Culture of fish food organisms

Unit 2 - Rotifers

2.3.Biology and life history of Rotifers
The body of rotifers is differentiated into 3 distinct parts head trunk and foot. Head carries the rotary organ corona due to which it gets its name. The retractable corona provides locomotive capabilities and whirling water movement which facilitates their nutrition. The trunk contains the digestive tract, the excretory organs and genital organs. The epidermis contains densely packed keratin proteins known as the lorica. Mastax is the characteristic organ in rotifers which help in grinding of the ingested particles. The shape of the lorica and the profile of the spines help us differentiate the different species and morphotypes. Mass production of rotifers is achieved by by encouraging them to reproduce asexually, since sexual reproduction results in male and resting eggs.

Rotifer morphology and life cycle
Rotifers are dioecious and reproduce sexually or parthenogenetically. They are sexually dimorphic, with the females always being larger than the males. The female reproductive system consists of one or two ovaries, each with a vitellarium gland that supplies the eggs with yolk. Males do not usually possess a functional digestive system, and are therefore short-lived, often already being sexually fertile at birth. Fertilisation is internal. The egg secretes a shell, and is attached either to the substratum, nearby plants, or the female's own body. A few species, such as Rotaria, are ovoviviparous, retaining the eggs inside their body until they hatch. Sessile species, however, are born as free-swimming larvae, which closely resemble the adults of related free-swimming species. Females grow rapidly, reaching their adult size within a few days, while males typically do not grow in size at all. The life span of rotifers has been estimated to be between 3.4-4.4 days at 25OC. Generally the larvae becomes adult after 0.5-1.5 days and females there after start to lay eggs approximately every four hours.