Anisogamy (also called heterogamy) refers to a form of sexual reproduction involving gametes of different sizes. The smaller gamete is considered to be male (sperm cell) whereas the larger gamete is regarded as female (egg cell).
There are several types of anisogamy. Both gametes may be flagellated and thus motile. Conversely, neither of the gametes may be flagellated. This situation occurs for example in some algae and plants. In the red alga Polysiphonia, large non-motile egg cells are fertilized by small, non-motile spermatia. In flowering plants, the gametes are non-motile cells within gametophytes.
The form of heterogamy that occurs in animals is oogamy. In oogamy, a large, non-motile egg cell (ovum) is fertilized by a small, motile sperm cell (spermatozoon). The large egg cell is optimized for longevity, whereas the small sperm cell is optimized for motility and speed. The size and resources of the egg cell allow for the production of pheromones, which attract the swimming sperm cells.