The Yellowline Arrow Crab (Stenorhynchus seticornis)

by B. N. Sullivan

One of my favorite macro photography subjects on Caribbean reefs is the Yellowline Arrow Crab (Stenorhynchus seticornis). At first glance, these creatures resemble pointy-headed, long-legged spiders. The way they skitter around on those long legs also lends to that first impression.

Once you see a good macro photo of the Arrow Crab, however, you will notice some features that you would never see on a spider. First there is that elongated pointy head, with the goofy eyes protruding on either side. Next you will notice that their legs are jointed, and their 'knees' are a bright yellow color. At the end of each of their forelegs is a little claw. The claws are violet, making them look as if they've just come from the nail salon.

Yellowline Arrow Crabs are abundant throughout the Caribbean, but they are very small, and they are sometimes hard to spot during the day. They tend to find resting spots in nooks and crannies where they stay put during daylight hours. They actively forage at night, so they are quite easy to find on night dives when they are roaming around the reef looking for a meal. By the way, they are carnivores, but you don't have to worry about getting chomped by an Arrow Crab. They eat tiny worms and other itty-bitty animals that populate coral reefs.

They don't seem to be afraid of divers -- even divers with bright lights and cameras -- so once you locate an Arrow Crab, it is fairly easy to photograph it. They don't spook when the camera strobes flash, so the photographer usually will have plenty of time to take a number of shots of a given individual.

I photographed the Yellowline Arrow Crab on this page during a night dive at Little Cayman. This is a 1:2 macro shot. The creature is only about two inches (5 cm) long.

Video: Mantis Shrimp vs. Crab

Last month we introduced readers of The Right Blue to the Mantis Shrimp. Among other features unique to this animal, we described their raptorial appendages. Some Mantis Shrimp species spear their prey, and others smash their prey with their raptorial appendages. The species in the photos that accompanied our article was a 'smasher'.

I wrote:

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.

Here is a video I came across on YouTube that shows a Mantis Shrimp in an aquarium doing exactly what I described. Although the Mantis Shrimp in the video is a different species from the one in my photos, it also is a smasher. Have a look (but don't blame me for the choice of background music):



(If the video does not play or display properly above, click here to view it on YouTube.)

Tip of the hat to YouTube user accubyrd, on whose page there are a few more Mantis Shrimp videos.

Sneaky Shark

Caption: Why underwater photographers should have rear-view mirrors!

Where: I took this photo of the sneaky shark during a dive
at Pulau Sipadan, off the coast of Borneo.

[Click photo for larger view.]

My previous Wordless Wednesday posts.

Trailing behavior in Risbecia nudibranchs

by B. N. Sullivan

You just never know what you're going to find out next about marine creatures. The photo on this page is a case in point.

We are planning to run a series soon on The Right Blue about nudibranchs. In case you are unfamiliar with the name, nudibranchs are marine gastropod snails without shells -- also known as sea slugs. In order to prepare, we have been going through our photos and notes, and looking up certain bits to make sure we have our facts straight. (After all, we don't want to misinform our readers.)

One thing I always do is double check that the species name I have recorded for a creature is correct before I publish it. In my notes, I had recorded the species of the two nudibranchs in this photo as Risbecia pulchella. I did a quick search on the name to see what would come up -- hoping to see images that looked like mine, or a detailed description. Sure enough, I found a few authoritative sources that confirmed that the nudibranchs in my photo were correctly identifed. They are indeed Risbecia pulchella nudibranchs. But I learned something else, too.

A page on the Sea Slug Forum -- a wonderful source of information about nudibranchs, run by the Australian Museum in Sydney -- detailed a behavior said to be typical of nudibranchs in the genus Risbecia. The writer called it 'trailing' behavior:

Also known as queueing or tail-gating, all species of the chromodorid genus Risbecia exhibit this behaviour where they seem to play "follow the leader". Perhaps its a behaviour which has evolved amongst relatively uncommon animals to ensure they find each other for mating. When tailing, one animal appears to follow the mucous trail of the other until they actually make contact. Then the following animal, as can be seen in thse photos, keeps contact by touching the 'tail' of the leader. Sometimes 3 or 4 animals can be seen together.

I have to tell you, my jaw dropped. That is exactly what the two nudibranchs in my photo are doing!

For the record, I photographed these two Risbecia pulchella nudibranchs engaging in 'trailing' in the Red Sea. More precisely, they were photographed at Ras Mohammed, near the remains of the wreck of the Jolanda, at a depth of about 13 meters (42 feet).

I took the photo a number of years ago, but only now did I realize the significance of the behavior that I just happened to record. As I said, you just never know what you're going to find out next about marine creatures.