The Question: I was swimming in the lake behind our house with a friend one day and we found a thing attached to our dock and it appeared to be brown fish egg sacks. But with closer observation it turns out to be a jelly like substance. What is it?

Submitted by: Kali, Tennessee

(click on photos and graphics to expand)

"Pectinatella magnifica 8568" by Jomegat

Freshwater bryozoan (photo by Jomegat)

The Short Answer: Without any other information or photos, it’s hard to be definitive about this, but usually, when people ask this question, what they have found is colony of the freshwater bryozoan Pectinatella magnifica. Freshwater bryozoans are tiny colonial animals that can form jelly-like masses, which are often found attached to sticks or docks.  Bryozoan colonies can be as much as a foot (30 cm) in diameter.

This Youtube video shows a couple of very nice bryozoans blobs: http://www.youtube.com/watch?v=g1H3e0Iynso

Pectinatella magnifica (photo by Triclops200)

Pectinatella magnifica (photo by Triclops200)

Pectinatella magnifica

Pectinatella magnifica

More Information: Though they are not closely related to corals, bryozoans are superficially similar in that they are tiny colonial aquatic creatures that effectively filter particles from the water. The large gelatinous species is native to North America and often grows on docks and other submerged wood. During the summer it releases small larvae that swim away and establish new colonies nearby. In the fall each colony produces thousands of tiny, seed-like disks that remain dormant over winter and germinate the following spring. Most other freshwater bryozoan species form branching tubules that resemble brown moss in the water (Bryozoa = “moss animal”). While freshwater bryozoans improve water quality, some species become a serious nuisance when they clog intake and irrigation pipes.  For further information on freshwater bryozoans, the site of Dr. Timothy Wood at Wright State is very helpful: http://www.wright.edu/~tim.wood/bryozoans.html. (My thanks to Dr. Wood for his help in preparing this answer.)


This Youtube video shows a very nice view of a bryozoan filtering food particles out of the water: http://www.youtube.com/watch?v=utPtyceR_YY&feature=related

Ask a Naturalist® (November 28, 2020) What is this Jelly-like Blob Under My Dock?. Retrieved from http://askanaturalist.com/jelly-like-blob-under-my-dock/.
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"What is this Jelly-like Blob Under My Dock?." Ask a Naturalist® [Online]. Available: http://askanaturalist.com/jelly-like-blob-under-my-dock/. [Accessed: November 28, 2020]