Pacific Coast Convergent Evolution

Long Beach, part of the Pacific Rim National Park, is an enormous stretch of surf swept sand on Vancouver Island. When you move south towards Wickaninnish there is an Interpretive Centre (accessible for wheelchairs) where exhibits and movies allow you to explore the natural and cultural history of the Pacific Rim.

Rocky Shore Animals

At first glance many of the animals that can be seen on and under the rocks look like their European (Atlantic) counterparts. Closer inspection reveals that they are actually different species that are adapted to the same niche, and therefore look very similar. Three examples of this phenomenon follow:

• Two Barnacles – Semialanus cariosus and Semibalanus balanoides occupy the same part of the shore in the Pacific and Atlantic respectively. They look almost identical, but naturally the Canadian species grows a little bigger (like everything in Canada!). The shell plates of the Pacific species tend to be a little more robust, and they also seem to be more ribbed (accounting for the common name – ‘Thatched Barnacle’). They both behave in the same way, filtering plankton and organic debris from the water that swirls past them, and both are adapted to survive the rigours of life in rough intertidal waters.
• Two Sea Urchins – Strongylocentrotus droebachiensis and Echinus esculentus both browse on algae in the same way, and although they are usually different colours (green in the Pacific and reddish in the Atlantic) they are otherwise almost identical. Here not two different species, but two different genera have adapted to similar conditions in the same way – habits and body plans are almost the same. (It is these, and other, Pacific urchins that can destroy Kelp Beds if not kept in check by Sea Otters.
• Two Porcelain Crabs – Petrolisthes cinctipes and Porcellana platycheles both live under rocks and in crevices where they filter-feed. Once again the Pacific version is slightly larger, but they look otherwise identical. (Incidentally, these are not true crabs – they are more closely related to the ‘Squat Lobsters’, but only a specialist (or someone with a penchant for counting legs) would realise this!).

Convergent Evolution

It is true that the three examples given above represent animals that each had common ancestors in the relatively near past (geologically speaking), but they demonstrate quite nicely the way that different animal species and genera (and less closely related taxa) can adapt to similar circumstances in very similar ways. Birds and Bats both fly, so they look superficially quite like one-another. Dolphins and Sharks both swim in the sea, and again have superficial resemblances. Anyone familiar with the typical zonation patterns of Atlantic rocky shores will feel at home on Pacific beaches – but while everything looks the same, in detail everything is different!

Pacific Beach

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