First deep-sea fish known to be a mouthbreeder

Most fish are broadcast spawners, casting their eggs and sperm in clouds and leaving their young to develop alone. But a tiny minority — about 2 percent — are “mouthbreeders,” keeping their fertilized eggs (and sometimes hatchlings) protected in their mouths.

Deep-sea fishes normally spawn externally, and their young migrate to more productive shallow waters before returning as adults to the food-scarce deep. But mouthbrooding is a comparatively costly investment. Some shallow water mouthbreeders eat with a mouth full of eggs, which is more difficult and costs more energy, and others abstain from eating entirely as the young develop, draining energy reserves.

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R. Singer, J. Moore, E. Stanley, NOAA Okeanos Explorer

This parazen fish (Parazen pacificus) was spotted some 500 meters deep, near Puerto Rico, by a remotely operated underwater vehicle.

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R. Singer et al/Scientific Reports 2020

Researchers counted over 500 eggs in a parazen fish’s mouth using CT scanning technology. Here, a side view of the head (left) shows a mouth full of eggs. Egg masses are colored red (right) to show their placement in the mouth.

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