Capitella

Geoff Read gread at actrix.gen.nz
Sun Oct 10 21:46:17 EST 1999


Some more thoughts on Capitella (capitata) species complexes.

The latest Capitella sp. I paper to cross my desk is the one by Horng & 
Taghon (1999) JMBE 242:41-57. They are looking at organic 
contaminants and the effects of fecal pellet formation thereon. Their 
Capitella sp. I culture came direct from Judith Grassle. That is probably 
fine (except for worries about changes due to inbreeding) but it highlights 
what is going on. It is difficult to experiment on your local Capitella sibling 
spp because they are not definable - or so people may think.

Horng & Taghon talk of "members of the Capitella sibling species complex" 
which is fine, but the previous sentence begins, "Capitella capitata, 
originally believed to be a single species, is now known to consist of a 
complex of species ..." Oops, we just parted company. Paper after paper 
after paper commits this mistake. In my scheme of things C. capitata will 
always be a single species. The question is which, if any, of the so far 
reported 'siblings' is C. capitata.  By the way I would rewrite the above 
along the lines of, "the morphologically similar capitellids, originally believed 
to be one species, Capitella capitata, are now known to be ..." It's not at all 
difficult to be more accurate in name usage.   

What annelids do Capitella species resemble? They resemble 
oligochaetes. What do oligochaetologists use routinely to distinguish 
externally similar species? They use internal anatomy. Have any of the 
students of the group of species similar to Capitella capitata proved  the 
internal morphologies are identical? Not as far as I am aware.   

Knowlton (1993) talks of sibling species being artifacts of ignorance. Amen 
to that. Usually they become pseudo-sibling species once more 
exhaustively examined. Adopting the term 'sibling species' should not be 
regarded as an end in itself, nor should the term be used as a crutch. It 
explains nothing. 

James Blake wrote:
> each of those four siblings can be readily separated from 
> one another on the basis of prostomial shape, nature of the peristomium, 
> shape of the posterior end, distribution of hooded hooks, and several of 
> the reproductive and developmental features.  

To which can be added genital hook morphology, sperm morphology, 
chromosome number and karyotypes - thanks to work by Judith Grassle 
and Kevin Eckelbarger.   

One thing that worries me is the developmental sequence of capillary setae 
replacing hooks. I find it hard to believe no one found any variation in 
setigers 1-6 in juveniles at least. I also note in passing that Capitella sp III 
(C. jonesi), with capillaries always only on the first three setigers, has 
never been a sibling species, although included in papers as such. It just 
doesn't fit C. capitata.   

Wilcox and Nickell recently (1988: Ophelia 49(2):141-145), and others earlier, 
report poecilogony in  Capitella capitata. For W&N it is on the basis of a 
difference in egg size in the brood tube of one specimen. I don't know 
what to make of this observation. Does anyone have any suggestions?

> What is needed is to acquire a collection of adult Capitella from Naples, 
> the type locality and to establish and fix a definition of C. capitata from 
> there.

Jim corrected himself to Greenland. Looking at Fabricius one can see the 
type locality appears to be a  place called 'Pullateriak,' which is helpful. 
However, one can imagine there might be now, if not in Fabricius's day, 
more than one species of the complex present there.  

--
   Geoff Read <gread at actrix.gen.nz>


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