Crustaceans are a diverse subphylum of arthropods, including crabs, lobsters, shrimp, and barnacles. They are predominantly found in aquatic environments, including marine and freshwater habitats, though some have adapted to terrestrial life. This group is characterized by a segmented body, jointed limbs, and a hard external covering, making them a successful and widespread component of global ecosystems.
External Anatomy
Crustaceans possess a rigid exoskeleton, primarily composed of chitin and proteins, often reinforced with calcium carbonate. This exoskeleton provides protection from predators and environmental stressors, while also offering structural support and attachment points for muscles. This rigid shell does not grow with the animal, necessitating molting, where the old exoskeleton is shed to allow for growth and the formation of a new, larger one.
The crustacean body is divided into segments, often grouped into three regions: the head (cephalon), thorax (pereon), and abdomen (pleon). In many species, the head and thorax are fused, forming a cephalothorax, which may be covered by a single, large shield-like structure called a carapace. Each body segment can bear a pair of jointed appendages, though their number and form vary among different crustacean groups.
Crustaceans have two pairs of antennae located in front of the mouth, distinguishing them from other arthropods. These antennae serve sensory functions, including touch and chemoreception. Mouthparts, such as mandibles for biting and crushing food, and maxillae for manipulating it, are also present. Beyond the head, appendages are adapted for diverse functions, including walking legs (pereiopods), swimming legs (pleopods or swimmerets), and specialized claws or chelae used for feeding, defense, and grasping.
Internal Systems
Crustaceans have an open circulatory system, where their circulating fluid, hemolymph, is not entirely contained within vessels. Hemolymph is pumped by a heart, which can be a simple tubular or muscular organ, into arteries. From these arteries, the hemolymph flows into open spaces within the body cavity, known as the hemocoel, directly bathing the tissues and organs before returning to the heart.
The nervous system of crustaceans consists of a brain, or supraesophageal ganglion, connected to a ventral nerve cord composed of ganglia, or nerve centers. In some species, nerves to the antennae may originate from a ring around the esophagus, while in others, they connect directly to the brain. While the ganglia can process information locally, allowing for rapid responses, a complex network of nerves and neurohormones also enables coordinated movements and modulation of internal organ functions.
The digestive system of crustaceans is a straight tube, though it can be coiled in some species. It comprises a foregut, midgut, and hindgut. The foregut often includes a chitinized structure called a “gastric mill,” consisting of calcified plates that mechanically grind ingested food. Processed food then moves to the midgut, where digestive glands, such as the hepatopancreas, absorb nutrients and secrete enzymes to further break down food.
Most aquatic crustaceans respire using gills, which are thin-walled structures, often modifications of appendages. These gills are located in branchial chambers, often protected by the carapace. Gas exchange occurs as oxygen from the surrounding water diffuses across the gill surface into the hemolymph, while carbon dioxide is released. Smaller crustaceans may lack specialized respiratory organs, performing gas exchange through their body surface.
Life Cycle and Reproduction
Crustaceans reproduce sexually, with distinct male and female sexes, though some species exhibit parthenogenesis. Fertilization is often external, with eggs typically laid and carried by the female until hatching. The life cycle often involves metamorphosis, with young animals passing through several larval stages that can look very different from the adult form.
A common early larval stage is the nauplius, characterized by an unsegmented body, three pairs of appendages used for swimming, and a single eye. As they grow, nauplius larvae undergo molts, adding segments and appendages, and may transition into other larval forms. For instance, many develop through zoea stages, which possess compound eyes and use thoracic appendages for swimming, followed by a megalopa stage, which uses abdominal appendages for propulsion and often resembles a miniature adult. These developmental shifts often involve changes in habitat and feeding strategies, moving from planktonic larval phases to benthic or free-swimming adult forms.
Sensory Capabilities
Crustaceans perceive their environment through specialized sensory organs. Many possess compound eyes, composed of numerous individual light-sensing units, similar to those found in insects. These eyes are effective at detecting movement and changes in light intensity, with some capable of adjusting to varying light conditions. Some crustaceans, particularly larval forms, also have a single “naupliar eye” that primarily detects light direction.
Chemoreceptors, which enable taste and smell, are distributed on the crustacean body, notably on the antennae and mouthparts. These receptors are important for locating food, identifying mates, and detecting chemical cues. Mechanoreceptors, specialized for detecting touch, vibration, and water currents, are found as hair-like structures or setae projecting from the exoskeleton across appendages and the body surface. Some crustaceans also have statocysts, balance organs located at the base of appendages, which contain small granules that shift with changes in orientation, providing information about gravity and body position.