DRILL HOLES IN LAND SNAIL SHELLS FROM WESTERN TURKEY

by Aydin Örstan

A summary of the paper published in Schriften zur Malakozoologie 13:31-36, 1999

The larvae of drilid beetles (Coleoptera:Drilidae) prey on land snails (Lawrence, 1991). In areas where summers are hot and dry land snails aestivate with their apertures sealed either with a thick epiphragm or against a rock. Under those conditions, the beetle larvae enter the snail shells by drilling a hole thru their shells. This was first mentioned by Roth in 1855 (cited in Gittenberger, 1999). It wasn’t until 1903, when von Martens discussed it again. It appears that after 1903 these observations once again fell into oblivion until 1994, when Schilthuizen et al., reported finding oval holes made by the larvae of drilid beetles (Drilus sp.) and circular holes made by an unknown organism in Albinaria shells collected on the island of Crete. I read the paper by Schilthuizen et al., in the Spring of 1998 and later when I was exploring in Turkey in August, I made it a priority to keep my eyes open for Albinaria shells. The result of this study was my 1999 paper.

If a large number of Albinaria shells with uniform oval holes are examined, one will notice that there are 2 types of holes (see the pictures below). In one type, the edges of the holes are beveled (inwardly narrowing), and in the other type, the edges are not beveled, but are more or less vertical. Schilthuizen et al., (1994) considered these to be entrance and exit holes of the larvae, respectively. In my paper, I showed that in Albinaria shells, the majority of the holes with beveled edges were in the body (ultimate) whorls, whereas the majority of the holes with vertical edges were in the penultimate whorls. This distribution makes sense because it indicates that the larvae enter an empty whorl (during aestivation, the snail withdraws beyond the body whorl), where they will be protected while consuming the snail.

One also notices that the dimensions of the holes are highly variable. This also makes sense because the larvae go thru several instar stages as they grow. In my paper I presented a frequency distribution plot of 63 oval holes that suggested that the distribution was tetramodal. I interpreted these results as indicating that either the subject drilid species has three larval instars and the fourth peak corresponds to the holes bored by the adult beetles when leaving the last host's shell or it has four instars and the adults emerge from the hole the last instar bored to enter the shell. To confirm (or refute) these results, size distribution of a larger number of drilid holes should be examined.

Right: Two specimens of Albinaria forbesiana from Turkey. The top shell has 2 partially overlapping circular holes that were drilled by an unknown organism. The bottom shell has 2 holes that were drilled by a larva of a drilid beetle. The hole in the body whorl is an entrance hole; the one above it is an exit hole. The scale is in millimeters.

There are many unanswered questions. For example, many shells have only exit holes. Does a larva sometimes enlarge the hole it came thru to go out of a shell? Where do female beetles lay their eggs? And how do the larvae find the snails? In many places, more than 50% of Albinaria shells may have been killed by the beetle larvae (Schilthuizen et al., 1994; Welter-Schultes, 2000). This brings up another question: has predation by the beetle larvae caused any evolutionary changes in the conchological characteristics (that is, dimensions and/or other properties) of Albinaria shells? Francisco Welter-Schultes (2000) has shown that in Albinaria idaea on Crete, beetle predation intensity was not closely related to shell diameter, shell shape or rib density. More studies are necessary to understand if the snails have evolved any defense mechanisms.

Moreover, we still don't know the identity of the organism that bores the circular holes. Some Albinaria shells I examined had multiple (up to 7) circular holes. They tend to be more common in the penultimate and antepenultimate whorls. Their nonrandom distribution and almost perfectly circular shapes and that they are frequently found on fresh shells rules out a physical causative agent. Francisco Welter-Schultes (in e-mail) has come up with the acronym CHO (circular hole organism) to refer to the mystery organism. The CHO could be another beetle, a nematode or some other predator or parasite. I have found similar circular holes also in the shells of snails in other families. Whatever it is, the CHO appears to be widespread. In the Delaware Museum of Natural History, while examining Cerion shells from 2 lots collected on different Caribbean islands I found 2 shells that had circular holes whose dimensions matched those from Turkey.

LITERATURE

Gittenberger, E., Basteria 63:164, 1999.

Lawrence, J. F., Drilidae (Cantharoidea). In Stehr, F.W., Immature Insects, 2:424, 1991.

Martens, E. von, Sitzungs-Ber. Ges. naturforsch. Freunde, 8:393-396, 1903.

Schilthuizen, M., Kemperman, TH. C. M. & Gittenberger, E., Bios, 2:177-186, 1994.

Welter-Schultes, F.W., Biological Journal of Linnean Society, 71:237-250, 2000.

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