Meals in the company of carcinologists are wont to descend into a discussion of segments utterly off the record of most diners. Once, to ease the tedium of peeling apart a plate of cereal-coated crustaceans in a quiet part of Chinatown many moons ago, our attention turned to the arrangement of lateral plates or pleura on the abdomen of the deceased decapod. It was pointed out that as the creature's first pleuron overlaps the anterior of the second, the beast was reliably a penaeid. In popular parlance, the animal was a prawn. Shrimp, in sensu stricto, possess a second pleuron that is visibly larger than the rest and overlaps the posterior of the first pleuron.
Size, however, is no indicator of shrimpiness. The tiny bodies of fragile glass that litter the sand of sunless shores are undoubtedly caridean shrimp, as are the armed culprits of coastal rackets and the pretty pairs that bed down on a carpet of stings. So far, no intertidal representatives of the so-called barbershop shrimp family have been encountered locally. But the caridean family Hippolytidae, members of which pop up on occasion, also counts shrimps with the habit of cleaning larger marine creatures for edible parasites and loose particles.
Hippolytids are also called 'broken-back shrimp' for the 'hump' formed by the rear of the third abdominal segment, which is rather pronounced in some species. The family includes candy-striped cleaners in the genus Lysmata, bizarre shrimp shaped like elongated saw blades as well as the mangrove-dwelling Merguia oligodon, the only known shrimp with semi-terrestrial habits. Also distinctive is the genus Saron: marble shrimp that provide divers with a visual feast likened to a "rich Turkey carpet" and exhibit such variability in colour patterns that some believe certain morphs to be discrete species. That much, and precious little else, is known about these robustly built shrimp that in local shores usually occur as pairs on reefy rubble after dark.
The crustaceans usually referred to as prawns (Americans, however, insist on calling them 'shrimp', reserving 'prawn' for freshwater carideans) in both culinary and taxonomical contexts are a quite different kettle of fish. Besides their distinguishing abdominal features, penaeids lack the enlarged chelae that distinguish many carideans and release their eggs directly into the sea, in stark contrast to carideans which carry their brood on their pleopods until hatching. They also tend to have a more streamlined profile, as befits their pelagic habits.
Despite having no leg paddles in the manner of portunids and matutids, these prawns are able swimmers and burrowers, using their barrage of pleopods to propel themselves forward or rapidly vanish under soft silt. The group includes many highly sought-after seafood items such as the Kuruma prawn, Chinese prawn and the alcohol-loving giant tiger prawn, the intensive aquaculture of which contributes to the widespread destruction of the very mangroves the creatures use as nurseries.
Other penaeids are secretive, even cryptic, shrimp of little commercial importance. The family Sicyoniidae, for one, consists of benthic prawns (often known as rock shrimp) too small and scarce to be of commercial importance. A possible member turned up in a tidal pool at Terumbu Raya months ago, which has been tentatively identified as Sicyonia komai, a species known only from two specimens collected in Japan and Thailand. Darting about over a carpet of zoanthids, the inch-long prawn was clearly a strange find, with its thick, 'furry' antennae and a body of marbled segments. Like its broken-back cousin, this little malacostracan of mystery keeps its cards close to its carapace and reveals just enough of itself to illuminate our ignorance of reefs in certain jeopardy.