Reproduction In Sponges

Sponges can reproduce asexually by forming small buds that break off and live separately. The sponge shown in Figure 33-1 has many buds. During droughts or cold weather, some freshwater sponges produce internal buds called gemmules (JEM-yoolz). Each gemmule is a food-filled ball of amoebocytes surrounded by a protective coat made of organic material and spicules. Gemmules can survive harsh conditions that may kill adult sponges. When conditions improve, sponge cells emerge from the gemmules and grow into new sponges. Sponges also have remarkable powers of regeneration, the regrowth of missing cells, tissues, or organs. A very small piece of a sponge can regenerate a complete new sponge.

Sponges can also reproduce sexually. As you can see in Figure 33-2, sperm released into the water from one sponge enter the pores of a second sponge. Choanocytes in the second sponge engulf the sperm and transfer them to amoebo-cytes. Amoebocytes carry the sperm to an egg located in the mesohyl. After the egg is fertilized, it develops into a larva. A larva is an immature stage of an animal that is usually very different in form from the adult. Flagella on the larva's surface enable the larva to leave its parent sponge. Eventually, the larva settles and attaches to an object. Its cells then reorganize to form an adult sponge.

Some species of sponges have separate sexes, but in most species, each individual is a hermaphrodite (her-MAF-roh-diet). Hermaphrodites produce both eggs and sperm. Self-fertilization rarely happens in hermaphroditic species. Instead, the sperm of one individual usually fertilize the eggs of another individual. Because each hermaphrodite produces eggs, the chances of fertilization are greater than in species in which only females produce eggs. Hermaphroditism is common in invertebrates that are sessile, that move slowly, or that live in low-density populations.

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