Crustaceans, a subphyla of arthropods, are quite mysterious creatures as most of them live in the sea, or under rocks, and are only seen rarely. The name crustacean means a sea-going creature, even though some are land-based. They all share the same structure, though, with hard exoskeletons composed of a hard protein called chitin. The different types of crustaceans feed on different things, but they can be classified into main groups.
Red rock crab (Grapsus grapsus). © Krunchky.
Types of crustaceans
The best-known types of crustacean are crabs, including the popular pet crab, hermit crab and lobsters, and these are typical of the family. They have eight legs or so and are aquatic animals. Crayfish and shrimp are also good examples, but you can also include barnacles, some plankton and even the common woodlice.
Predators and scavengers
Most crustaceans, like lobsters, are scavengers or predators. They use their powerful claws to catch their prey, then tear it apart. Their prey includes small fish and smaller crustaceans. The pistol shrimp is an amazing creature. It has a specialized claw that it uses like a weapon. It whips this claw very rapidly and this sends a bubble shock wave to attack its prey and stun it. The punch from this claw is enough to break a human finger! The pistol shrimp spends most of its life hiding, waiting for passers-by.
A Northern lobster eating a dead skate fish. © Andrew Martinez.
Scavengers and detritivores
If hunting is proving unsuccessful, then most crustaceans are not too proud to resort to scavenging. They’ll look for the remnants of a shark’s meal, or an old fish that’s died and sunk to the bottom of the sea. It won’t be long before a host of crustaceans turn up and start nibbling away at this feast. On land, woodlice like to eat decaying plant matter, and are especially fond of bark.
A signal crayfish eating a pacific lamprey, head first. © FishBio.
Barnacles go their own way in life, and have a very different feeding mode to their relatives called filter feeding. These animals find a home on one place, usually a rock, and start feeding on the tiny plankton that float though their space. We see them at low tide, closed and dormant, waiting for the water to come back and serve dinner. When the tide comes back in, they open their shells and wave their feet in the water. These feet are covered in thousands of tiny hairs, or cilia, and these cilia capture plankton and maneuver it into the animal’s mouth.
Red cherry shrimps eating a dead fish. © EssexSpark (watch the video on YouTube).
Copepods are also specialized crustaceans. These tiny creatures have become parasitic, and they will attach themselves to a host, staying there until the host dies. You’ll see them feeding on the eyes of small fish like sprat, or locked onto huge animals like whales. These are quite simplified animals, with their body parts divided into latching and eating, and not much else.
One of the best-known examples of symbiosis is the cleaner shrimp and its friend fish. People who own tropical fish tanks often buy these animals because these shrimp climb over fish and other tank mates to clean them of parasites and so on. These shrimp tend to hide in cracks to escape predators, and come out when their friends park themselves outside their homes waiting for their valet service. The fish appreciate the removal of parasites, and actively seek this service out.
Learn more about specific species
Continue reading: arthropod’s diet.
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