We have two more photos from reader Stephen Barnard in Idaho, who appears to still be alive despite his ownership of a new, high-powered sports car. His customary pair of eagles, Desi and Lucy, have returned as well, and appear ready to make more eaglets.

Yet another Red-tailed Hawk (Buteo jamaicensis) in flight:

The bald eagles (Haliaeetus leucocephalus): Lucy (on the left) looks pregnant. It’s about that time. Last year she laid eggs in March. Desi is the male on the right.

From reader Tim Anderson in Oz:

Here are two pictures of sunset looking west towards the town of Tumut, New South Wales:

Reader Joe Dickinson sent some photographs called “More from Moorea.”

 Tours to see, touch, and snorkel with stingrays (Himantura fai?) and blacktip reef sharks (Carcharhinus limbatus) are widely offered in French Polynesia.  They often are in shallow water on sandy bottoms, so pretty good views are possible without going in the water. A snorkel mask and waterproof camera provide better clarity (photo #2).  The rays, in particular, are fully habituated to frequent visitors, so up close and personal contact is easy. Note the remora on the ray’s ventral surface in the second photo.  The sharks usually have one or more remoras either swimming just below catching a ride. I like the final photo for the distorted reflections from the surface.

Note that the remora’s “sucker” is on its dorsal (top) side, so the one on the shark is riding upside down, while the one on the ray is upright.

Before we go back to the photos, we have a brief biological announcement, for there’s an evolutionary lesson here.

The “sucker” of the remora is remarkable, for it’s simply a modified dorsal fin. That was first conjectured from its morphology, but is now supported by analyzing its development. The following photos and info are from London’s Natural History Museum.

First, here’s the sucker; isn’t it bizarre?

But it’s a modified fin. The reason we know its evolutionary history is that the “sucker” begins developing just like a normal dorsal fin, which you can see by comparing its development to that of the dorsal fin in a sucker-less fish. Here is a sharksucker (Remora sp.) in early development, with the bones stained red. The sucker begins developing just like a normal fin, complete with the fin spines. Bottom photo is closeup of spines:

And then, as the Museum page notes, the structure begins growing and moving forward:

Then, over a series of small changes, the dorsal fin in the Remora begins to expand andshift towards the head.

By the time the Remora has reached around 30mm in length, the dorsal fin has become a fully formed 2mm sucking disc. It still has the components found in the dorsal fin, the tiny fin spines, spine bases and supporting bones, but the spine bases have greatly expanded.

So, the sucking disc is formed by a massive expansion of the dorsal fin through small changes while the fish is developing. It is not the result of the evolution of a completely new structure.

Here’s the diagram of the dorsal fin of a regular suckerless fish (a bass, Morone sp., top) with that of a remora (bottom). The equivalent (“homologous”) parts are given the same numbers. In the remora, the internal bases of the spines (#1) have gotten much longer, the plates anchoring the spines (#2) have become large platelike structures, and the spines themselves (#3) are the lateral structures in the sucker:

Here’s the adult remora with the bones stained red. You can clearly see the bony spines, homologous to the regular dorsal-fin spines:

So, as is usual in evolution, remarkable and useful new structures don’t arise out of nowhere: they’re simply modifications of things that were there before. One can only speculate about the steps taken by evolution to convert a dorsal fin into a sucker. If this is a product of natural selection, and it surely must be because of its complicated design and usefulness, then each intermediate step in the transition between a normal dorsal fin and the highly modified sucker fin must have been adaptive. I leave it to readers to think about how this might have happened gradually, with each modification conferring a reproductive advantage on individuals in the evolving lineage. (Hint: if you give up, read the article by Carl Zimmer highlighted by reader Glenn Butler in the comments.)

One more point: the sucker is so effective that in some places remoras are used to catch turtles—even large ones. They put a line around the remora’s tail, toss it into the water near a turtle, and it promptly heads for and fastens onto the turtle’s shell. (Remoras hate not being fastened to something.) Small turtles are simply reeled in with the fish, and large ones hauled near the boat where they can be harpooned.

Now back to our regular program: the blacktip sharks photographed by Joe: