Porcupine Newsletter Number 25, Winter 2008/09.

CrotchetsThe taxonomy of polychaetes depends ona subjective comparison of their morphologyand particularly the chaetae. In Dodecaceriacrotchets (hook-like structures) are particularlyimportant in this respect 2 . They have a slightserpentine profile and a subterminal tooth, araised region or ledge. The hook frequently hasa distal depression or spoon (Figure 1a).Dodecaceria diceria and D. laddiThe geographical range of D. diceriaextends northwards from about 25° N andis common in the North Atlantic (Gibson,1996). D. laddi is found between 35° N and35° S (Gibson, in preparation). Because thetwo species are so similar, if not identical,they could be the same species. Hartman firstdescribed Dodecaceria laddi from the MarshallIslands (Hartman, 1954) and D. diceria from offthe Florida Keys (Hartman, 1951) 3 . Since shedescribed both species she might have beenexpected to have noted their similarity. As shedid not, one should be cautious in saying thatthey are same species. Further sampling mayshow their ranges overlap.In addition to looking the same both havesome 40-50 chaetose segments. Therefore onbalance they may be thought to be the samespecies. If they were then the name D. diceriashould take priority.Figure 1. Drawings of the crotchets of Dodecaceria usinga X40 objective, a) D. fimbriata and D. berkeleyi and b)D. diceria and D. laddiIn D. fimbriata and D. berkeleyi thecrotchets appear to be the same as one another(Figure 1a). Again, in D. diceria and D. laddithey are apparently identical (Figure 1b) toeach other. In the second pair of species thedistal region, although hooked, does not havea depression and is without as marked a toothor raised region seen in the first pair of species.In D. diceria and D. laddi the base of the hookruns smoothly into the shaft. The lower regionof the hook is corrugated although this is onoccasions only visible with an oil immersionobjective. The corrugation can appear as lobesor blunt teeth.The problem in using a characteristiccrotchet for identification is that a speciesnormally also has many forms of crotchet andthese are found in other species in the genus.Therefore identification of a specimen dependson searching through the crotchets to find thedefining type.Convergent evolutionFor Dodecaceria fimbriata and D. berkeleyito show convergent evolution they musthave been subject to the same environmentalinfluences. However, the environment coulddirectly influence development. If this wereso, it must be limited since the crotchets of D.fimbriata and D. concharum (in sensu Gibson)differ, yet the species coexist in the samehabitat. To know whether such an apparentlyLamarckian effect is possible the developmentof crotchets has to be understood.Chaetal developmentChaetae are composed of closely packedtubules which are seen macroscopically asfibrils (Hausen, 2005). These fibres are tubulesor channels. They appear to be formed from amatrix secreted around microvilli at the base ofthe chaetae. The shape of a chaeta is probablydetermined by the length of the microvilli(Gibson & Stoddart, 2005). 4 That is, theamount of matrix and the time taken to secreteit. The growth of tubules may be constrainedby the growth of adjacent tubules in a mannersimilar to the arcing of a bimetallic strip whenheated (each having different coefficients ofexpansion). Faster growing tubules, thosewith more matrix, cause a chaeta to bend as14PMNHS Newsletter No.25 Winterer 2008/09