There is no generally accepted definition of algae. It ranges from single cell diatoms to giant kelp which grow to 50 meters tall. Recent estimates claim nearly 72,500 species worldwide.

Crabs are decapods (have 10 feet, 2 of which are claws and some function as mouthparts), with a short tail and thick exoskeleton. There are about 6,800 species.

These marine animals have a leathery skin and elongated body. There are about 1,700 species, many of which are consumed by humans.

About 1,500 species of starfish exist. Starfish or sea stars are only found in a marine environment and very few stars are suitable for a home aquarium.

There are over 2000 shrimp species. They have a laterally compressed body divided into three parts: head, thorax, and abdomen. The head and thorax are fused and covered by a shell called the carapace which encloses the gills. Shrimp are great scavengers and are useful for keeping our aquarium free of uneaten food.

There are thought to be between 35,000 to 150,000 species of snails. Other than insects snails make up the largest animal group.

There are about 5000 species of sponges. Sponges with calcareous spicules are in the class Calcarea. Most other sponges are in the class Demospongiae. These are typically siliceous spicules or protein.

Urchins are small, spiny, globular animals of the class Echinoidea. About 950 species are known.