Introduction

Ancient lakes and their unique faunas present unparalleled opportunities for studying the patterns and processes of evolution and speciation. Lake Tanganyika is an ancient lake, 8-12 million years old located in southeast Africa. It is well known for hosting a large, endemic species assemblage including many species of fishes, crabs, sponges, and of course gastropods (Fig. 1).

The heavily decorated and thickened shells of Lake Tanganyikan gastropods are atypical for freshwater gastropods and have been reported to comprise as many as 4 layers of crossed- lamellar crystal architecture (aragonite crystals arranged in sheets) (West & Cohen, 1996). In contrast, non-lacustrine gastropods are often smooth and thin shelled with few crossed lamellar layers. Previous studies (West et al., 1991; West & Cohen, 1994, 1996) have identified predator/prey co- evolution between the gastropods and their crab predators as the source for the unusual shells. In particular, West & Cohen (1996) concluded that lake gastropods with 3 and 4 shell layers have evolved repeatedly to strengthen the shell as a specific adaptation in defense against shell-crushing predators. However, these studies included only a small subsample of the species, and did not examine possible ontogenetic variation in shell microstructure which would have important implications for this co-evolutionary scenario. Thus, the goals of this analysis are to:

• Carry out a broad comparative survey of gastropod shell microstructure among Lake Tanganyika species and two outgroups
• Document ontogenetic variation in shell microstructure within these species

Materials and Methods

Paramelania imperialis cross section

• Shells of 18 species were selected, cleaned in bleach and embedded in epofix resin.
• Large shells were cut with a diamond saw parallel to the long axis of the shell prior to embedding (see Fig. 2). Resin blocks were polished using three different grit sizes (240, 360, and 600) until the desired cross section was achieved.
• To enhance visualization of microstructure, polished resin blocks were etched for various times using 2% and 1% acetic acid, and washed with distilled water.
• Resin blocks were mounted on stubs, sputter coated with 10-12 nm 60% : 40% gold : palladium alloy and carbon, and photographed on the Leica 3350 Stereoscan SEM.
• The number of layers of crossed-lamellar structure was quantified for each species throughout their ontogeny.

Results and Discussion

Consistent with previous studies, the number of crossed lamellar layers was found to vary from a single comarginal layer (parallel to the shell margin), to a maximum of 4 alternating comarginal and radial (perpendicular to shell margin) crossed lamellar layers. However, previously unreported is that, rather than alternating comarginal and radial layers, some species possessed two comarginal layers that differed in the angle of the crystals (Anceya giraudi, Syrnolopsis lacustris). In addition, layers were often found to be bounded by a thin layer of regular prismatic microstructure; in some cases, numerous prismatic layers were intercalated within individual crossed lamellar layers.

Most importantly, significant variance in shell microstructure during ontogeny was found. Some species possessed only a single comarginal crossed lamellar layer (Martelia tanganyicensis), but the intercalated prismatic layers varied and fluctuated in thickness. In species with numerous crossed lamellar layers, typically shell microstructure was simple early in ontogeny (one to two layers), expanded to three or four layers during young adult stages, then decreased again to usually a single comarginal layer in the adult. In species with significant spiral ornament, the outer shell layer was invariably constructed of radial crossed lamellar structure (Tiphobia, Lavigeria, Paramelania).

A-D: Ontogenetic microstructure sequence of Mysorelloides multisulcata. Arrows point to changes in microstructure. A: Larval shell. B, C: Juvenile. D: Adult. Scale bar is 10 µm.

A-D: Ontogenetic microstructure sequence of Tanganyica rufofilosa. Arrows point to changes in microstructure A: Larval shell. B, C: Juvenile. D: Adult. Scale bar is 20 µm.

Conclusions

• The number of crossed lamellar layers varies from one to four.
• Early in ontogeny, microstructure is typically rather simple, with one or two layers
• In species with many layers, the layers do not persist throughout ontogeny. Instead, shell microstructure becomes simplified again in the adult, usually with only a single comarginal crossed lamellar layer.
• This finding is inconsistent with a predator-prey model of coevolution driving repeated origin of numerous crossed lamellar layers to strengthen shells in defense of shell crushing predators.
  

Acknowledgments

This project was conducted as a part of the Research Training Program and funded through the Alice Eve Kennington Endowment. Special thanks to: Jerry Harasewych (NMNH), Megan Schwartz (Seattle University), and Scott Whittaker (NMNH).

Work Cited

West, K., Cohen, A., Baron, M., 1991. Morphology and behaviour of crabs and gastropods from Lake Tanganyika, Africa: implications for lacustrine predator-prey coevolution. Evolution 45: 589-607.

West, K., Cohen, A. 1994. Predator-prey coevolution as a model for the unusual morphologies of the crabs and gastropods of Lake Tanganyika. In: Martens, K., Coulter, G., Goddeeris, B., (Eds.), Speciation in ancient lakes. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart. Pp 267-283.

West, K., Cohen, A. 1996. Shell microstructure of gastropods from Lake Tanganyika, Africa: adaptation, convergent evolution, and escalation. Evolution 50: 672-681.