Saturday, April 12, 2014

Ask An Expert: Tidal Pools


I was on the rocky shore to the west of Kits beach (down the steps at the end of Trutch Street) and I noticed that there is very little life in the tide pools other than algae and mussels. I didn't see any snails or hermit crabs at all. Is this natural for this area or have the snails, hermit crabs and other littoral zone life been killed by pollution?

Thank you,

Answer (by Tom Carefoot)

The beach in question is part of a sandstone outcropping that extends from the Alma Street area eastwards.  It is characterised by shallow depressions with loose rocks (and other "moveable objects").  There is nothing wrong with the water quality in the area, and further east the shore becomes quite rich with intertidal flora and fauna.  Still, with rocks and sediments moving in the waves, there is little chance of anything delicate surviving on the Trutch-street beach, and hermit crabs and crabs in general would also not like the shifting substratum.  As you note, shells would likely be a resource in short supply for any would-be hermit-crab colonisers.  You may have noticed at least a few winkles Littorina spp. higher up on the shore, but the larger dogwhelks Nucella lamellosa, whose shells would be of a more suitable habitable size than those of winkles, are absent from most Vancouver shores.  They used to be extremely common in the Harbour side of Stanley Park, but tributyltin present in the water likely killed them off.  You may know that this latter, a component of older anti-fouling paints, and quite effective in preventing barnacles and such from settling, is now banned from inshore waters of most countries in the world.  Its problem, discovered only in the 1980s, was that it created a condition in whelks known as imposex.  This refers to the masculinisation of females, or imposition of maleness in the females leading, within a few generations, to total sterilisation of a population.  I haven't been down to the Park to check on them for several years, so a visit to the beach near the HMCS Discovery site might be something useful to do.  The winkles mentioned above comprise two common species Littorina scutulata and L. sitkana.  These are high-intertidal dwelling herbivorous species, sometimes even supratidal, and are too small to eat (winkle-"picking" is a respectable profession on Atlantic shores and in Europe).  However, thanks probably to release by winkle-eaters who can purchase live east-coast Littorina littorea from several seafood-supply stores in Vancouver, this much larger and quite tasty species can now be found on some local Vancouver beaches.   It lives in the intertidal zone, much lower than the indigenous species just mentioned, and (should you be wondering) is unlikely to enter into direct competition with them.  Well, you probably have much more information here than you really wanted, but it's easy to ramble on about topics dealing with marine invertebrates.

If you have the inclination, you can get more information on imposex on A Snails Odyssey website.

Also, you can read more about littorines & relatives on A Snail's Odyssey. The information presented there mainly relates to information published by Dr. Chris Harley of the University of British Columbia who, along with his students, first noticed the presence of L. littorea on local beaches.

Tom Carefoot is Professor Emeritus, Department of Zoology, University of British Columbia and is also the author of A Snail's Odyssey, a comprehensive, research-based site dedicated to marine invertebrates.  Tom is a frequent contributor to E-Fauna BC.  

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