Crabs, lobsters and prawns are familiar to most people as seafood but these decapod crustaceans are also very important at all steps in the food chain. Few realise just how many different types there are and how difficult it can be to identify them. A newly published book provides tools for the task!
When most people think of a crustacean, they think of a decapod. Know it or not, you’ve probably seen or even eaten a decapod crustacean – maybe at a Christmas lunch. Mud Crabs, Blue Swimmers, Rock Lobsters and King Prawns are certainly some of the best-known decapods because they are popular seafood, but they are by far not the only ones. In fact, some 15,000 species of decapods occur around the world, with new species discovered almost daily.
What are decapod crustaceans?
Crustaceans have a body covered by a hard exoskeleton (shell) with many articulated segments, jointed limbs and two pairs of antennae. As decapods, they also have a carapace (main shell covering the front half or most of the body), a tail section with usually seven segments and, as their formal name Decapoda (Latin for ten feet) suggests, five pairs of limbs – of which one or more pairs are usually pincers. Despite these commonalities, decapods sport an amazing array of very different forms, lifestyles and sizes. They include tiny crabs fully grown at only 2–3 millimetres in shell length to the largest known arthropod, the Giant Japanese Spider Crab with a shell length of 370 mm long and leg span of 4 metres.
In nature, decapods are both predator and prey. Most roam or swim freely, but many are commensals on corals and sponges, and some are parasitic inside mussels and oysters. Some have even become serious invasive pests in parts of the world, such as the European Shore Crab (Carcinus maenas), which has been introduced to North America, Australia and Japan. Some species live on land or in freshwater creeks and rivers but most are marine, found from the shore down to ocean depths exceeding 7,000m. Apart from familiar crabs, there are long tailed swimming forms such as shrimps and prawns, armoured crawling forms like yabbies, crayfish and lobsters, the slender burrowers such as mud lobsters and ghost shrimp (often used as fishing bait, under name ‘nippers’), and peculiar hermit crabs, which carry snail shells as a portable hideout.
How do we classify decapods?
Decapods are classified into 12 major subdivisions called suborders and infraorders, each containing a series of nested groups called families, followed by genera (singular: genus) and species. For example, the Blue Swimmer Crab (Portunus armatus) and Flower Crab (Portunus pelagicus) are species in the genus Portunus, within the family Portunidae and infraorder Brachyura (the major subdivision containing true crabs). At present, there are some 189 families and more than 2,100 genera of marine decapods worldwide. Being abundant, diverse, of conspicuous size and often strikingly coloured, decapods are very often met by students, researchers, naturalists, divers, fishers and almost anyone visiting the sea. Because of their sheer diversity however, decapods can be difficult to identify for specialists and non-specialists alike.
In identifying and documenting decapods for our scientific work and assisting students and co-workers, my colleague Dr Gary Poore (Museums Victoria Research Institute) and I have often relied on our own unpublished research knowledge or had to consult numerous (often obscure) scientific papers inaccessible to most non-specialists. Knowing where to start with identifying an unknown decapod can be a daunting proposition, especially with no single source with worldwide scope. To this end, we embarked on a major collaboration several years ago to create a single volume identification guide, now newly published as Marine Decapod Crustacea: a Guide to Families and Genera of the World. The book synthesises knowledge of the taxonomy of all marine decapods and provides dichotomous identification keys to all marine families and genera worldwide. We introduce the reader to the anatomical features used for distinguishing decapods, include short diagnoses and illustrations for each family and genus, summarise knowledge of biology, depth and geographic distributions, and show living colours with hundreds of photos.
Our main aim is to provide the tools to identify decapods and to stimulate further research and public interest, as well as showcase these amazing animals. Decapods have held our interest for decades, and hopefully we can open some eyes to see why we find them so compelling!
Professor Shane Ahyong, Head, Marine Invertebrates, Australian Museum Research Institute.