Spermcast Mating by the Pacific Gooseneck Barnacle Pollicipes polymerus

Something Darwin Didn't Know About Barnacles:
Spermcast Mating In A Common Stalked Species

Proceedings of the Royal Society of London B
(Jan. 16, 2013)

Marjan Barazandeh*, Corey S. Davis, Chris J. Neufeld*, David W. Coltman & A. Richard Palmer*

Department of Biological Sciences
University of Alberta
Edmonton, AB, Canada
   and also
*Bamfield Marine Sciences Centre
Bamfield, BC, Canada


Clever headlines
What's the story?
What do Pollicipes look like?
Where was the study done?
How did we sample barnacles?
Why did we suspect spermcasting?
What are we doing next?
Barnacles feeding in breaking waves (video)

More about Marjan Barazandeh
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(Orange sticks mark individuals of Pollicipes polymerus leaking sperm)

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Clever Headlines: Media headlines have become as much of the story as the study itself:   
THE PUZZLE: How do barnacles too far away from a mate to copulate end up with fertilized eggs? Barnacles, which are hermaphrodites, were always thought either to copulate with a neighbor using their long penises or (in some species) to self-fertilize if no partner was nearby. But even though some isolated individuals bear developing embryos, the Pacific gooseneck barnacle (Pollicipes polymerus) was thought not to self-fertilize.

A SOLUTION: We used genetic markers to show that there's a third way: spermcast mating (capture of sperm from the water) that may supplement fertilization by traditional means.

THE SIGNIFICANCE: No one ever suspected that barnacles (or any crustacean for that matter) were capable of spermcast mating, although it makes a lot of sense for barnacles that are permanently fixed to the bottom. Surprisingly, even individuals with a mate nearby still captured some sperm from the water, so sperm capture is not limited to individuals outside of penis range from a partner.

Fascinating questions arise about how Pollicipes actually capture sperm and whether other barnacles can do so as well. Furthermore, we believe that this is the first evidence of sperm capture in any crustacean, a hugely diverse group that includes crabs, shrimps, lobsters, water fleas, and copepods.

Might any other crustaceans have this ability as well?

What do Pollicipes look like?

Figure 1. Pollicipes polymerus is a stalked or gooseneck barnacle. It typically lives in dense clumps permanently attached to the bottom. The body, which is surrounded by a cluster of calcareous plates (white), sits on top of a fleshy stalk (brown) that can sometimes be many times longer than the length of the body. Pollicipes extend feathery feeding legs (cirri, inset) out into the water to capture prey dislodged by breaking waves. They also possess an extensible penis (arrow, inset) that is unusually short for barnacles (only ~0.7 body lengths compared to >2X body lengths for most species).

Where was the study done?

Figure 2. At one of our wave-exposed field sites (Seppings Island, Barkley Sound, West Coast Vancouver Island, Canada; near the Bamfield Marine Sciences Centre), Pollicipes occurred in dense beds (A), distinct small clumps of individuals (B) or as scattered, isolated individuals. Several areas (yellow circles) were searched for a) isolated individuals that were two or more body lengths from their nearest neighbor (well beyond penis range of ~0.7 body lengths) or b) isolated pairs, where two adjacent individuals were two or more body lengths from any nearest neighbor.

How did we choose barnacles to sample?

Figure 3. Careful searching revealed isolated individuals (to left) or isolated pairs (to right) that were too far away from any other Pollicipes to copulate with them (ruler divisions in cm). Surprisingly, many isolated individuals carried developing embryos. This could happen in only one of two ways: either 1) self fertilization (thought to occur in other barnacles because barnacles are hermaphrodites) or 2) capture of sperm from the water. We used DNA markers to test these alternative hypotheses and confirmed that embryo masses in these isolated Pollicipes contained many embryos bearing genetic markers from other individuals, so somehow they must have captured sperm from the water. Even more remarkably, embryos in 24% of individuals that had an adjacent partner (isolated pair) nonetheless carried embryos fertilized by distant individuals. So even when a potential partner is nearby, Pollicipes still appear to capture sperm.

Why did we suspect spermcasting?

Figure 4. We do not know the whole story about how sperm are released and captured. We first observed an individual Pollicipes leaking what appeared to be sperm in a cave on Tatoosh Island in 2006, but we didn't know what to make of that observation. However, more recent careful searches of the shore regularly revealed scattered isolated individuals leaking sperm at low tide (left, arrow) and occasional individuals leaking sperm near another leaking individual (right, arrows). We suspect sperm released this way may be captured by downshore barnacles when washed away by the first breaking waves on an incoming tide (see video below), but we have yet to confirm this. Significantly, we are not suggesting that Pollicipes don't exchange sperm via copulation, only that they have the capacity to capture sperm. We are currently trying to test whether, and how often, Pollicipes copulate.

What are we doing next?

Figure 5. To learn more about mating behavior, and about sperm release and capture, we have begun filming clusters of Pollicipes as the tide comes in using a waterproof digital camera mounted to the end of a long pole.

Barnacles feeding in breaking waves
Video1.jpg Video 1. (7.8MB Quicktime 34s; by M. Barazandeh; or view on YouTube; HD version, 58MB)

Video2.jpg Video 2. (9.9MB Quicktime 45s; by M. Barazandeh; or view on YouTube)

As the tide comes in, waves begin to break over barnacles that have been exposed to air. Pollicipes then extend their feeding legs into the water to capture any potential food items dislodged by breaking waves. We suspect sperm released by leaking up-shore individuals (above) may be captured from the water at this time.

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