What is a fish? Part 2 - On lampreys and hagfish
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
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