Development of a Fish Egg

Fish begin life inside an egg. As we have seen, for many coral reef species the exact moment of fertilization occurs in the water column amid a swarm of predators — not the easiest start in life! From this point on the pace of development is relentless; eggs are an excellent and nutritious food source for a huge number of different predators and it is a race for each developing embryo to hatch before it is found and eaten.

How long does it take for an egg to hatch?

There is quite a range of hatching times among coral reef fishes, but they fall into two basic categories: fish that spawn in the open waters produce eggs that hatch rapidly, whereas the eggs of demersal spawners can take several days. For example, the eggs of larger, pelagic, broadcast spawners, such as queen angelfish, hatch in 15 to 25 hours depending on temperature. By contrast the comparatively large eggs of demersal-spawning clownfish take around one week to hatch in some species, again depending on temperature.

Why does the hatching time depend on temperature?

Almost all fishes are poikilothermic — in simple terms, cold-blooded — so their body temperature, and therefore their metabolic rate, depends on the temperature of the water they live in. Inside the egg, the rate of development depends on water temperature in the same way. Within the normal temperature range, the warmer the water, the faster the egg develops.

How does predation affect hatching times and numbers?

Predation on the free-floating eggs in the plankton layer of species such as angelfish is extremely high. Each day, about a third of all the eggs are eaten. Over evolutionary history, fish have responded to this by laying more and more eggs, and many broadcast-spawners produce tens or even hundreds of thousands of tiny eggs. The eggs of Centropyge angelfish are less than an inch in diameter. Even in such large numbers, the eggs themselves are defenseless.

Although a newly hatched larva could scarcely be described as well protected, it can at least move around and its ability to avoid predators improves with every passing day. Therefore, rapid hatching is likely to be an adaptation to reduce the risks of being eaten. Thanks to the attention of their parents, egg mortality is much lower in demersal spawners, so they develop at a more leisurely rate.

What's going on inside the egg during development?

Each egg is a complex chemical mix of proteins, fats, and sugars, plus a genetic code to fit them together, all contained within a protective, but permeable, shell. Fertilization is the starting point for an incredibly frantic period of activity in which a single cell develops into a functioning animal in mere hours. At first the cells divide to produce identical copies of each other — one becomes two becomes four becomes eight and so on.

Eventually differentiation occurs — cells are guided by genes to form specific tissues and organs and the nervous system and the brain develop. Shortly before hatching, tiny muscles are twitching, the heart is beating, and eyes are clearly visible. The embryo is almost ready to hatch. It is curled tightly around the yolk sac, so the egg now offers little room for further growth. The next stage of life occurs in open water.