Biology of Finfish and Shellfish (2+1)

4.8.4. Different types of fish eggs

Pelagic eggs

Isolated eggs (mostly) - The spawned eggs are isolated, not forming any mass.

Agglutinated eggs (Lophiidae) - The spawned eggs are embedded in a gelatinous ribbon/ballon, or agglutinated to each other forming a mass

Pelagic eggs: The pelagic eggs of most species are small in size, measuring about 0.7 mm to 1.5 mm in diameter. A few species have larger eggs between 1.6 mm and 2.6 mm in diameter. All pelagic eggs are transparent and are practically spherical, except for those of anchovies which are oblong (longer than broad). Occasionally eggs are found to be slightly ovoid.

Pelagic eggs are floating type, smaller in size compared to demersal eggs. These eggs do not have adhesive membrane. They are buoyant; the buoyancy is maintained by single oil globule. If the oil globule is not there, high percentage of water is present which helps in floating. During floating stage, dispersion of eggs takes place. The pelagic eggs are subjected to high mortality mainly due to two factors. i. Predation, ii. Eggs are exposed (carrying) to unfavourable conditions. But this is compensated by increased fecundity and protracted spawning season.

Demersal eggs

Adhesive eggs (Exocoetidae, Gobiidae) - The spawned eggs adhesive to substratum with adhesive egg membrane or filaments

Non –adhesive eggs (Salmonidae)

The demersal eggs are generally larger than pelagic eggs which may be laid in masses or singly. These eggs are heavy or dense. Since they are heavy, they link to the bottom. The eggs are provided with adhesive membrane. They stick on to other objects with filamentous structure. Normally there is no relation between habitat and type of eggs produced. i.e. pelagic fishes can produce demersal eggs and demersal fishes can produce pelagic eggs. (Generally most common pelagic food fish have pelagic eggs)

The pelagic sardine produces pelagic eggs, whereas herring is a pelagic fish but the eggs are demersal. Similarly angler fish which is a demersal fish but produce pelagic eggs. Deep sea wolf herring is a demersal fish and produces demersal eggs.