Mrs. RickJay and I recently got the special 30th anniversary edition of “Jaws” and watched it over two bowls of popcorn. After enjoying the movie I fired up some pictures of Great Whites (the shark, not the band) on the Internet. It suddenly crossed my mind that sharks, as well as their relatives the rays and skates, are in some ways not at all like fish.
Specifically, of course, they don’t have bones made of bone.
I also found that sharks, rays, and skates are in an altogether different class (Chondrichthye) than other fish.
So is a shark actually a fish? Or is it a different kind of animal?
Well, “fish” is not really a technical scientific term. According to Merriam Webster, a fish is:
“Fish” thus normally includes three different living classes of vertebrates: the Osteoichthyes, or bony fish; the Chondrichthyes, or cartilaginous fish (sharks and rays, as well as some odder groups such as chimeras), and the Agnata, or jawless fish (lampreys, hagfish, plus many early fossil forms). There is yet another class, the Placoderma, which includes armored fish that are all now extinct.
So yes, sharks are indeed fish, but a quite different kind of fish than a trout. The two are about as closely related as an elephant is to a turtle.
Wouldn’t any two land vertibrates be much more closely related than a trout and a shark, if we measure relatedness by the amount of time since the two species diverged? I realize you weren’t making an exact comparison, but it might have sounded that way…
Yes, after I posted that I thought better of it. Elephants and turtles are much more closely related to each other than sharks are to trout. And trout are more closely related to elephants than they are to sharks.
Sharks are most certainly fish, belonging to class Chondrichthyes (and according to this article subclass Elasmobranchii, though that doesn’t appear in traditional taxonomy). They are distinct from what we generally think of as fish in that, not only are their skeletons cartilaginous (though the jaws are typically proper bone) but that they do not have scales, preferring instead a surprisingly rough skin. (The roughness actually reduces drag underwater, which seems counterintuitive to most people not versed in fluid dynamics.)
Sharks are typically considered to be “primitive” in that their evolutionary “progress” has essentially halted–most sharks have effectively the same form and features today as their ancestors tens of millions of years ago–but it is more properly considered that they are so finely adapted to their environment that selective pressures have virtually ceased to have any net impact upon them. Having dispensed with the need for baggage like temperature regulation, unnecessary appendages, and an unwieldy oversized brain, they have proceeded to dominate more than three quarters of the planet, which compares favorably with the Mongols, the Romans, and the Moonies, not to mention Donald Trump. Amazing creatures, they are.
Chondrichthyes (to which sharks and their allies belong) is a sister clade to Teleostomi (to which more traditional fish belong, as well as ourselves). The common clade (Chondricthyes + Teleostomi) is called Eugnathostomata, and its ancestors (within Gnathostomata - the jawed fishes) had bony skeletons, just as the teleostomes do. This means that sharks, etc., are secondarily cartilaginous, having evolved from bony ancestors.
Actually, they do. They are called “placoid scales”, and have an outermost layer of enamel, dentine and pulp. Very much like teeth, in fact.
I thought I read somewhere that some of the creatures called eels were fish and some were a more distinctly seperate species. (Who I guess may still be fish given the broadness of the characterization.)
Are there “real” eels and “faux” eels? Or am I completely off on this?
(Sorry for the hijack, but it’s 2 am, I saw this thread, and now I must know what I ate at that sushi place the other night.)
Is there a standard regulating organization for taxonomies? Looking even briefly online I’ve found at least three distinct and mutally incompatible taxonomies for sharks, any my rather old (and not really authoritative) reference is incompatible with all of them.
That’s very interesting. Is this some kind of developmental retardation of the process of endochondral ossification or is skeletal development replaced by something completely different?
Your link isn’t working for me, but I’m guessing that placoid scales are the same as denticles which, while they are the precursors to cycloid and ganoid scales, don’t function the same way as the what are commonly thought of as scales, just as scales don’t confer the same benefits as feathers.
Generally, it seems to be whatever is accepted by most folks is…whatever is accepted. There are regulatory agencies for binomial nomenclature (e.g., the International Code of Zoological Nomenclature, and it’s botanical brother, the International Code of Botanical Nomenclature), but none for higher groups, so far as I know. For online sources, I generally use palaeos.com, the Tree of Life Web (tolweb.org), or Mikko’s Phylogeny Archive. Those are all cladistic sites (I’m no fan of Linnaean taxonomies, but then I 'd guess there are many who aren’t fans of cladistic ones).
Most likely, the former. While shark bones don’t ossify, they do typically calcify, providing some strength (though not as strong as true bone).
I’m not sure why that would be relevant; scales are scales. The general evolutionary trend is thought to be placoid -> cosmoid -> ganoid -> ctenoid/cycloid (some fishes can even have multiple scale types, e.g., flounders and other flatfish).
And feathers are no longer thought to be derived from reptilian scales. The feather is a whole new animal. So to speak.