Tuesday, May 09, 2017

A snail family and a photo experiment

kw: species summaries, natural history, natural science, museums, research, photographs, digital darkroom

A few weeks ago I finished a project to clean up the data for all of the specimen lots of freshwater snails at the Delaware Museum of Natural History. We loaded almost 10,000 data records to the new database product, and linked them to the InvertEBase site (the link opens the Collections page; we are #3). Then I began working my way through the land snails (called terrestrial gastropods in most literature, and they include tree snails). The Curator and I decided to work taxonomic family by family, or in groups of related families, working with about 1,000-2,000 records at a time.

The two great groups of terrestrial gastropods are the Pulmonates (infraclass Pulmonata of class Gastropoda) and the Operculates (in class Caenogastropoda along with many freshwater and marine species). "Pulmonate" means they have a lung; "Operculate" means they have a small, separate shell with which they can block the aperture of their main shell when they pull inside, and this allows them to survive periods of dryness and also blocks most predators.

The majority of land snail families are Pulmonates, so we began with the family Ellobiidae and two related families, Carychiidae and Amphibolidae. Digging into current taxonomic research, I found that the family Carychiidae is now a subfamily of Ellobiidae, and is now named Carychiinae. I also found that, while a few species in family Amphibolidae are terrestrial, most are marine, and our collection contained only marine species. Nonetheless, having extracted the records, I proceeded with both families, dealing with 75 lots of Amphibolidae and 1,236 lots of Ellobiidae.

It is instructive to survey the family Ellobiidae, which tend to have a certain appearance no matter what environment they inhabit. Some genera are all terrestrial, some are all marine, a few genera are estuarine (adapted to brackish water), and others contain species found in various habitats.

We'll first look at Ellobium chinense (Pfeiffer, 1856). Lawrence Pfeiffer originally described the species under the name Auricula chinense, thus the parentheses around his citation, indicating the reclassification of the genus.

These are medium-sized, up to 3 cm long and 1.4 cm wide, and rather ovate or cigar-shaped. In the closeup below notice the small lump on the inner lip of the aperture. The apertures of nearly all species in this family are variously decorated, probably depending on the kinds of predators these snails encounter.

The genus Ellobium is primarily Asian, and this species occurs in Japan.

Here we have another strictly terrestrial species, Pythia pantherina (A. Adams, 1851). Though the genus of this species has been changed to Pythia, I don't have information what its earlier name was. (Scarabus) on the older label indicates a subgenus, now no longer used.

Shells of this genus have the most elaborate dentition in the aperture, which indicates they have more severe predation at the aperture, probably by birds. Such a wiggly aperture allows the soft-bodied snail to emerge and crawl about, but prohibits entry to all but the slenderest of bird beaks. Other predators have other ways in: see the shell at lower left in the closeup, with two tiny pinholes in its lower left area (I didn't notice them until I looked at the closeup). They are from a predatory drilling snail, which uses its abrasive radula (sort of like a tongue with tiny teeth) to scrape a hole through the shell. It then injects a nerve poison. The animal inside relaxes, and enough of it extrudes through the wiggly aperture that the predator can either dismember it in place or pull it out to be consumed.

The genus Pythia is found throughout the Indo-Pacific region, typically on mangrove roots above the high tide line, and a little further inland. These specimens, six of the eleven in this lot, are from the Sulu Archipelago of the Philippines.

The third species of interest is Auriculastra subula (Quoy & Gaimard, 1832); the genus was formerly Auricula. These are quite small, seldom exceeding 1 cm in length.

The closeup below shows a single tooth in the aperture on the inner lip, simlar to the Ellobium specimens above. This is an estuarine species, found on mangroves in tidal marshes. It is not fully marine and cannot tolerate ocean water for any length of time.

The genus Auriculastra occurs throughout the Indo-Pacific and South Pacific. These are from Fiji. They show more wear than the prior two species, indicating that they get roughed up in the sandy lagoons, probably during the frequent storms.

The fourth and final species is Ovatella algerica (Bourguignat, 1864), originally called Alexia algerica. These are very small, just a bit bigger than those called "minute", seldom exceeding 0.6 cm in length. Note, however that like the others they have the ovate/cigar shape characteristic of the family.

The closeup shows that they also have small teeth, in this case two of them, partially blocking the aperture. These are 8 of the 20 in this lot. The original label also shows a better example for future shell collectors than the other three: the town, the beach ("Quarry Beach" in Las Palmas), and the country, plus the month and year of collecting. Is lacks only the collector's name. I am not sure why an earlier version of the database placed these in Argentina, but I am glad it has been corrected ("Argentina" on the oldest label is in someone else's handwriting. The handwritten correction on the older DMNH label is from the 1990's.)

These species are found near-shore Europe and north Africa. Their habitats are fully marine to salt marsh. The Canary Islands are offshore from north-western Africa.

The photographic experiment I mention in the title is a matter of spacing for the sake of good focus. The typical way to photograph museum mollusk specimens is to arrange the specimens on black velvet, velveteen, or (as in 3 of the 4 cases above) on black felt, and then to arrange the labels and the scale indicator around them. But even the small shells of these Ovatella specimens have a significant thickness when being photographed close-up. One may either focus on the upper surface of one of the shells, or on the labels, with the consequence that the other will be a little out of focus. One way around that is to use a lens aperture of f/8 or f/11 for greater depth of field. I tried something different.

I cut a number of small pieces of corrugated cardboard, either 3/4" x 2" (2x5 cm) or 1/2" x 1.5" (1.3x3.5 cm). I put these under the labels to bring them up to a plane close to the tops of the shells. In the case of the Ellobium specimens above, it took three thicknesses of cardboard, and you can see the spacers under the smallest label in that photo and the first photo of the Pythia specimens. It worked very well! Everything I want to see is in good focus, using f/4 to f/5. I was also using a +2 closeup lens on my camera's 18-105 mm zoom lens.

With the camera back fixed at 24" (60 cm) above the table top, I used focal lengths of 36 mm and 105 mm. I used a spot focus just below the top of a central shell in each group for the autofocus to work with, and I am pleased with the results.

Each of the original images was cropped, and the color levels clipped to remove the grayness the camera allowed in the black background, and the gamma and saturation were adjusted so the colors in the image matched the colors of the shells. The images above are all re-sized to 1620x1080 pixels, so you can click on them to get images that fill most screens. I also used just a little bit of Unsharp Masking to emphasize the shell decorations, also to make the images look more "eyeball true". My Nikon camera under-sharpens its images, which I like. I usually prefer its usual, softer look, but when I want to sharpen back to "normal", UM is the most versatile way of doing so.

Finally, although felt is cheaper than velvet, the clean, smooth blackness of genuine velvet makes for much better final images, as can be seen by comparing the first two images with the other six.

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