A few days ago I was watching some Giant Barnacles (Balanus nubilus) feeding in a tidepool. I was trying to get a photo of the feeding activity — the extended appendages (cirri) forming a broad net to capture zooplankton from the water:
Below, look closely to see the fine setae (bristles) along the cirri (appendages) that form a sieve for capturing food particles as the water passes through. [Click on the image for a larger version.]
I continued to scan for individuals that were feeding when I noticed this:
!!! This is something that you don't get to see very often — copulation in barnacles! The long tubular structure extending from the left hand barnacle to the right hand barnacle is a penis. Most barnacle species are known for having very long penes. Barnacles are sessile (attached to the substrate); to mate with a neighbor, it's helpful to have a long reach!
Barnacles are hermaphrodites. One barnacle deposits sperm into the the mantle cavity of another individual. The receiving barnacle then extrudes a batch of eggs which are fertilized. The packet of embryos is moved to the base of the barnacle (inside the mantle cavity) where the brooded embryos continue to develop. The timing varies, but after a couple of weeks, tiny larvae hatch and are released from the adult barnacle. They're free-swimming, planktonic larvae that will eventually return to the adult habitat and undergo metamorphosis into juvenile barnacles.
I captured several photos of Giant Barnacle copulation events, and because it's relatively rare to see this in the field, here are two more examples:
I know...you probably didn't expect to see barnacle sex when you checked into this blog today. There's always something different at the Natural History of Bodega Head! :)
These photos were taken along the Oregon coast on 30 May 2017.