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.

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.

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

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.