Wayfaring barnacle sperm – more contrariness:
I have mentioned barnacles recently and recently mentioned that I am always on the lookout for evidence against my belief.  Here go both at the same time.

I must confess that I never made much of a distinction between barnacles and limpets.  I just filed them both under, “More reasons not to walk barefoot on underwater rocks at the sea’s edge.”  They are little rough things you wouldn’t want to put your weight on.  But a limpet is a snail, so it does saunter about, and is indeed able to return exactly to its home perch or “home scar.”  In this case “scar” seems to come from the Nordic “protruding rock” or when I have heard it used really what I would call a cliff rather coming from the French word for scab, but I would not swear to it.  The shape of the limpet’s shell adapts itself exactly to the shape of the place to which it habitually returns. 

Roughly speaking, if it’s in the right environment and has a cone shaped shell somebody has called it a limpet.  If it has a little opening at the top, it’s a barnacle, a crustacean, and it stays put.

This sedentary habit causes a social problem.  How is the male to go calling on the ladies?  Some species are known to develop a penis that is (Rethinking Barnacle Reproduction SCIENCE vol. 339 no. 6117 January 18, 2013 page 258 reviewing an article in Proceedings of the Royal Society B http:scim/ag/barne) up to eight times total body length.  So when those lads knock the ladies know it’s cut-to-the-chase without a chase.  But some relatively less well endowed species augment that by releasing sperm into the water. 

Among those species it turns out that most of the fertilized contain sperm from “distant” barnacle males.  This seems to be a surprise.  Just about everybody, I included of course, thought that reproduction would be local.

Well one case does not make a general principle, but it does raise the issue of whether reproduction in all species must have significant consanguinity in the long run is in fact universal. 

A couple of things come to mind.  For one, just how distant is “distant.”  I am sure it means more than eight body lengths, but how much more, ten, a thousand, more? 

Then of course there is a question of the long run.  Absolute pre-zygotic infertility appears never to be the case in any animal species.  Problems with fertility only arise when the outbreeding is stubbornly maintained.  It is quite possible that indeed in any one generation those distant matings produce most of the fertilized ova, but generations down the road they decline in favor of more consanguineous coupling. 

I have no independent data, but there are those that do and I suspect we shall be hearing more on the subject.

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