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I am delighted to share the first of two guest posts from our dear friend Tom Feild. Read on today to learn why we are more closely related to trout than trout are to sharks!

What is a fish?

Many of us remember learning five classes of vertebrates: fish, amphibians, reptiles, birds, and mammals. While this traditional arrangement is generally still followed for the latter four groups, what are typically called fish have now been divided into at least four distinct classes. This is due to the richness and diversity of the world’s fish fauna, with nearly 34,000 living species, more than half of all vertebrates. This diversity is driven by the long history of vertebrate evolution.

When Linnaeus first defined the classes of vertebrates, relationships were based on structure and behavior, which can be subjective. The theory of evolution provided a more objective framework, and modern taxonomy strives to define taxa (e.g., species, genus, family, etc.) according to their evolutionary relationships. Each taxon should be defined to include all descendants from a common ancestor. Thus, every member of a taxon should be more closely related to every other member of that taxon than they are to any member of a different taxon.

One of the major developments in vertebrate evolution was the development of non-cartilaginous bones. Sharks and Rays have cartilaginous skeletons; this branch of the vertebrate tree first appeared prior to the evolution of bones. The other major group of fish, called bony fish, appeared after the evolution of bones. Amphibians, reptiles, birds and mammals, collectively called tetrapods, also have bones. This shows that the sharks diverged from the other “fish” before the tetrapods diverged from the bony fish. We are more closely related to trout than trout are to sharks! If we maintained the class Pisces as defined by Linnaeus to include sharks and bony fish, this taxon should also include mammals, birds, amphibians and reptiles – a fishy arrangement indeed!

Instead sharks and rays are placed in class Chondrichthyes and bony fish in class Actinopterygii. For this reason, MBP places the sharks and rays on a separate page from the bony fish. The taxon Gnathostomata is defined to include all jawed vertebrates, so sharks and rays, bony fish and tetrapods are all within Gnathostoma.

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Join us tomorrow for Part 2 of this guest post, where things get even stranger!

• Bill

Enjoy part 2 of Tom Feild's excellent summary of the amazing evolutionary history of what we call "fish" (and a bit on "birds")!

Yesterday's post discussed the classification of the jawed vertebrates, including sharks and rays (Chondrichthyes) and bony fish (Actinopterygii). There are two much smaller, but equally interesting classes of fish found in Maryland: the eel-like hagfish (Myxini) and lampreys (Cephalaspidomorphi or Hyperoartia).

Hagfish and Lampreys are from lineages even more ancient than the sharks. These branches appeared prior to the evolution of jaws. Instead of jaws, lampreys have concentric circles of bizarre rasp-like teeth that can be used to latch onto fish and to abrade the flesh. They also feed on carrion and filter-feed. Hagfish have similar feeding habits, but they have teeth arranged in two rows. To increase the strength of their jawless bite they sometimes tie themselves in an overhand knot as they feed and pull their head through the loop this forms, squeezing the head as it goes through to push the teeth together and assist in taking a bite from their prey.

The Sea Lamprey is pelagic, but other species can sometimes be seen in shallow coastal plain streams in the spring. In our area hagfish are generally seen only seen at sea, where they are sometimes seen by fishermen when they prey on fish that have been caught on lines or in nets.

Hagfish lack true vertebrae, but it is believed that they evolved from ancestors that had them, rather than branching off the evolutionary tree prior to their evolution. So, hagfish have no vertebrae, but are considered vertebrates! Strange, but in accordance with the goal of defining taxa consistent with evolutionary relationships.

A final note on the tetrapods: mammals, birds, reptiles and amphibians. The goal of ensuring these taxa are consistent with evolutionary relationships was complicated by the discovery that birds descended from therapod dinosaurs. It is currently believed that the closest living relatives of the birds are the crocodilians. Birds are more closely related to alligators than alligators are to lizards, snakes, or turtles. To be consistent, birds should be placed within the class Reptilia. Some authorities have taken this step (See ‘Reptile’ in Wikipedia), but traditions are hard to change!

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Thanks, Tom! This was fantastic. - Bill

Least Brook Lamprey (Lampetra aepyptera) photo courtesy of Ben Springer. More at Maryland Biodiversity Project: https://www.marylandbiodiversity.com/view/160

Today's photo is a Fin Whale (Balaenoptera physalus) in Maryland waters in August 2009. The Fin Whale is second only to the Blue Whale as the largest creature that has ever lived. Think of what wonders persist in the world - creatures larger than any dinosaur in our oceans!

Maryland waters are traversed by at least 23 species of cetaceans (Order Cetacea) - the whales, dolphins, and porpoises. Our species list includes Blue Whale, Sperm Whale, Orca and others. It may be possible to add other species to our list as we mine data sets and publications. Here's the MBP checklist: https://www.marylandbiodiversity.com/viewChecklist.php?order=Cetacea

Please support ocean conservation in addition to our local efforts. I try to balance my conservation/environmental donations with a mix of local and global/international. Healthy oceans are critical to our long-term survival on this planet. We

Have a great week!

Bill

Photo by Bill Hubick. More at Maryland Biodiversity Project:
https://www.marylandbiodiversity.com/view/826