by Greg Mayer

My Okinawa correspondents spent Boxing Day at the Okinawa Churaumi Aquarium, and sent me a bunch of pictures. The aquarium is a sprawling complex on the coast in northwestern Okinawa, and includes large areas of gardens and park land, and a recreation of traditional Okinawan homes and buildings, as well as the aquarium proper.

It reminded me, as I’m sure it did many of you, of the Sausalito Cetacean Institute. That’s Ie Shima island in the background.

One of the main attractions at the Aquarium is the Kuroshio Sea Tank. It’s enormous.

When my correspondents told me they were going to the Aquarium, they mentioned something about “whale sharks”, but I didn’t query them further. It turns out the Aquarium actually has whale sharks (Rhincodon typus), the world’s largest species of fish!

And not just one!

Although whale sharks are, for sharks, specialized feeders– they feed on plankton– they are “typical shark” shaped.

Sharks are cartilaginous fishes (Chondrichthyes), which are of two main types: the Holocephali, comprising the ratfishes and chimaeras (we’ve mentioned them here before at WEIT), and the Elasmobranchi, comprising sharks and rays. Most people have a good idea of what sharks and rays look like. Here are some more typical sharks (I don’t know what species– any shark people out there?) Note that the gill slits are on the side of the head; the fellow in the middle is male, as you can tell by the large claspers medial to the pelvic fins.

And here’s a typical ray (again, no ID). Note the flattened shape, and the spiracles (whitish bits) behind the eyes– these are the vestigial first pair of gill slits. The flat body of the ray is mostly the greatly enlarged pectoral fins.

Most people also know the manta ray (Manta birostris). It’s a little unusual for a ray, being pelagic and filter feeding, so the mouth is at the front tip of the body– and, it’s got those crazy cephalic fins or “horns”, from whence it gets the alternative vernacular name “devil fish”. Do note that the gills are on the bottom of the head.

There is more diversity among sharks and rays than most people realize. Sawfish, which look a lot like sharks with a saw strapped to their snout, are actually rays, but shouldn’t be confused with the similar looking saw shark, which is a shark. There are also angel sharks, which look a lot like rays, and guitarfish, which are rays that look a lot like sharks– in fact, more shark-looking than angel sharks.

I’ve never seen either angel sharks or guitarfish in any aquarium, and thus was delighted to find that Okinawa Churaumi has guitarfish (which, remember, are rays). Here’s a guitarfish surrounded by three sharks, with a typical ray off to the right (and a shadowy form below and to the right). If you look carefully, you can see the spiracle (again, whitish looking) on top of the head, behind the eye.

In the following picture, we get a really good view of why it’s a ray. Note that the gill slits are on the bottom of the head, as is the mouth (the latter is typical, but not diagnostic, of rays). And, the pectoral fin is joined seamlessly to the head– at a point above, in fact, of the gill slits (which is why the slits are on the bottom of the head). The spiracle, already spatially distant from the other gill slits in sharks, is thus, in rays, separated from the other slits by the interposition of the enlarged pectoral fin.

In the next (and last) picture, note that the dorsal, caudal, and pelvic fins all are at least passably shark-like, but that the enlarged pectoral fin is being flapped for locomotion in the manner of a ray. (Also, it’s a male– you can see the free distal ends of the claspers below the second dorsal.)

More on the Okinawa Churaumi Aquarium tomorrow.