the number of eggs that ripen in a female’s ovaries between two spawning periods, and batch fecundity, or how many eggs a female can lay in a single spawning. As a female fish prepares to spawn, she hydrates some egg cells, or oocytes, to increase their volume and help future embryos develop correctly.

The researchers calculated total fecundity in the trapped Physiculus cyanostrophus females by counting all the eggs in the ovaries, and batch fecundity by only counting hydrated oocytes. They found that among older fish females, the fish with regenerated tails had decreased total and batch fecundity. In other words, older females may prioritize tail regeneration over spawning lots of offspring.

Brand-new tails for expecting mothers

Regeneration of a large portion of the body demands lots of energy. It seems that injured Physiculus cyanotrophus females allocate their resources to regrowing their tails and staying alive, rather than to developing eggs.

The study authors interpreted this finding as a benefit to maternal fitness of the Physiculus cyanostrophus species. Even with lower egg-laying abilities, more surviving mothers still mean more potential spawning events, and more fish offspring in the long run.

Mothers with regenerated tails may also contribute to the fitness of their species in another way, as they may be genetically predisposed to successful regeneration. If they survive and procreate, their genes get passed on to the offspring. When the time comes for their baby fish to flee a hungry predator, they may then have an easier time regenerating their own tails.

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