Thursday, October 16, 2014

Hungry flowers

My hermit crabs love hydroids, so I bring home "messy" eelgrass, covered with assorted hydroids and diatom fuzz. Usually, they clean all this off overnight, but this time, they've left me a small cluster of Obelia, right beside the glass wall of the tank.

The tallest of these plant-like animals is about an inch high, and the individual "flowers" are barely visible without a lens. They sway constantly in the current, or bounce as hermits walk up and down the eelgrass. And out of the water, they collapse, so I've been trying to take their photo through the glass.

A tangled mess, growing from the end of a blade of eelgrass also coated with pink tunicates.

Zooming in. Among the hydroids, some small critters have laid their egg masses. To the naked eye, these are just specks of white dust.

Some of the taller stalks, showing the polyps in different stages of development. The feeding polyps (the ones with tentacles) sting tiny swimming critters, such as copepods and smaller plankton.

Life cycle of an Obelia. Image from Kent Simmons, U of Winnipeg.
The reproductive polyps are more difficult to distinguish in these photos. The empty cone near the top of the last photo may be one of them.

An empty reproductive polyp. Photo from 2011.

The hydroids will release the next stage, the medusas, from these reproductive polyps. I have seen them swimming near the parents, but in this tank, they end up in the pump, and don't survive.


  1. Have you ever tried setting up a plankton tank, possibly next to a sunny spot to generate some phytoplankton? Chances are, you'd probably just get green sludge after a while. But you could possibly use that as a nursery for some of your beasties.

  2. Tim, I had a tank outside, and it did ok for a while, then local pollution killed it. It's a bit difficult since we don't have sunny spots, except for a few minutes a day in mid-summer. And raccoons come to fish in any container I leave outside with seaweed in it.

    I do have a small tank inside without a pump, just the bubbler. I move delicate and tiny things over there. Some survive.


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