Functional morphospace analysis of molluscan shells
Takao Ubukata

Kyoto University

ubukata@kueps.kyoto-u.ac.jp

Morphospace is a powerful tool for large-scale comparison of morphology across a number ofspecies.

Morphospaces are geometric spaces in which a particular organic form can be plotted withreference axes chosen to represent morphological parameters.

Abstraction of essential

morphological traits allows us to summarize complex morphological variation into a few
dimensional morphospace. If a functional performance can be assessed for a set of organic shapes
accommodated in the morphospace, the relationship between the functional performance andmorphological traits can be visualized as a surface on the morphospace which mimics an adaptive
landscape. The functional morphospace allows us to predict which shape is common or rare fromthe view point of functional morphology. Such a prediction can be readily tested for
paleontologists by referring actual fossil records. The present talk introduces a couple of casestudies on evolutionary morphology of fossil and extant molluscan shells using morphospaces basedon theoretical morphologic models.
In the first topic on bivalve shell morphology, geometric constraints of bivalve shell form
derived from space conflict between umbones of both valves are discussed. A functional
morphospace analysis of bivalve shells using a theoretical morphologic model revealed that thespace conflict between umbones restricts morphological diversity of bivalve shell of each groupdepending on hinge morphology. A characteristic type of ligament evolved in a heteroconch
bivalve in the late Mesozoic may has allowed the clams to develop an adaptive shell shape forburrowing through their macroevolution.
The second topic on shell form of gastropod snails focuses on a functional trade-off between
postural stability and efficiency of shell construction. The result of a functional morphospace
analysis of gastropod shells revealed that mechanical stability of a snail while creeping and theamount of shell material necessary for shell construction both depend on shell shape and are hardly
compatible with each other. The aperture of snail is often inclined to the coiling axis particularly inthe species living in the environment with less buoyancy and calcium carbonate. The resultssuggest that the aperture inclination downward relaxes the functional trade-off between posturalstability and efficiency of shell construction.
The subject of the third topic on cephalopods focuses on the functional properties for theirnektonic mode of life. The functional morphospaces constructed using a theoretical model
representing planispiral cephalopod shells clearly shows a functional trade-off between hydrostaticand hydrodynamic efficiencies: a shell form which minimizes a form drag acting on the shell whileswimming readily maximizes the density of the totally produced shell. The results ofmorphometric analyses indicates the followings. Nautilids tend to be optimized for hydrostatics incomparison with other groups.

Some ammonoid groups seem to be optimized for hydrodynamicefficiency. Tarphycerid cephalopods tend to have high scores of postural stability. The results
suggest that the macroevolutionary pattern is clearly different among cephalopod phylogenies.
Morphospaces constructed through geometric morphometric and multivariate analyses appear
to be potentially useful for a functional morphospace analysis.

Evolutionary morphology based onmorphospace analysis can be facilitated by big-data analyses using morphological databases.
Combining the conventional functional morphospace analyses and their potentially relevant
techniques is an interesting subject of future research.

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