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Mantis Shrimps - The Lurking Horror of the Deep

Text and photographs by Andrea & Antonella Ferrari

 

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Peacock Mantis shrimp Odontodactylus scyllarus, colorful male, a fierce benthic predator. Pulau Mabul and Kapalai sandbank, Sabah, Sulawesi Sea, Borneo, Malaysia
Peacock Mantis shrimp, Odontodactylus scyllarus, colorful male, a fierce benthic predator. Pulau Mabul and Kapalai sandbank, Sabah, Sulawesi Sea, Borneo, Malaysia Image #: 145408

Many fresh divers are scared of sharks. Others are afraid of morays. Some again are intimidated by barracudas... Little they know that some of the scariest, most fearsome and probably most monstrous creatures of the deep lurk a few feet below the surface, silently waiting, coldly staring at their surroundings, waiting for the opportunity to strike with a lightning-fast motion to cruelly impale their prey or smash it to smithereens! Luckily, most of these terrifying critters are just a few inches long – otherwise diving on coral reefs might be a risky proposition indeed for every human being...But stop for a moment, and consider those cunning predators of the seabottom, the mantis shrimps: an elongated, segmented and armored body, capable of great flexibility and yet strong enough to resist the bite of all but the fiercest triggerfish; a series of short, parallel, jointed legs under the thorax to swiftly propel it among the reefs rubble bottom; a pair of incredibly large, multifaceted dragonfly-like eyes, mounted on sophisticated swiveling joints, capable of giving the animal an absolutely unbeatable 3-D vision on a 360° field of vision, immensely better than our own and enabling it to strike with implacable accuracy at its chosen target. And above all, consider those incredible front raptorial claws, articulated exactly like a switchblade or like those of the terrestrial predatory insect, the so-called praying mantis, from which these fascinating marine crustaceans take their common name in divers’ circles: able to shoot out and grab their prey, impaling it alive and writhing on sharp spikes or smashing its shell to a pulp, with a three-millisecond strike which is almost invisible to the eye. Behold, my friends, the true alien of the reef, the beautiful monster of the muck, the implacable raider of the rubble – the mantis shrimp!

Spearers and Smashers

Mantis shrimps can be roughly separated in two groups – the “spearers” and the “smashers”. Those belonging to the first group are generally sedentary ambushers, preferring to patiently lurk in wait at the entrance of their vertical mucus-lined burrow dug in the sand or silt of the bottom, attentively peeking out of the hole with just their stalked eyes and their folded raptorial spiked claws showing. Should an unwary fish or squid pass directly above (or quite often just close enough to be withing grasping distance) the alert crustacean shoots out and grabs it with a lightning-fast strike, flicking out its three-segment articulated spiked claws around it. Those who have witnessed this act – as we have in several occasions – cannot avoid being but amazed (and a bit scared) by the speed, efficiency and unfailing accuracy shown by the mantis shrimp - and by the raw strength of its grip, with the fish prey being often broken in two by the violence with which the predator drags its down its burrow to consume it. Given their habits, “spearers” are generally able to reach a greater size, with the quite large Giant mantis shrimp Lysiosquillina lisa – commonly found on SE Asian coral reefs – reaching a total length of about 35-40 centimeters. “Smashers” are usually smaller and much more active, being often observed rapidly scooting among coral heads and under overhangs, hunting for crabs in the open but always within reach of their more or less horizontal U-shaped tunnel, often half-built and half-excavated among the coral rubble but always featuring two entrance holes – so that in the case of an emergency the mantis shrimp will be able to bolt out from one while danger looms at the other. Well adapted to hunting and consuming hard-shelled crustacean prey like large reef crabs, “smashers” are armed with modified front raptorial claws which show no spikes on their edges but feature a blunt, rounded, mace-like tip at their extremity instead. Flicking these with great speed and violence at fleeing crabs – just like a boxer on a ring would do at his adversary – the prey is rapidly disorientated, stunned and crushed, its shell soon giving way to the armored fists of its tormenter. The best known of the “smashers” living in SE Asian reef waters is the spectacularly beautiful and very showy Peacock mantis shrimp Odontodactylus scyllarus, commonly observed in shallow, well-lit waters while it fearlessly runs about like a little runaway mechanical wind-up toy. While “spearers” simply disappear down their burrow when they feel threatened, “smashers” cornered in the open will instead tend to roll on their backs, turning into an almost impregnable ball and offering their thick armored telson (the fan-shaped “tail” at the posterior end of their body) to the attacker. Several other species can be observed on coral reefs and muck bottoms with a little patience – several of them quite beautiful and all equally interesting, as small bright green ones often inhabit algal or seagrass mats – but these two are quite probably the two most common ones. Interestingly, despite being fearful and implacable predators, large “spearers” often host commensal shrimps at the entrance of their burrow – we’ve seen Lysiosquillina lisa being attended by several different species including Blue boxer shrimp Stenopus tenuirostris and Squat shrimp Thor amboinensis, the cleaners quite clearly not afraid of being grabbed and consumed by their much larger and rather horrifying host.

Phototips

Great attention and care are needed to shoot good portraits of mantis shrimp. Peacocks make gorgeous camera subjects with their bright blue-green bodies and their orange, blue and dark red decorations, being also much easier to observe and approach in the open: look carefully at olive-tan females, much lighter than males, as with a bit of luck these can be occasionally observed carrying a precious, wine-red mass of tiny eggs under their belly, cradling it in their thoracic arms and constantly fanning it to circulate oxygen-rich water around it. Beware of hounding “smashers” too close with your camera – the much-circulated story of their strike being equal in power to that of a 22-caliber bullet might be a bit far-fetched, but a good blow from their mace-tipped claws might very well smash or crack a polycarbonate or glass lens dome. They’re known as “thumb-splitters” with good reason, you know! “Spearers” need a stealthier approach, as they will disappear in a flash down their burrow if they feel threatened. Most feature a strongly banded lower body, but that is generally well-hidden in the burrow; especially showy is the quite uncommon and all-over bright fluorescent orange Lysiosquilloides mapia, only recently described. Some experienced dive guides have learnt to bait “spearers” out of their burrows offering chunks of fish or prawn, but do not try to do this by yourself if you care about your fingers! Look for the “spearers” tell-tale round, rubble-rimmed holes in the sand while floating above, and, when you see one, quietly drop on the bottom a few meters away. Approach it skimming as close as you can to the bottom – then lie down on the substrate and slowly inch your way forward. If you’re slow and patient enough you can get real close and be able to shoot show-stopping close-ups as we have often done!

Where?

Most common mantis shrimps can be observed almost anywhere in SE Asian waters from Thailand and Malaysia to Indonesia, Borneo, the Philippines and Papua New Guinea. They are usually encountered on coastal, shallow (1-20 meters) and well-lit bottoms, “spearers” being more common on muck, sand and silt while “smashers” are more easily observed on coral rubble. They are all diurnal but in general rather shy.