Friday, February 19, 2016

Aquatic Worms


While collecting pond water to feed some damselfly larvae I came across some of these transparent worms.  They slither around in the decaying leaves like earthworms, and indeed they are in the same family, Oligochaete.  They are found in almost any freshwater pond, preferring the shallow areas either on top of sediment or deeper into it. 

They avoid bright light which they detect with primitive photoreceptors all over their body.  Movement is almost peristaltic and each segment has chaetae, bristles with muscles, so they can pull them in or extend them to grip the mud or simply swim as seen in this video.

Some Oligochaetes are predatory, feeding on other worms and tiny crustaceans such as cladocera (water fleas), while others eat detritus such as decomposing organisms and bacteria.  They eat by extending their pharynx like a tongue, in some cases even turning it inside out like a suction cup to pull food into their gut.

Planaria - Click to enlarge
Most reproduction is asexual.  They can split in half or even multiple pieces, each then regenerating into a full worm similar to the planaria we have been finding this winter.  Another mechanism is producing buds where new worms grow out of their body before separating as  new individuals.

Sexual reproduction is a mutual act with flexibility as they are hermaphrodites, each having male and female parts.  One may fertilize the other or they may fertilize each other at the same time.  The egg and sperm form a cocoon which is deposited in the pond bottom where they will emerge shortly or overwinter to come out in the spring.  If the pond dries up they can survive for a while as cysts.

This shallow pond is 150 feet above the creek valley and a quarter mile from other ponds.  In spite of this it is teeming with life, likely brought in on the feet of wood ducks which visit frequently.  Oligochaetes are close to the bottom of the food chain.  They are spared predators like fish and crayfish but have to dodge newts, salamanders, frogs and even tadpoles.

Coming soon, more strange pond life.


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