Dive Dry with Dr. Bill

#633: Float Like a Feather, Sting Like a Bee

I'm not a big fan of boxing, but I do remember lying in bed and listening to a radio I built myself to one of Cassius Clay's matches. Who, you youngsters ask? Well, you may know him as Muhammed Ali. For today's column, I'm going to paraphrase one of his famous sayings. My subjects thing week "float like a" feather "and sting like a bee." Have I taken too many punches to the head? Hardly. I'm talking about one of the most vicious beasts in our dive park... the feather hydroids.

Now hydroids are members of the phylum Cnidaria along with jellyfish, corals and sea anemones. What do they have in common? Stinging cells known as nematocysts. Earlier in my diving career I used to wear a 3mm shorty wetsuit during summer and fall. I stopped doing that when I began shooting video and noticed how much the camera was shaking in my shots! I also noticed that my legs and arms were sometimes covered in irritating red welts. The feather hydroids were to blame.

I'm going to write about two fairly common ones in our local waters, the ostrich plume hydroid and what many call the white feather hydroid. Both are stinkers... er, I mean stingers. Both possess the tiny stinging cells common to the phylum and they are capable of penetrating my thick epidermis (and yours too)!

The ostrich plume hydroid in our waters is probably Aglaophenia latirostris or A. struthionides. I don't carry my microscope when I'm down under and even specialists get confused. It is found from British Columbia to San Diego, so we are at the southern end of its range. The white feather hydroid (Plumularia sp) is even more common and is easily overlooked... until you get zapped by the little buggers.

These hydroids are actually colonies of "individuals." Feeding in feather hydroids is accomplished by tiny polyps looking like miniature sea anemones that dot the branches. They use their tentacles to trap or filter food in the form of tiny plankton or organic matter as it passes by in the current. The feeding polyps are accompanied by the stinging defensive polyps which lack mouths to feed and look like a single tentacle. These hydroids are probably hated by my friends on the political right since they represent the "finest" in socialism. All the polyps are connected by a common gut and food gathered by the feeders is shared.

Their reproduction is very interesting although I doubt it will ever be featured in Playboy or Playgirl, only in scientific journals (and my columns!). There is an alternation of generations between the polyp colonies and tiny jellyfish-like medusae which reproduce sexually. In some hydroid species the medusae are released and dispersed, but in Aglaophenia they are contained in structures known as corbulae. Male sperm is released and fertilizes the egg inside the female where it develops into a wormlike larva known as a planula. The planula crawls around for a short while before attaching and growing into a new colony.

I know some female divers might be disturbed to read that these hydroids are often home to spiders... sea spiders known as pycnogonids which feed on the hydroid polyps. Another predator found clinging to them are caprellid amphipods, also known as skeleton shrimp.

© 2015 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Ostrich plume hydroid and "bumps" known as corbulae containing the reproductive polyps;
and the white feather hydroid and large colony. .

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia