Alvania

It's a general rule with organisms that species diversity increases as size decreases (at least down to about the millimetre range, below which things get a bit more complicated). That's certainly the case with molluscs, whose range clearly favours the tiny.

Alvania cimex, copyright Alboran Shells.

Alvania is a cosmopolitan genus of marine gastropods, found in most parts of the world except the Antarctic and sub-Antarctic (Ponder 1984). The average Alvania species is less than five millimetres in total length, and other members of the family they belong to, the Rissoidae, are similarly wee. The shell of Alvania species varies from elongate-conical to more squatly conical in shape, and generally has a sculpture of both axial and spiral ridges. In some species the axial and spiral ribs are both similarly prominent; in others, the spiral ridges are more strongly developed.

Rissoids may be found crawling on seaweed or sheltered amongst stones or other rubble. Alvania species seem to be more likely to be found in the latter habitat than the former. Alvania have a smaller mucous gland on the rear of the foot than species of Rissoa, a related genus that is more likely to be found on the weeds. The mucus produced by this gland assists rissoids in clinging to their substrate or the surface film, and its reduction in Alvania is presumably connected to their preference for the low life. Rissoids are grazers on microalgae or deposit feeders; those species found on seaweeds will feed on diatoms and the like growing over the seaweed rather than on the seaweed itself. Among European species, A. punctura is known to selectively pick out diatoms and dinoflagellates from among detritus when feeding whereas A. jeffreysi may be less discriminating in what it swallows.

Alvania subcalathus, copyright H. Zell.

The greater number of Alvania species are planktotrophic as larvae, and as described in some of my previous posts on turrids, their shells have protoconches to match. Nevertheless, the genus also includes some direct-developing species with fewer protoconch spirals. The Mediterranean species A. cimex and A. mammillata are almost indistinguishable when mature except by features of the shell apex, which is broader with fewer spirals to the protoconch in the latter (Verduin 1986). If A. mammillata is a direct developer while A. cimex has a planktotrophic larva, it would tally up with the situation elsewhere seen among turrids.

REFERENCES

Ponder, W. F. 1984. A review of the genera of the Rissoidae (Mollusca: Mesogastropoda: Rissoacea). Records of the Australian Museum Supplement 4: 1–221.

Verduin, A. 1986. Alvania cimex (L.) s.l. (Gastropoda, Prosobranchia), an aggregate species. Basteria 50: 25–32.

The Lithoglyphidae: Let's Get Fresh

Live individuals of Lithoglyphus naticoides, copyright Jan Steger.

In previous posts on this site (see here and here), I've introduced you to members of the Hydrobiidae, a diverse family of mostly freshwater gastropods. Hydrobiids have long been recognised as a tricky group to work with, because of their small size and general shortage of distinctive shell features. In recent years, an understanding has developed that the 'hydrobiids' may include a number of lineages that became independently adapted to fresh water, and a number of previously recognised subfamilies of the Hydrobiidae have come to be recognised as their own distinct families. One of these ascended subgroups is the Lithoglyphidae.

Flat pebblesnails Lepyrium showalteri with eggs, copyright Friends of the Cahaba River National Wildlife Refuge.

The Lithoglyphidae are a family of about 100 known species, mostly found in the Holarctic region (Strong et al. 2008), though they have also been recorded from South America. Most lithoglyphids have distinctively squat, relatively thick shells, and for a long time this was treated as one of the main defining features of the group. However, Thompson (1984) pointed out that the sturdy lithoglyphid shell was probably an adaptation to living in fast-flowing streams and rivers, and could also be found in other 'hydrobiid' groups. As well as reducing the shell profile, the lithoglyphid shell possesses a broad aperture that allows for a proportionately large foot, increasing the snail's clinging power. Thompson (1984) identified a number of other features characteristic of lithoglyphids, including a spirally sculptured protoconch and a simple, blade-like penis that lacks accessory lobes or glandular structures. As the soft anatomy of many 'hydrobiids' has not yet been described, it is still possible that some taxa currently identified as lithoglyphids are in fact impostors. Conversely, confirmed lithoglyphids now include some taxa more divergent in shell shape, such as the limpet-like Lepyrium showalteri from Alabama. This species is distinctive enough that when first described it was identified as a neritid, a member of a group of gastropods not even closely related to lithoglyphids (imagine a new species of rodent being identified as a ratfish). Sadly, Lepyrium is also now endangered, being extinct in one of the two river catchments it was historically known from (see here). Thompson (1984) notes that another North American lithoglyphid genus, Clappia, may be entirely extinct. For at least one species, the cause of extinction was pollution from coal mining; no cause was specified for the other species, but according to Wikipedia its native habitat in the Coosa River has been modified by the construction of hydroelectric dams.

Shells of Benedictia baicalensis, from Baikal.ru.

Also closely related to the lithoglyphids are the Benedictiinae, a group of 'hydrobiid' gastropods endemic to Lake Baikal in Russia. A single species of benedictiine has been described from Lake Hövsgöl in Mongolia, but has not been collected there since; it seems likely that its original location was an error (Sitnikova et al. 2006). Baikal is a remarkable place: one of the world's largest freshwater lakes (and easily the largest in terms of the volume of water it contains), it is basically a freshwater sea. While other large lakes such as the Rift Lakes of Africa are poorly oxygenated at deeper levels, effectively restricting most animal life to the surface layer, Baikal has oxygen-rich deeper waters allowing a rich deep-water animal community (this may also be related to the numerous hydrothermal vents in the depths of Baikal). Some of you may have heard of the endemic Baikal seal Phoca sibirica, but Baikal is also home to a wide diversity of endemic fish (including a dramatic radiation of sculpins), a remarkable array of endemic amphipods, and even its own endemic family of sponges. The Benedictiinae are currently classified as a separate subfamily of Lithoglyphidae, with the remainder of species in the Lithoglyphinae (Bouchet et al. 2005), but as the relationship between the two subfamilies has not yet been examined in detail it is possible that the lithoglyphines are paraphyletic to the benedictiines. The benedictiines generally have thinner shells the lithoglyphines, possibly related to the differences in their usual habitats.

REFERENCES

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Sitnikova, T., C. Goulden & D. Robinson. 2006. On gastropod mollusks from Lake Hövsgöl. In: Goulden, C. E., T. Sitnikova, J. Gelhaus & B. Boldgiv (eds) The Geology, Biodiversity and Ecology of Lake Hövsgöl (Mongolia), pp. 233-252. Backhuys Publishers: Leiden.

Strong, E. E., O. Gargominy, W. F. Ponder & P. Bouchet. 2008. Global diversity of gastropods (Gastropoda; Mollusca) in freshwater. Hydrobiologia 595: 149-166.

Thompson, F. G. 1984. North American freshwater snail genera of the hydrobiid subfamily Lithoglyphinae. Malacologia 25 (1): 109-141.