Although they often lay hundreds or even thousands of eggs, some fish do build nests to protect their young. The stickleback, found in European ponds and rivers, builds a nest of plant fibres in which the male guards the eggs until they have hatched, chasing away even the female that laid them.
The most interesting habits of fishes are their parental behavior in guarding the eggs and caring the young ones. In Chondrichthyes, young ones hatch out in a fully developed condition. But, in Osteichthyes larvae hatch out from eggs and then metamorphose to young adults. In most cases these larvae are quite numerous and so chance will favor at least few of them to tide over adverse condition.
Nest and nursery building Nest building is the commonest method adopted by fishes to protect their eggs and young ones. It is exhibited mostly by fresh water fishes and also by marine fishes having demersal eggs. Nest building involves active participation of either males or females or both. The simplest form of nest building is exhibited by salmons, darters, sunfishes, cichlids, etc. Salmons select gravelly shallows of running streams as their spawning ground. Here they assemble in shoals. Female will make a nursery in the form of pit to lay eggs. After fertilization, she will cover them with layer of gravel. Similar method is adopted by Australian fresh water, Arius. In case of darters , sun fishes and cichlids males make shallow basin like dipressions on the bottom.
The male of N. American bowfin (Amia calva) constructs a crude circular nest of soft weeds and rootless amidst aquatic vegetation. Spawning takes place in the “weedy castle”. Sometimes later the young ones leave the nest in a swarm escorted by their watchful father. The male of N. American bowfin (Amia calva) constructs a crude circular nest of soft weeds and rootless amidst aquatic vegetation. Spawning takes place in the “weedy castle”. Sometimes later the young ones leave the nest in a swarm escorted by their watchful father.
The male of two-spinned stickle back builds an elaborate nest in fresh water, using twigs and weeds, fastened together by the threads of a sticky secretion from the kidney.
Nest building by female is rarely known. Female Heterotis has been shown to make nests in swamps. There are instances in which both the parents take part in nest building. Eg: Labrus Labrus.
Carrying the eggs on the body some fishes ensure protection to their eggs by carrying them safely either in the mouth or anywhere in the body. In Aspredo and Platystacus, the skin of the lower part of the female becomes smooth and spongy during breeding seasons. Fertilized eggs get attached to it.
In some fishes like Arius and Tilapia mouth-brooding or buccal incubation is a characteristic property both the male and female carries eggs and young ones in their mouth. In oral incubation, the parent will not feed until young ones hatch out. male nursery fish krutus has a cephalic horn upon which female deposits grape like egg clusters.
Keeping the eggs in brood chambers in most species of Syngnathus, Hippocampus, siphonostoma, male has a brood pouch for the deposition of eggs. In Hippocampus, some sort of” placenta” may be formed for gas exchange between the father and the developing young ones. In syngnathus, brood pouch has a highly vascular spongy lining from which the developing young ones may draw nourishment. in unique pipe fish of Indian and pacific oceans, Salenostomus the inner side of the ventral fin of female coalesces with the integument forming a large pouch for keeping the eggs.
Coiling round the eggs The British gunnel or the butter fish has a peculiar way of parental care. The female roll the eggs to a ball and then curls around them. Often male may assist her in this process. Butter Fish.
Keeping the eggs in egg capsules Some chondrichthyes, gives maximum protection to eggs by enclosing them in an egg capsule. In Scyllium, Raja etc, fertilized eggs are kept in a specially designed horny egg capsule, popularly called “Mermaid’s purse”. Egg filled capsules get attached to aquatic weeds with the help of tendril like filaments. Development is completed inside the capsule, utilizing the yolk reserve. Young ones hatch out by breaking the capsule.
Oviposition means, act of laying or depositing eggs. It is mostly exhibited by central European bitterling. In this the genital papilla of the female serves as an ovipositor. With its help eggs may be introduced to the gill chamber of a pond mussel. During this female takes to a vertical posture and spawning. Male swims around her and discharges sperms in to the mussel. Fertilization and development take place in the gill chamber and young ones leave the host later.
Several fishes provide maximum pre-natal protection to their embryos by adopting ovoviviparity. Here the development is internal and the special portion of the oviduct serves as an unspecialized uterus. A true mammalian type of placenta is absent. Nutrition is given either by yolk reserve or the uterine milk which is secreted by uterine wall. Eg: for ovoviviparous chondrichthyes are scoliodon, sphyrna, pristis, stegastoma, squalus, mustelus, myliobatis, trygon pteroplatea,etc. Eg: for ovoviviparous osteichthyes are Gambusia,poicilia,blennis,allis,zoarces,cymogaster etc.