Dia de los Bryos

Okay, I know my timing is a bit off for a reference to el Día del los Muertos, but something I saw recently made me think of the celebration that I associate most with skulls. I’ve always had a fascination with skulls, what they can tell us about evolution and adaptation, but also how they represent death or perhaps something ‘other,’ something less tangible, even sinister. Yet, skulls are one of the last remains left by vertebrate animals that people can easily put a name and a face to.

The skull I saw, however, did not represent death. True, it was made of calcified material, much like our bones are…


Small Membranipora membranacea colony growing on bull kelp. Bryozoans grow clonally, budding off new individuals from a founding zooid that settled as a larva from the plankton. In M. membranacea, the larva quickly develops into two zooids, so this species has two founders, or ancestrulae, which are the eyes of the “skull.” This colony is about 0.5mm wide.

…but this isn’t a skull at all. Rather, it’s an exoskeleton inside of which three tiny, genetically identical animals, known as zooids, reside. The animals are called bryozoans, and they are amazing, though diminutive, creatures. The name translates as “moss-animal,” and they are so-called because they grow as colonies that can coat hard surfaces, such as rocks, animal shells, and underwater plants.

The species is Membranipora membranacea, which is part of one of the coolest groups of bryozoans because it spends a long time (months1) feeding in the plankton as a microscopic swimming baby (It’s really called a cyphonautes larva, but I can’t resist). One outcome of this planktonic life history phase is that bryozoan babies can potentially spread over wide distances along coast before they ‘settle’ on a surface and undergo metamorphosis to start life affixed to a surface.

In the last week of November 2013, we had a huge influx of Membranipora settlers on kelp along the Northern California coast. My labmate had collected bull kelp (Nereocystis luetkeana) near Fort Ross, CA, to use in feeding experiments with urchins and abalone, and we noticed that the blades were peppered with tiny spots.

Bryo Skull_far

Numerous colonies on a small piece of a bull kelp’s blade. Note the colony with five zooids in the lower-right corner of the photo.

You can see in the photo above some colonies are slightly larger, which suggests that they settled earlier or grew faster. You can even see the faint outlines of colonies on the opposite side of the blade. Hundreds of colonies were scattered over the blades of a single kelp individual.

One of my professors, Dr. Eric Sanford, mentioned seeing the same thing on giant kelp (Macrocystis pyrifera) at Point Reyes around the same time. I find it fascinating to think about what will happen to all of these colonies. Will they be eaten by sea slugs? Will they overgrow one another? Will their kelp hosts get ripped off the rocks and wash ashore in a big storm?

Unfortunately, I haven’t gone back to see what the kelp looks like now, but you can bet that most of the animals that settled will die before reproducing. And, imagine how many of the babies were eaten or “lost” in the plankton! Fortunately for Membranipora membranacea, they produce many offspring and the fact the larvae spend so much time in the plankton may actually give them access to good food resources that can prepare them for a chance to settle on preferred substrates.

One last photo shows of the zooids extending its tentacles to feed (sorry it is a bit blurry):

The feeding structure, or 'lophophore', of the bottom zooid is extended

The feeding structure, or ‘lophophore’, of the bottom zooid is extended


1Yoshioka, P.M. 1982. Role of planktonic and benthic factors in the population dynamics of the bryozoans, Mebranipora membranacea. Ecology 63: 457-468.