Hagfish have a skull, but no spine. They diverged from vertebrates prior to the origin of many other structures that are widespread within the group, including jaws. Hagfish are extremely important for understanding the origins of these key structures, but they are also famous for an unusual behavior—tying themselves in knots.
Hagfish have an eel-like body. They lead a bottom dwelling life and have a great sense of smell, but lack well developed eyes. When stressed, hagfish release a secretion that contains special filaments from glands along the body. When it contacts water, this secretion forms a massive slime and makes the hagfish an unpleasant bite for potential predators. To sneak out from this slimy shelter, the Atlantic hagfish, Myxine glutinosa, makes knots, which wring off the layer of slime. The knot, traveling along the body column, can provide a surface for the hagfish to push off. This enables the animal to pull its body out of the hole it makes in it’s prey’s flesh, or escape the grasp of a scientist.
Pictured above are the heads of two Atlantic hagfish, whose bodies are burried in soft sediments. They were caught in the Gullmarfjord on the Swedish Westcoast.
Botryllus schlosseriis a colonial tunicate (or sea squirt), so named because it lives in colonies that are communally covered by a leathery tunic. Its larvae bear a striking resemblance to vertebrates, and are even called tadpoles. The resemblance is not superficial or coincidental, tunicates and their kin are the closest living relatives of vertebrates. Each tadpole attaches itself to a rock, pier, or other hard surface in the sea, and metamorphoses into a sack-like adult that will spend the rest of its life stuck in that one spot.
Tadpoles are produced sexually–they arise from an egg that is fertilized by a sperm. Like many other animals, though, Botryllus also reproduces asexually by budding off clones of itself. Each adult (also called a zooid) produces a bud, and this bud in turn begins producing another bud even before its own heart begins to beat. These clones remain attached to each other in a star shaped group with common central opening, called a siphon, and continue to share resources through their connected circulatory systems. Once the colony is large and robust enough, usually with 5 to 10 members, each adult forms a pair of ovaries and testes, and the next generation of tadpole larvae can be produced.
This is the first in a series of illustrated lifecycles I’ll be posting to CreatureCast.
