Agnathans are known as the “jawless” fish, with some characteristics such as:
- cranium
- internal ossification (placing new bone)
- protective functions such as scales and fins which make an exoskeleton
- Lacking a vertebrae in some (Hagfish)
- gills
- body similar to eels
CLASS MYXINI
This class includes the Hagfish which is a deep sea marine vertebrate, who feeds on annelids, molluscs, crustaceans or on dead or dying fish.
They have an olfactory sac located near the nostril and brain, which is to sense smell. They also have a notochord instead of a vertebrae.
They have teeth on their tongue which is keratinised, and this can be folded to act like pincers.
They are blind but the olfactory sac helps them have excellent smell. The eyes are covered by skin and are significantly reduced as the deep sea doesn’t need eyes!
They can gain leverage on their meals by tying themselves into a knot! Such weird things.
The feeding occurs when the food is enclosed by mucooid bag. The digestive enzymes then diffuse through the mucous and the undigested food will be drained away in the mucoid bag.
One thing that should be noted about these creatures is the slime levels = way too high. They have slime glands which goes along the body and they can even kill fishes by letting it choke on their slime. Eww.
There is poor osmoregulation which is because the deep sea is usually stable in temperature so the body fluids would be iso-osmatic with the sea wate.
There is a low pressure circulatory system with 3 accessory hearts in the liver and tail. They also only have a single nasal opening which is used to pick up chemical cues.
The reproductive aspect is very unknown as there is not much found on them since the habitat is after all so deep in the sea.
CLASS PETROMYZONTIDA
This is the class with lampreys which are horrific looking creatures. There are 41 species with all of them being fish parasite. They have the structures of a vertebrate, and they have good osmoregulation as they live in fresh water and marine habitats.
They breathe through 7 gill pouches, with the oxygen flowing in through ventilation.
When they are parasites they can move to many environments including tidal! As parasites they scrap away some flesh and suck up fluid and tissues. They secrete anticoagulant to prevent clotting and their gut is simple so eating, digesting and excreting is fairly straight forward.
The lapreys first creates nests by moving rocks and the females grab onto a rock while the males attach via the oral disk. They then die after mating.
The ammoceotes larvae then burrow into the tunnels and can develop for 3 to almost 7 yeas (some even longer)!
The metamorphosis occurs during this time and then they are ready to migrate to the lake.
They then become a parasite and grab on to the fish and the host will continue to bleed through out the process of them being a parasite.
These fishes are fairly odd but so cool, but next time we will look at a more familiar bunch: Gnathostomes or the jawed fishes.
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