Abalone Radulae

Introduction
One of the characters I am using is the radula of abalone. It had been stated by earlier investigators that the rhipidoglossan radula of abalone is of no use to species level distinction within the family. After spending considerable time on these organisms I have come to a quite different conclusion, which I illustrate below with some selected examples. I will partition the radula into three areas:

• The central field
• The lateral teeth 3-5
• The marginal teeth

Overview
Above you see the full width of the radula of Haliotis dalli Henderson, 1915, from the Galapagos Islands. The radula in this species has very narrow laterals 1 as you will be able to compare in the picture below, showing some additional species. Otherwise you can clearly see, that the two sides of the radula from a zipper, i.e., the teeth of one half of the radula fold neatly in the space between two rows on the other side. Back to top of page

Central Field: Rachidian and Lateral Tooth 1
In these four sample pictures you can note a number of differences between the species. In the top left corner Haliotis kamtschatkana has the normal width of the rachidian tooth which does not have a basal projection as in H. roberti to the right. H. elegans has an extremely wide rachidian tooth, however, this character state is currently based on a single specimen of this rather uncommon species, hence needs verification. Note also, that neither of the two species at the bottom have a basal projection of the rachidian tooth.

On the lateral tooth 1 you note in H. kamtschatkana a concave depression on the ridge which is a continuation of the cutting edge of the rachidian and the lateral tooth 1. This depression cannot be found in either of the two species shown at the bottom of the picture; these are rather convex. As already mentioned, H. dalli has a special condition, in that the lateral tooth 1 is extremely narrow, a synapomorphy it shares with H. roberti McLean, 1970, from the Cocos Island. Back to top of page

Lateral Teeth 3-5
On the lateral teeth 3-5 only a single character could be extracted so far: The presence and absence of denticles on the outer margin of these teeth. I have illustrated this character with two pairs of examples. In the top two panels the denticles are visible, whereas the lower two panels do not show them. Back to top of page

Marginals
On the marginal teeth I found a single three-state character. In most species the denticles are symmetrical, i.e., the first denticle on either side of the cusp are on the same level as viewed on the longitudinal axis of the cusp (panel A). In some species the denticles are off by a count of one, and in a single species so far (Haliotis asinina Linné, 1758) it is off by two: the first denticle on one side is at the same level as the third on the other side (Panel D). Note, that the number of denticles increased progressively while they become located further from the middle of the radula. Panel C shows a intermediate marginal in lateral view, and panel D shows outer marginals with very fine brush shaped denticles even surrounding the tip of the cusp. Note however, that the median denticle is a bit stronger developed into a bristle. The outermost marginals have a further derived morphology not illustrated here. Back to top of page

Conclusion
I hope I have successfully dispelled the myth, that the rhipidoglossan radula of the Haliotidae is of no use for species level systematics. So far only a dozen characters could be extracted from the radula, so a phylogenetic analysis of the 55 species cannot be based upon the radula alone. However, I has shown its usefulness as part in a total evidence cladistic analysis. Back to top of page

Acknowledgments
I would like to acknowledge the support by the Labor für Rasterelektronenmikroskopie of the University of Basel, where I could do some of the initial observations on the intra- and inter-specific variability of the radula of abalone. Further work in conjunction with my ongoing research for my dissertation project at the University of Southern California was supported by a Student Research Grants from the Hawaiian Malacological Society and the Lehrner-Gray Fund for Marine Research (American Museum of Natural History). Back to top of page