Red Crab Anatomy 101: A Comprehensive Guide

Red King Crab Anatomy: A Complete Guide to Paralithodes camtschaticus

Red king crab (Paralithodes camtschaticus) is one of the largest and most commercially important crustaceans in the world. Understanding their anatomy — from the asymmetric claws to the vestigial fifth leg pair that distinguishes them from true crabs — deepens appreciation for both the animal and the seafood it produces. This guide covers the complete external and internal anatomy of red king crab, with notes on how each anatomical feature relates to the seafood experience: which parts are edible, which are not, and why the meat quality varies between different parts of the body.

Taxonomic note: red king crab are not true crabs (infraorder Brachyura); they are anomurans (infraorder Anomura), more closely related to hermit crabs than to Dungeness or blue crab. This distinction is reflected in several anatomical features, most notably the vestigial fifth leg pair described below.

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External Anatomy

The Carapace

The carapace is the hard, calcified dorsal shell that covers and protects the cephalothorax (the fused head and thorax): (1) Size: the carapace of a mature male red king crab typically measures 6–11 inches (15–28 cm) in width; the legal minimum carapace width for commercial harvest in the Bering Sea is 6.5 inches; (2) Shape: the carapace is roughly triangular or heart-shaped when viewed from above; it is wider than it is long; the surface is covered with spines and tubercles (small bumps) that provide additional protection; (3) Color: the carapace of live red king crab is brownish-red to dark red; the distinctive bright orange-red color that consumers associate with king crab develops during cooking, when heat denatures the proteins and carotenoid pigments in the shell; (4) Composition: the carapace is made of chitin (a polysaccharide) reinforced with calcium carbonate; it is rigid and provides structural support as well as protection; (5) The carapace and seafood: the carapace itself is not edible; it is removed during processing; the body meat (inside the carapace) is edible but less abundant than the leg meat; (6) Spines: the carapace bears several prominent spines, particularly along the anterior (front) margin; these spines are a distinguishing feature of red king crab and provide protection against predators.

The Chelipeds (Claws)

Red king crab have one pair of chelipeds (claw-bearing appendages), which are asymmetric: (1) The crusher claw (major cheliped): the larger of the two claws; used for crushing hard-shelled prey (mussels, clams, barnacles, sea urchins); the crusher claw has rounded, molar-like teeth on the inner surface; in most individuals, the crusher claw is on the right side, but left-handed individuals exist; (2) The cutter claw (minor cheliped): the smaller of the two claws; used for cutting and manipulating food; has sharper, more pointed teeth; (3) Size: the chelipeds of a large male red king crab can span 3–4 feet from tip to tip; the claw meat is among the most prized parts of the crab; (4) Claw meat and seafood: the claw meat is dense, sweet, and slightly firmer than the leg meat; it is considered by many to be the most flavorful part of the crab; the claw section is often sold separately as “crab claws”; (5) Regeneration: red king crab can regenerate lost claws; a crab that loses a claw will grow a replacement over several molting cycles; the regenerated claw is initially smaller than the original but grows to full size over time.

The Walking Legs

Red king crab have four pairs of walking legs (eight legs total), plus the cheliped pair and a vestigial fifth pair: (1) Walking leg structure: each walking leg consists of seven segments: coxa (base), basis, ischium, merus, carpus, propodus, and dactylus (the pointed tip); the dactylus is sharp and curved, providing grip on the seafloor; (2) Leg span: a large male red king crab can have a leg span of 5–6 feet (1.5–1.8 meters) from tip to tip; this is the measurement that gives king crab their dramatic visual impact; (3) Leg meat and seafood: the walking legs contain the majority of the edible meat; the merus section (the largest segment) contains the most meat; king crab legs are sold in clusters (a section of the body with several legs attached) or as individual legs; (4) The vestigial fifth leg pair (the anomuran distinction): red king crab have a small, vestigial fifth pair of legs that is folded under the carapace and not visible from the outside; these legs are used for grooming the gills; this vestigial fifth pair is the anatomical feature that distinguishes anomurans (king crab, hermit crab) from true crabs (Brachyura), which have no fifth leg pair; (5) Why king crab appear to have only 6 legs: consumers sometimes notice that king crab legs come in clusters of 6 legs (3 pairs of walking legs + 1 cheliped); this is because the vestigial fifth pair is too small to be included in the cluster and is removed during processing.

The Eyes and Antennae

Red king crab have stalked compound eyes that can be retracted into protective sockets in the carapace: (1) Compound eyes: each eye contains thousands of individual optical units (ommatidia) that provide a wide field of vision; compound eyes are particularly effective at detecting movement; (2) Eye stalks: the eyes are mounted on movable stalks that allow the crab to look in multiple directions without moving its body; (3) Antennae: red king crab have two pairs of antennae; the longer pair (antennae) are used for sensing the environment; the shorter pair (antennules) contain chemoreceptors used for detecting food and chemical signals from other crabs; (4) Sensory importance: in the dark, cold waters of the Bering Sea (100–600 feet depth), chemical sensing (smell) is more important than vision for finding food and mates.

