Jon R Stone, State University of New York

The genus Lambis is a member of the family Strombidae, a group of marine gastropods known as `true conchs' and renowned for their elaborated labia. In addition to the siphonal canal, the apertural lips of species of Lambis are adorned with circumapertural projections (CAPS), fingerlike digitations emanating from the edge of the aperture. The number of CAPS exhibited by the nine species currently classified as Lambis ranges between 5 and 11 excluding 7. Two aspects of CAPS have vexed conchologists: how did CAPS evolve and what is their function (i.e. are they adaptations)?

R. Tucker Abbott [1] classified the 9 species of Lambis, predominantly on the basis of shell morphology. Implicit in the classification is the assumption that this group of gastropods with similar shells share a unique common ancestry. The classification, and the historical relationships it implies, has persisted with little modification for more than three decades. CAPS have been postulated to have evolved in an "escalating arms race" as a response to predation [9], and they have been shown to confer protection against crushing predators by increasing shell diameter (thereby decreasing the mechanical advantage of the predators' crushing apparatus), distributing stress over a large area of the shell, localising stress at the thickest points of the shell, or creating enormous reactionary forces upon predators[3]. They have also been hypothesized to provide stability on substrates (by permitting 'snowshoeing') and to be a means of circumventing geometric constraints during growth[4]. It is also possible that CAPS enhance stability via hydrodynamic processes (e.g. drag). A recent study of the morphological evolution of species of Lambis [7] yeilds hypotheses concerning the origin and function of CAPS. The study combines cladistic methodology, mathematical modelling, and morphospatial analysis. In cladistic analysis[2], information is used to construct a branching diagram that can be interpreted as depicting phylogeny. Mathematical modelling of shells has evolved into the discipline of `theoretical conchology', wherein forms are described by only a few parameters . Morphospatial analysis has been used to represent a wide spectrum of organismal forms (usually as points) within mathematical spaces [8]. The combination of these three tools can be used to define trajectories in mathematical spaces that describe ontogenies of extant taxa with reference to phylogeny. The result is that an hypothesis of the history of morphological evolution of the extant taxa and the forms of their ancestors can be reconstructed.

The study of the morphological evolution of Lambis revealed that the current classification of the genus is untenable. Three species of the genus Strombus are inferred to be among the most recently evolved members of a monophyletic group containing all members of Lambis. The number of CAPS among the 12 species in the monophyletic group is inferred to have varied during the course of evolution, generally (and most recently) decreasing. This information can be used to test the hypotheses of adaptation mentioned before: the adaptive significance of CAPS can be addressed through experiments that are designed to falsify each hypothesis as well as by reference to the hypothesised phylogenetic relationships among species of Lambis and genera of Strombidae. For example, if CAPS have evolved in an escalating arms race, changes in the number of CAPS (and the resistance to predation which they confer) and changes in the crushing ability of predators on independently derived cladograms should be correlated. The number of CAPS can be altered experimentally and the modified shells subjected to crushing tests to verify the causal aspects of any correlations revealed. The results of such practical experiments might have a significant bearing on `thought experiments' of theoretical conchology, such as whether `heptaCAPS' lambids (shells of Lambis with 7 CAPS - see figure 1) ever existed as ancestors en route to fewer CAPS (as implied by the cladograms) or could have evolved.

 This work was supported by a Malacological Society of London Centenary Award to the author.

 
References

[1] Abbott R T (1961) The genus Lambis in the Indo-Pacific. Indo-Pacific Mollusca 1: 147-174.

[2] Hennig W (1966) Phylogenetic Systematics. University of Illinois Press, Urbana.

[3] Palmer A R (1979) Fish predation and the evolution of gastropod shell sculpture: experimental and geographic evidence. Evolution 33: 697-713.

[4] Savazzi E (1991) Constructional morphology of strombid gastropods. Lethaia 24: 311-331.

[5] Stone J R (1995) CerioShell: a computer program designed to simulate variation in shell form. Palaeobiology
21: 509-519.

[6] Stone J R (1996) The evolution of ideas: a phylogeny of shell models. The American Naturalist 148: 904-929.

[7] Stone J R (1997a) The Shell Game: Who's under What? Morphological evolution and trajectories through morphospace exemplified with species of Lambis. Ph.D. Dissertation. University of Toronto.

[8] Stone J R (1997b) The spirit of D'Arcy Thompson dwells in empirical morphospace. Mathematical Biosciences
142: 13-30.

[9] Vermeij G J (1989) Evolution and escalation: an ecological history of life. Princeton University Press, Princeton.

Fig. 1

A shell with 7 'circumapertural projections'.

A putative ancestral form unobserved in extant species of the genus Lambis. The image, which does not depict the siphonal canal, was simulated using the computer program CerioShell [5].