by B. N. Sullivan
Let me introduce you to one of our favorite marine creatures: the mantis shrimp. Why are they a favorite? They're cute (sort of), they're very smart (for a crustacean), and they're fun to watch. Unfortunately, they're also difficult to photograph -- more on this a little later!
The individual in the photos on this page is a Shortnose Mantis Shrimp (Odontodactylus brevirostris), from Hawaii. I know that the critter looks pretty big in these photos, but it's not. These are macro images. The mantis shrimp in these pictures was only about two inches (5 cm) long. You can click on any of the photos to see a larger view.
Despite the name, mantis shrimps are not true shrimps. I guess I don't need to explain that they are not mantises, either -- although they apparently acquired their name because they resemble praying mantises somewhat. Like crabs and lobsters and true shrimps, mantis shrimps are Crustaceans.
All mantis shrimps belong to the order Stomatopoda. Stomatopods have a number of features that set them apart from other crustaceans. Among the most notable -- and noticeable -- are their raptorial arms, and eyes that are highly evolved.
Mantis Shrimp Eyes
I've always wondered what the undersea world looks like through the mantis shrimp's eyes, because their eyes are like no other eyes. Take a look at the photos, and notice that the eyes are set at the end of stalks. Each eye is independently movable -- and they seem to move constantly, giving the critters a hyper-alert look. Each eye is trinocular, that is, each has three separate perceptual regions. This would be sort of like having built-in trifocal lenses, except that instead perceiving things at three different levels of magnification, each eye region is specialized to perceive a different kind of visual information -- but all at once.
Mantis shrimp eyes are capable of hyperspectral vision. In other words, not only can they see the visible light spectrum, like we can, but they also can see spectra of light that we cannot, including infrared and ultraviolet. Very recently it was discovered that mantis shrimps have the ability to perceive circular polarized light, too.
The mantis shrimp's raptorial appendages actually are legs that have evolved into claw-like arms, specialized for killing prey. There are two types: some species have thin, barbed raptorial arms that can spear prey; others have club-like raptorial appendages that smash their prey. (The mantis shrimp species in the photos is a 'smasher'.) When mantis shrimps are at rest or walking about, they keep those killer appendages folded up like closed jack-knives against their bodies, just as the creature in these photos is doing. When they go after prey, those appendages unfold at an incredible speed and they spear or whack the prey.
The spearers tend to choose soft-bodied prey like worms and little fish. The spearers are ambush predators. That is, they sit quietly and concealed until prey comes along and then they attack. In contrast, the smashers usually pursue their prey -- and that is one of the mantis shrimp behaviors we love to watch, when we get the chance.
The smashers use their raptorial arms not just to kill prey, but also to break it apart to eat. The species in the photo eats things like small crabs, and gastropod snails that live in shells. When the mantis shrimp sees one of those and goes after it, first he punches it silly, then he uses his raptorial arms to crack open the shell so that he can pick out the meat with his little forward legs, which are also specialized for that purpose.
The Star of the Puako 'Petting Zoo'
The mantis shrimp in the photos on this page lives off the coast of Puako, Hawaii in an area where we have made countless dives. When you dive in the same area again and again and again over a period of years, you become aware of things that a visiting diver probably never would notice in the course of just a few dives. One thing you learn is where all the different types of creatures live. Except for pelagics that roam the open sea, most marine creatures have a relatively small range. Once you spot where they live, you can usually count on seeing them in the same general area any time you go there. Once you know where to look, there they will be.
Puako has a wonderful fringing reef that parallels the shoreline for a couple of miles. The coral reef area is beautiful, and very accessible, but there are areas beyond the reef that are just as fascinating, if not quite so pretty. Where the seaward edge of the reef ends there is a steep slope. The top of the slope has many rocks and is still covered with quite a lot of coral, but deeper on the slope where less sunlight penetrates there is little live coral. There are more rocks, plus lots and lots of coral rubble -- lumps of dead coral washed down the slope from the reef over time. At the base of the rubble slope the terrain levels out into a sandy plain.
There is an area near the base of the rubble slope, at a depth of about 100 ft (30 meters), that we named the Petting Zoo. We called it that because it teems with small creatures. Sometimes we would go directly to the Petting Zoo, plop ourselves down in one spot, and spend the entire dive watching all the little creatures go about their business. You can learn a lot about marine creatures' behavior that way.
On one of our visits to the Petting Zoo, we noticed this particular mantis shrimp. Some mantis shrimp species are nocturnal, i.e. they only come out at night. This species works the day shift. We first saw him when he emerged from his burrow and began scurrying about. Mantis shrimps can swim a bit. If you look at the second photo above, you'll notice that this creature has a tail not unlike that of a lobster. They can use their tails to propel themselves through the water for short distances, but their more usual method of locomotion is to run about. Notice I said run. They move quickly -- usually too quickly to get a good macro shot.
Mantis shrimps live in burrows. This one had excavated a double-ended tunnel under the sand. I have no idea what it's like inside the burrow -- we used to joke that he probably had an overstuffed chair and a TV in there for all we knew. We did notice that outside the entrance to the burrow we would sometimes see pieces of broken shell in little piles. A few times we saw the critter in the process of housecleaning -- literally throwing bits of shell out the door.
On successive visits we saw this mantis shrimp stalk and kill prey -- usually a small crab. One time we saw him smack a crab that was bigger than he was, dismember it, and drag the body to the opening of his burrow. The mantis shrimp disappeared inside the burrow for a few minutes, then re-emerged and whacked the shell of the now legless crab a couple of times to crack it open and began to dig out the 'meat' for his feast. We watched him until it was time for us to begin our ascent. He was still working on the crab when we left. When we came back a day or two later we looked for the remains. Sure enough, there was a recognizable piece of the crab's empty shell not too far away from the entrance to the mantis shrimp's burrow.
I mentioned in the first paragraph of this article that mantis shrimps can be difficult to photograph. That's because they seem to be in motion constantly when they are outside their burrows. It's not too difficult to get a shot of one peeking out of its burrow, but capturing an image of the whole animal had eluded me for years.
Then one day at the Puako Petting Zoo, this little mantis shrimp came out of its burrow and just stood there looking at us. And -- Hallelujah! -- not only did I have my camera, it was set up for macro photography. Moving as slowly as I could, not to startle the critter, I lay down on my belly on the sand at the edge of the rubble slope, and sort of inched toward the mantis shrimp. He didn't run away.
I focused carefully on his wonderful eyes and pressed the shutter release, figuring this would probably be my one and only shot. So often in underwater nature photography, the flash of the strobe startles a photo subject and it quickly leaves, so one shot is all you get. But no, not this mantis shrimp. I expected him to high-tail it to his burrow, but instead he actually came toward me a bit, stopped and turned sideways, almost posing. Now practically touching him with the end of the lens port, I took another shot, and a third.
Then he turned the other way and positioned himself as if to say, "How's this? Can you see my tail a little better now?" I shot once more, and then he finally scampered over to his burrow and disappeared inside. I did press the shutter one more time while he was in retreat, but his movements had stirred up the sand by then.
I've often wondered what it was that made that mantis shrimp decide to be such a cooperative photo model that day. I've never again had an opportunity like that.