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Internal Anatomy

The Gills

Red king crab are fully aquatic; they breathe exclusively through gills and cannot survive out of water for extended periods: (1) Gill structure: the gills are feathery, highly vascularized structures located in the branchial chambers on either side of the body, under the carapace; (2) Function: the gills extract dissolved oxygen from seawater and release carbon dioxide; water is circulated over the gills by a specialized appendage (the scaphognathite); (3) Aquatic dependence: unlike some crab species (such as Christmas Island red crabs, Gecarcoidea natalis, which can breathe air), red king crab are fully marine and cannot breathe air; they will die if kept out of water for more than a few hours; (4) Cold water preference: red king crab require cold, well-oxygenated water; they are found at depths of 100–600 feet in the Bering Sea where water temperatures are typically 28–35°F (-2–2°C); warmer water holds less dissolved oxygen and is physiologically stressful for king crab.

The Hepatopancreas (“Crab Butter”)

The hepatopancreas is a large, yellowish-green gland that fills much of the body cavity: (1) Function: the hepatopancreas serves as both the liver and pancreas; it produces digestive enzymes, absorbs nutrients from digested food, and stores energy reserves (primarily as lipids); (2) The “crab butter”: the hepatopancreas is the yellow-green material inside the body cavity that consumers sometimes call “crab butter” or “tomalley”; it has a rich, intense flavor that some people enjoy; (3) Food safety note: the hepatopancreas can accumulate environmental contaminants (heavy metals, biotoxins including domoic acid) at higher concentrations than the muscle meat; the FDA advises caution with consuming large amounts of tomalley; the leg and body muscle meat is safe; (4) Commercial processing: the hepatopancreas is typically removed during commercial processing; commercially sold king crab legs and clusters do not contain the hepatopancreas.

The Digestive System

Red king crab are omnivorous scavengers: (1) Diet: worms, clams, mussels, snails, sea urchins, sea stars, barnacles, fish, and organic detritus; the crusher claw is used to break open hard-shelled prey; (2) The gastric mill: the stomach contains a “gastric mill” — a set of calcified teeth used to grind food before it passes to the digestive gland; (3) Digestive efficiency: the hepatopancreas produces enzymes that break down proteins, fats, and carbohydrates; nutrients are absorbed directly into the hemolymph (the crab’s blood equivalent).

The Circulatory System

Red king crab have an open circulatory system: (1) Hemolymph: instead of blood, crustaceans have hemolymph — a fluid that circulates through open body cavities (sinuses) rather than a closed network of vessels; (2) Hemocyanin: the oxygen-carrying pigment in hemolymph is hemocyanin (copper-based), which gives the hemolymph a blue color when oxygenated; this is in contrast to vertebrate blood, which uses hemoglobin (iron-based) and is red; (3) The heart: a muscular heart pumps hemolymph through the body; the heart is located dorsally (on the top side) under the carapace.

Reproductive Anatomy and Male vs. Female Identification

Red king crab reproductive anatomy: (1) The abdomen (pleon): the abdomen is the segmented tail section folded under the body; in red king crab (as in all anomurans), the abdomen is asymmetric — it curves to one side; (2) Male identification: the male abdomen is narrow and triangular; males have paired gonopods (modified appendages used for sperm transfer) on the underside of the abdomen; (3) Female identification: the female abdomen is wider and more rounded than the male’s; females carry their eggs attached to pleopods (feathery appendages) on the underside of the abdomen; a brooding female can carry 150,000–400,000 eggs; (4) Commercial harvest: only males above the minimum legal size (6.5 inches carapace width in the Bering Sea) may be commercially harvested; females are protected to ensure population sustainability; (5) Mating: mating occurs in spring; the male carries the female under his body for several days before and after she molts (sheds her shell); mating occurs when the female’s new shell is still soft; eggs are fertilized externally and attached to the female’s pleopods, where they are brooded for 11–12 months before hatching.

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The Molting Cycle

Like all crustaceans, red king crab grow by molting (ecdysis) — shedding their old shell and growing a new, larger one: (1) Frequency: juvenile red king crab molt several times per year; adults molt once per year or less frequently; (2) The molting process: before molting, the crab absorbs calcium from the old shell back into the body; the old shell splits along the posterior margin of the carapace; the crab pulls itself out of the old shell; the new shell is initially soft and pliable; (3) Vulnerability during molting: a freshly molted crab (called a “soft-shell” crab) is extremely vulnerable to predators; the crab typically hides in a sheltered location until the new shell hardens (2–4 weeks); (4) Growth increment: each molt increases the crab’s size by approximately 20–25%; (5) Molting and meat quality: a recently molted crab has a larger shell relative to its body size; the meat fills the shell less completely; experienced buyers and processors can identify recently molted crab by the relative softness of the shell and the lower meat yield; (6) Lifespan: red king crab can live 20–30 years; males grow larger than females throughout their lives.

