Differences Between Sponges and Mollusks

Sponges, known scientifically as phylum Porifera, and mollusks, or phylum Mollusca, are both invertebrates, meaning they have no back bones. Mollusks are the second most numerous of the invertebrates, behind arthropods, with over 50,000 known species, while sponges have about 5,000 species. Both are mainly marine animals, with few freshwater species, but that is basically where the similarities end.

  1. Diversity

    • Mollusks, with seven different classes of species, are much more diverse than Porifera, which have only three. While sponges range in size, shape and color, recognizing them as members of the Porifera phylum is much easier than recognizing the vast array of mollusks as such. Mollusks range from tiny, hard-shelled chitons to clams, scallops, squid and octopuses. Sponges cannot live on land, while mollusks such as snails and slugs do. Sponges cannot be eaten, while many species of mollusks can.

    Movement

    • Sponges do not move, but lay mainly on the bottoms of the oceans, attached permanently to a solid location. Some mollusks, especially the bivalves, like clams and mussels, do not move, and either burrow into a sandy bottom, or attach themselves to something, as sponges do. Others, like scallops, do not attach themselves to anything; they can move, if need be, by squirting water through their mantle. Some mollusks, like chitons, move extremely slowly, moving maybe only 10 feet per year. The fastest moving of the mollusks are the cephalopods, such as octopuses and squid, which move quickly by jet propulsion.

    Body

    • The bodies of sponges consist mainly of two cell layers surrounding a large central cavity called a spongocoel. The body wall has many holes that allow water, which carries nutrients and oxygen, into the spongocoel, where different cells process them. Sponges have no mouths, brains, organs or nervous systems. Mollusks have three main body parts. The head contains the brain and nervous system, which is as developed as those of fish. The body contains the internal organs, including those for digestion, a heart and kidneys. The "foot" allows for motion. Many, but not all mollusks have shells. Sponges only eat by filtration, while mollusks eat with mouths or through filtration.

    Reproduction

    • Sponges can reproduce either asexually, meaning they make clones of themselves, or sexually with a partner. Sponges have no determined sex and can play either a male or female role. In sexual reproduction, sperm is released by the male and taken in by the female, the same way food is. After internal fertilization, larvae is released and floats around for a few days before settling on the bottom and growing. Mollusks mostly have separate sexes, but snails can evolve into hermaphrodites. Most mollusks spawn sperm and eggs into the water, followed by a larvae stage, where they swim around for a short period then settle and mature. Male squid produce a special arm that transfers the sperm to the female, which lays eggs that produce larvae.