The ensnaring predator

Last night I introduced the unusual hydroid, Candelabrum fritchmanii.  Tonight I promised to share a little more about its remarkable feeding behavior.

It may be hard to tell from the photographs, but this hydroid can change shape dramatically.  Sometimes it contracts and may only be ~5 mm long, but other times it elongates and becomes very slender and vermiform (worm-like), up to ~30 mm long.

When it's extended, the hydroid polyp acts as a "sensitive contractile snare" (Hewitt and Goddard 2001).  It's deceiving, because at first it just looks like a soft, pink "worm" waving around slowly.  But remember that this hydroid is completely covered with stinging tentacles.  Each tentacle tip is loaded with harpoon-like nematocysts that will discharge upon contact with prey.

What we didn't appreciate at first was just how fast the polyp would react to prey.  We read that Candelabrum eats small amphipods and isopods.  [Amphipods and isopods are small shrimp-like crustaceans.]  So we innocently placed an amphipod nearby (Sorry, Rebecca!). 

The polyp wrapped around the prey so quickly that we could only see a blur of coils and tentacles.  In the next image, you can barely see the blue-gray amphipod at the lower edge of the hydroid's coils.

 

The tentacles became elevated and started flicking rapidly (sometimes in small waves), in contrast to how they mostly lie flat against the body at other times.

 

The hydroid then began to engulf the amphipod whole.  (This amphipod was ~4 mm long.)  The body wall of the hydroid is partially transparent so you can see the amphipod even though it's now inside the hydroid.  Look for the dark black eye of the amphipod.

 

 

The hydroid repositioned the amphipod so that it was narrow-end down and then started to swallow it.

 

Through some intense contractions, the hydroid moved the amphipod towards the base of the polyp.  Here are two more pictures, one with the amphipod about mid-way down and then another when it's near the bottom. 

 

This is Candelabrum with its "belly full."

 

When Candelabrum is in a resting position, it's hard to imagine the extensible serpent-like coils and the sudden ferocity of this extraordinary predator!