When zebrafish eggs are laid, they are surrounded by a tough chorion. Sperm are unable to get through this chorion and must find the tiny tunnel called a micropyle that provides a channel to the egg cell membrane. This channel is so tiny that only one sperm is able to fit through at a time. The micropyle most likely.

Aids in preventing polyspermy.

Explanation:

Polyspermy is a process that involves multiple fertilization of an egg by multiple sperm, which leads to the embryo inheriting multiple centrioles from the parents. Polyspermy is often considered lethal, and there are several mechanisms in most animals that prevents multiple fertilization of eggs. One of such mechanisms is the possession of a micropyle by zebrafisg eggs which limits the chances of multiple fertilization, as only one sperm is able to pass through the micropyle at a time. This micropyle most likely aids in preventing polyspermy.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.