Tiger Sharks Don't Floss

by B. N. Sullivan

One day we were diving at Puako, Hawaii in the general vicinity of the area we call the Petting Zoo (described in a previous post). We were swimming close to the bottom edge of the rubble slope, where the sandy plain begins, looking for little creatures and seashells.

I spotted what I though might be the edge of a shell sticking out of the sand. When I picked it up, I realized that it was not a shell at all. It was a shark's tooth -- the first I had ever found in the water. I put it into the small mesh drawstring bag I always carried with me, and we continued our dive.

When we got home that day, I took the shark tooth out of my little mesh bag, washed it, and set it aside to dry out. That's it in the photos on this page. We didn't know what species of shark the tooth had belonged to, but we joked that whatever kind it was, he obviously didn't floss!

A few weeks later, an old friend of mine from Honolulu came to visit, along with her young son. To entertain the little boy while his mother and I chatted, I got out a few books that had a lot of pictures and gave them to him to look at. One of those books was about sharks. After awhile, the boy interrupted our conversation, telling us he needed to show us something right away. It was a picture of a Tiger Shark tooth, and it looked exactly like the one I had found. This was an unexpected piece of information.

Don't get me wrong -- we always knew that there were Tiger Sharks (Galeocerdo cuvier) in our waters. We knew that fishermen in the area caught one occasionally, and local surfers reported seeing them from time to time, yet in all our years of diving we had never seen one. In a way, this was surprising. Since we dived so frequently in the same area, we reckoned there was a high likelihood that sooner or later we would cross paths with at least one of everything that was out there. We had seen plenty of sharks, but no Tiger Sharks.

Much as birdwatchers do on land, we always have kept a record of every marine species we've encountered. We call it our Critter Log. Most of the sharks we had seen were reef sharks: Whitetips, Blacktips, and Grays. From time to time we would see a pelagic shark, like a Hammerhead, or a Whale Shark. We also had come face to face with one Galapagos Shark and one Thresher at Puako, but never a Tiger Shark. Still, the tooth I had found was proof that the Tiger Sharks definitely were there in what we thought of as our territory.

Surfers we knew who claimed to have seen Tiger Sharks in the water said they only appeared during dawn patrol, or just as the sun was setting. We began a lot of our dives very early in the morning, and when we did night dives at Puako, we usually entered the water at dusk. Thus, we surmised that if we ever saw a Tiger Shark at Puako, it probably would be either very early in the morning, or at the outset of a night dive. This thought was reinforced by the fact that our sighting of the Galapagos Shark was very early in the morning, and it was as we were descending for a twilight dive that we had our single encounter with a large Thresher Shark.

I had found the Tiger Shark tooth well offshore, and at a depth of more than 100 ft. Thus, in addition to believing there was a good probability that one day we would cross paths with a Tiger Shark at Puako, we imagined there also was a strong likelihood that we would see it well away from shore, in deep water. After all, these are large, pelagic animals -- and anyway, that was where I had found the tooth!

From time to time we discussed all this, usually in conjunction with showing what we now knew was a Tiger Shark tooth to another diver. As it turned out, our first and only encounter with a Tiger Shark was nothing like we had anticipated. It happened in the early afternoon, inshore, in relatively shallow water. So much for probabilities and likelihoods!

In the next post we'll tell you all the details of that first encounter with a Tiger Shark.

WW #45 - Emperor Angelfish (Pomacanthus imperator) - Red Sea

[Click photo for larger view.]

My previous Wordless Wednesday posts.

Wordless Wednesday home page.

Another Side of The Right Blue

This will be quick: I just have to share something.

As I do most days, this morning I opened my Google Reader and began to browse the feeds of some blogs I follow. When the feed for Go Visit Hawaii loaded, I was taken aback by what I saw. It was a photo of the view from my window!

Sheila has been running an interesting series on Go Visit Hawaii called "The Colors of Hawaii." She asked readers to submit photos of Hawaii that highlight a particular color. Unbeknownst to me, Jerry submitted one in the blue category (of course!), and Sheila published it today.

Click on over and have a look at Colors of Hawaii: Big Island Blues on the Go Visit Hawaii blog. And while you're there, you might enjoy looking at some of the other Colors of Hawaii, too.

P.S. I hear that Sheila is still accepting photos for The Colors of Hawaii, so if you have one you'd like to share, send it to her. The details are here.

UPDATE Aug. 6, 2008: Sheila has put a new post on her Go Visit Hawaii blog that lists all the Colors of Hawaii posts so far. She's still accepting more photos.

Meet the Mantis Shrimp

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.

Raptorial Appendages

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.