Arthropods Anyone?

The Creature Feature aims to introduce you to an inhabitant of the Indian Ocean every month. Many of these creatures live not far offshore from some of our favorite beach side destinations. Let’s find out a little more about them.

The word arthropod translates to “jointed foot”. Arthropods form a large phylum of inverte brate animals that have appendages with joints, segmented bodies and external skeletons or exoskeletons.Insects, spiders and scorpions are arthropods, and so are crustaceans! What insects are to land, crustaceans are to the oceans.

Under the phylum Arthropoda, the world of crustaceans is a vast one. It includes the crabs, prawns and lobsters that we know so well from the menus of our favorite sea food joints. It also includes less familiar krill, shrimp, cray fish and even barnacles.

This peacock mantis shrimp (Odontodactylus sp.) has exceptional,highly mobile eyes mounted on stalks. The ability to accurately gauge distance combined with a lightning fast strike of their clubbed ‘forelimbs’ make them formidable reef predators

The smallest known arthropod in the world, Stygotantulusstocki, is a crustacean. The arthropod with the largest leg span– a whopping 12 feet from claw to claw – is the Japanese spider crab, Macrocheirakaempferi. It is also a crustacean and, with some specimens reaching 20 kilograms in weight, is one of the heaviest arthropods, second only to another crustacean – the American lobster, Homarusamericanus.

This lively coral crab will defend its host coral against crown-of thorn starfish, which would otherwise eat and eventually kill the coral

Beyond just a large size range within the group, crustaceans are found in every part of the world’s oceans. Krill and cope pods, animals with probably the greatest biomass on the planet, form a crucial part of the food chain for Antarctic and temperate water animal communities. Crabs and shrimp have been observed around thermal vents in some of the deepest explored recesses of the Pacific. Most of us have encountered crabs and hermit crabs scuttling along beaches across the world – the relatively small part of the crustacean community that are terrestrial inhabitants. All in all, crustaceans are as ubiquitous in the oceans and inter tidal areas as insects are on land. And they play a crucial role in the health and balance of the ecosystems they inhabit.

In Indian waters, snorkelers and divers who shift their gaze to the smaller and more cryptic denizens of the sea will often encounter an unlikely crustacean right at the surface.Barnacles cluster around buoys, Styrofoam floats and the mooring ropes that attach these objects to the dive site below.These sessile creatures are effectively crustaceans nestled inside a protective shell, fixed to a substrate. The animal inside sticks its limbs, or cirri, out of the opening in the protective shell. These cirri spread out into the water and move to create a current. Water flows into the shell, allowing the crustacean inside to feed on trapped plankton.

A whip-coral spider crab (Xenocarcinus sp.)sits still on a whip-coral. When the polyps of the coral are extended, they form the perfect hiding place for these cryptic crabs

In the water column, a keen eye may decipher tiny crustacean larvae in various stages of development. They form part of the zooplankton soup that feeds millions upon millions of marine creatures, from coral polyps to planktivorous fish to filter-feeding marine mammals.

A cleaner shrimp sits on the face of a moray eel and picks off dead skin and parasites, often from deep inside the moray’s mouth

Along the reef itself, the number, diversity and behavior of the resident crustaceans are mind-boggling. Porcelain crabs live on the edges of anemone and use fan-like appendages to net plankton from the water. A closer look between the branches of some coral species often reveal two or three different kinds of crabs co-existing on their coral host. The coral strikes up symbiotic relationships with these creatures that play a crucial role in warding off the coral’s predatory crown-of-thorns starfish.At night, huge crabs of different kinds emerge from underneath coral and rocks to feed under cover of darkness. Some of the mare hermit crabs that increase their protection by fixing certain types of anemone to the shells they inhabit. The stinging cells of the anemone deter predators from attacking the crab, and the usually sessile anemone achieves a degree of motility on the backs of the hermit crabs.

In similar fashion, shrimp, too, inhabits all corners of the reef.On the sand, partner shrimp strike up relationships with gobies. The goby with its sharp eyes acts as the watchman, while the shortsighted shrimp maintains the burrow in which both disparate housemates live. At ‘cleaning stations’ on the reef,different kinds of shrimp provide cleaning services to fish that need to rid themselves of parasites and dead skin. The sharp and dexterous claws of the shrimp help clean the fish while the crustaceans eat everything they pick off. On gorgonian fans and whip coral, cryptic, well-camouflaged shrimp share space with equally well-camouflaged crabs. These coral generally grow in areas with strong currents and the crustaceans on them simply pick plankton out of the seawater.

Crustacean larvae have very distinct characteristics among zooplankton. This arthropod larva is almost at the end of its development, most likely in its Megalopa stage

Crustaceans are everywhere on a reef. Divers and snorkelers miss most of them for two reasons. Firstly, many of these creatures are so well camouflaged that only a movement on their part would reveal their presence to a keen observer.Secondly, the crustacean world truly comes alive at night.Lobsters emerge from underneath rocks and sit out in the open;the eye-shine of hundreds of crustaceans is revealed in the light of a diver’s torch. These benthic dwellers eat almost anything,from scavenging dead marine life to scraping algal matter off the substrate. In doing so, they also provide the crucial service of breaking down organic matter and keeping the reef free of decay and disease.

Goose barnacles cluster along a rope leading down to a dive site

This article was first published in the December 2015 edition of SAEVUS magazine

About the Author /

Umeed Mistry began diving in the Maldives in 1996 – an experience that has significantly shaped his life and work. Now a PADI Staff Instructor, over the last 16 years he has introduced people with a range of personal, academic and scientific interests to the reefs in the Indian Ocean. In 2005 he began photographing underwater and tries to spend as much of his time shooting in the field. His work has appeared in a number of national and international publications – including Saevus, Asian Diver, UW360, Scuba Diver AustralAsia, Outlook Traveler and NatGeo Traveler. He is the recipient of several national and international photography awards, starting with the all India Better Photography Photographer of the Year 2007. With a keen interest in marine and freshwater ecosystems, and a multi- disciplinary approach to creating awareness of these ecosystems, Umeed also facilitates art residencies and education programs with his partners at Earth CoLab ( All of Umeed’s varied work – as a dive instructor, underwater photographer and cameraman, writer and educator – is driven by the desire to spark in others the same love he has for marine and freshwater spaces. His photography work, both terrestrial and marine, can be seen on Instagram @umeed.mistry

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