Crabs, Shrimp, and Relatives (Phylum Crustacea)
This enormous phylum of animals contains at least 50,000 species, with many more undoubtedly waiting to be discovered. Although the vast majority dwell in marine waters, there are numerous freshwater species and many that are completely terrestrial. The diversity of these creatures is truly amazing, in terms of lifestyles and appearances. Many are microscopic, while the largest, the giant spider crab of Japan, has legs that span a distance of over 10 feet. A large number are of great interest to both freshwater and marine hobbyists.
Like most invertebrates, crustaceans are housed in a protective exoskeleton that is composed of various calcium salts and is shed as the animal grows. The tail, or telson, of crayfish, lobsters, and shrimp is hinged and serves as an escape mechanism. When threatened, the tail is thrust forward, creating a powerful current of water that propels the animal backwards, in the opposite direction that the predator might expect it to go.
The hard exoskeleton of crustaceans protects them from many predators, but it must be periodically shed. Immediately after shedding, the animal is very soft to the touch and vulnerable to attack. Shedding generally occurs at night, and hardening time varies. Be sure to provide your crustaceans with secure hiding spots and water of an appropriately high pH.
The brachiopods are considered to be the most primitive of the crustaceans. Their members include animals such as daphnia, brine shrimp, and copepods, all of which are extremely valuable food sources for captive and free living fish. Despite their classification as the least advanced of the crustaceans, most of these tiny creatures are quite successful and have colonized in a wide variety of extremely inhospitable environments. Brine shrimp, for example, thrive in highly saline waters that few other creatures can tolerate. Their tiny eggs survive in dried form for years and are shipped all over the world to be hatched by aquarists and used as food for tiny fish fry.
Krill are small, shrimplike crustaceans that float about in uncountable billions in the open seas. They are the primary food for a number of large creatures, including certain whales, basking sharks, and manta rays, and form the basis of the food chain in many of the world's oceans.
It is among the decapods that you find the crustaceans most of interest to both freshwater and marine hobbyists, as well as some of the world's most commercially important food animals. Decapods have much to recommend them as aquarium subjects, including the fact that they are active by day, have easily satisfied appetites, display spectacular coloration, and have fascinating habits. Many are, however, quite aggressive toward their own kind and are efficient predators of a number of other invertebrates and fish.
A vast array of shrimp is suitable for marine and freshwater aquariums.
Many have evolved quite unique lifestyles, such as living commensally with sea anemones and other sessile invertebrates. Other species clean parasites and debris from the scales and even from within the mouths of fish that unhesitatingly consume other species of shrimp of the same size as the “cleaners.”
Among the lobsters and crabs is an enormous range of creatures suitable for maintenance in aquariums. They too have evolved a nearly unbelievable range of appearances and lifestyles. Crabs are to be found living in association with the sea vents 1.7 miles below the surface of the ocean, as well as in completely terrestrial habitats. Some species camouflage themselves with debris from their environments, while others store food on their carapaces or in empty mollusk shells, which they use as homes. One species, commonly known as the coconut crab, grows to a massive size and is reputed to be capable of opening coconuts with its powerful claws.
Many crab and lobster species engage in extensive migrations and complicated mating displays. Some release sperm and eggs into the sea and leave the young to their fates, while in other species the females hold the eggs below the telson and protect them during their development.
The larvae of most crabs and lobsters are free swimming and undergo several distinct developmental stages before reaching the familiar adult form. So bizarre and different in appearance from the adults are immature crabs that many were formerly classified as entirely different species.
Crayfish, the freshwater relatives of the crabs (one family of which also dwells in freshwater) and lobsters, have successfully colonized in an extremely wide variety of habitats. Some species build extensive tunnel systems in wet meadows, while others grow quite large and are formidable predators within their aquatic habitats. A number of species are farmed commercially for food. Escapees from farms and bait buckets (crayfish are widely used as fishing bait) have established feral populations in several countries, much to the detriment of local plants and small animals.