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🔗 Also see: Red Crabbing 101: The Complete Beginner’s Guide | Red Crabbing Gear: The Complete Equipment Guide | 5 Red Crab Recipes: Crab Cakes, Garlic Butter, and More

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FAQs: Red King Crab Anatomy

Why do king crab legs come in clusters of 6 legs when crabs have 10 legs?

King crab leg clusters and the anomuran leg count: (1) The leg count: red king crab technically have 10 appendages: 1 pair of chelipeds (claws) + 4 pairs of walking legs + 1 vestigial pair = 10 total; (2) The vestigial fifth pair: the fifth pair of walking legs is tiny and vestigial (reduced through evolution); it is folded under the carapace and used for grooming the gills; it is not visible from the outside and is too small to contain edible meat; it is removed during processing; (3) Why king crab are not true crabs: true crabs (infraorder Brachyura, including Dungeness crab and blue crab) have 5 pairs of walking legs (10 legs total, not counting the chelipeds); anomurans (king crab, hermit crab) have only 4 functional pairs of walking legs plus the vestigial fifth pair; this is the anatomical distinction that separates anomurans from true crabs; (4) The cluster: a king crab leg cluster typically contains 3 walking legs + 1 cheliped (4 appendages) from one side of the body; the cluster is cut from the body during processing; the other side of the body produces a second cluster; a whole king crab produces 2 clusters; (5) Why 6 legs in a cluster: some clusters include the shoulder section of the body with 3 walking legs + 1 cheliped = 4 appendages; other processing cuts produce different configurations; the specific number of legs in a cluster depends on the processing cut.

What is the “crab butter” (tomalley) inside a king crab, and is it safe to eat?

Crab butter (tomalley) in king crab: (1) What it is: the yellowish-green material inside the body cavity of a crab is the hepatopancreas — a gland that functions as both the liver and pancreas; it produces digestive enzymes, absorbs nutrients, and stores energy reserves; (2) Flavor: the hepatopancreas has a rich, intense, briny flavor that is more concentrated than the muscle meat; some people consider it a delicacy; others find the flavor too strong; (3) In commercially processed king crab: commercially sold king crab legs and clusters are processed (cooked and frozen) without the body cavity; the hepatopancreas is not present in commercially sold leg clusters; you would only encounter it if you purchased a whole, unprocessed crab; (4) Food safety: the hepatopancreas can accumulate environmental contaminants (heavy metals, biotoxins) at higher concentrations than the muscle meat; the FDA advises caution with consuming large amounts of tomalley from any crab species; the leg and body muscle meat is safe; (5) Domoic acid: domoic acid (a marine biotoxin) accumulates primarily in the hepatopancreas; this is why West Coast crabbing advisories focus on the viscera (hepatopancreas) rather than the leg meat; the leg meat is safe even when hepatopancreas levels are elevated; (6) The practical answer: for commercially processed king crab legs and clusters, there is no hepatopancreas to worry about; the product is pure muscle meat.

What is the difference between red king crab and Christmas Island red crab, and why are they sometimes confused?

Red king crab vs. Christmas Island red crab: (1) Red king crab (Paralithodes camtschaticus): a large marine crustacean found in the cold waters of the North Pacific (Bering Sea, Gulf of Alaska, Sea of Okhotsk); fully aquatic; cannot breathe air; does not migrate on land; does not burrow; the species sold commercially as “king crab”; (2) Christmas Island red crab (Gecarcoidea natalis): a land crab found on Christmas Island (Australian territory in the Indian Ocean); primarily terrestrial; can breathe air through modified gills; famous for its annual mass migration to the ocean for spawning (tens of millions of crabs crossing roads and beaches); burrows in the forest floor; not commercially harvested; not edible in the commercial sense; (3) Why they are confused: both species are red and are called “red crab”; the Christmas Island red crab migration is one of the most spectacular wildlife events in the world and receives significant media coverage; articles about “red crab anatomy” sometimes conflate the two species, attributing the Christmas Island crab’s terrestrial adaptations (air breathing, land migration, burrowing) to king crab; (4) The key anatomical differences: king crab are fully marine; Christmas Island red crab are primarily terrestrial; king crab have fully aquatic gills; Christmas Island red crab have modified gills that can extract oxygen from air; king crab do not migrate on land; Christmas Island red crab are famous for their land migration; (5) Commercial relevance: when you purchase “red king crab” from a seafood supplier, you are purchasing Paralithodes camtschaticus from Alaskan or Russian waters; Christmas Island red crab are not commercially harvested.