Final answer:
Male crustaceans use their first two pairs of swimmerets to transfer sperm directly to female seminal receptacles, ensuring successful fertilization. This differs from external fertilization common in other aquatic species, where sperm is released into the water near the eggs.
Step-by-step explanation:
First 2 pair of swimmerets in male crustaceans form a channel to transfer sperm to a female's seminal receptacles. In multicellular animals, one common mechanism for sperm transfer occurs in aquatic environments, where males release sperm into the water near the eggs released by females. This is seen in a range of marine species and amphibians. Unlike these, crustaceans have specialized appendages - the swimmerets - which in males, are modified to aid the direct transfer of sperm to the female during copulation. This ensures a higher chance of successful fertilization by placing the sperm closer to the female's eggs.
Sperm Transfer in Marine Animals
In marine animals like molluscs and fishes, as well as amphibians, external fertilization is common where the male releases sperm into the water. Courtship behaviours often precede this to sync the timing. For instance, some fish exhibit courtship behavior that allows females to select a particular male, triggering a tightly concentrated release of eggs and sperm (spawning).
Internal Fertilization in Crustaceans
Certain crustaceans use the first two pairs of swimmerets for internal fertilization. Male crustaceans transfer sperm into the female seminal receptacles using these specialized structures, adapting their reproductive method to a more direct approach, unlike the external fertilization seen in other marine species.