Aquatic Invertebrates of Montana

Gastropoda

Introduction

Snails. The largest and most diverse group of mollusks. The phylum Mollusca in a ancient group that is second in size only to the arthropods. There are over 50,000 living species and about as many fossil species known. Mollusks are unsegmented, coelomate protostomes with a ventral muscular foot, a shell-secreting mantle, and a radula as the feeding organ. They have a trochophore larval stage similar to the annelids and a later veliger larval stage. The circulatory system is open and excretion is by metanephridia. Gastropods make up 70% of all mollusks and they are well represented in marine, freshwater and terrestrial habitats. About 500 species, 350 prosobranchs and 150 pulmonates, in the freshwaters of North America, north of Mexico.

Recognition

Mollusks with increased cephalization and an asymmetrical, univalved, usually spiral shell serving as a retreat rather than a shield. The class is characterized by torsion, a 180 degree counterclockwise rotation of the body behind the head.

Life Cycle

Marine forms have a free-swimming veliger larva, freshwater forms usually pass that stage within the egg and miniature adults emerge. Pulmonates are monoecious, copulation usually involves mutual sperm transfer, but it can be one-way only or they can be self-fertile. Freshwater prosobranchs are dioecious, except for Valvatidae. Some are parthenogenetic, some are egg brooders, some are both. Mating and egg laying may be seasonal or occur throughout much of the year. The species may be semelparous or iteroparous. Individuals of some species may live for several years. The life cycle is non-emergent.

Trophic Roles

Freshwater species are mostly phytophagous scrapers that feed by way of the file-like radula. Many also feed on detritus or vascular plants. They may feed from the surface film. Many can digest cellulose directly. A few use their gills to filter feed, at least in part.

Communities

Gastropod communities in our area are usually rather simple with few species, but some of these may be very abundant. Lentic and lotic communities are very different. Most pulmonates can use atmospheric oxygen directly, but prosobranchs always respire aquatically. Some species occur in deep water. None are present in cold mountain streams in rapid current. Some occur in temporary habitats, some are amphibious. More diverse gastropod communities occur in warmer areas near the oceans. The species are usually separated by substrate (rocks, vegetation, sand or silt), depth and current velocity. Gastropod communities are often linked with the fish, macrophyte and periphyton communities in complex ways.

Economic Importance

Snails are important in many food chains and they are frequently used by fish and wildlife as food. The importance of thin-shelled snails in animal diets is often underestimated. Snails may serve as pollution and acid raid indicators. They are intermediate hosts for many important parasites.

Zoogeography

Most of the families are boreal, austral or worldwide. Pulmonates are distributed by climate and habitat rather than by river courses, many genera and species are very widespread. Many estivate in the soil under temporary water bodies. Overland transport of eggs and estivating adults is known. Prosobranchs are distributed by river drainage, many genera and species are very restricted. Overland transport of prosobranchs is more difficult. Few species occur toward the center of the continent. Twenty-four species of prosobranchs from the Southeast are thought to be extinct, many more are threatened. Human introduced species are common and these can be hard on the native communities.

Collecting

Most are found clinging to various substrates or by dredging up vegetation and the surface layers of the sediment. All types of substrate should be checked. Some species burrow in sand. Some species drop to the bottom if even slightly disturbed. Many species can be separated from debris by panning in a bucket. Pulmonates may not be heavy enough to pan from debris. These can be extracted from dense debris by allowing them to surface in a bucket containing the debris.

Preservation

A good snail collection consists of two parts- soft parts and shells. The animals must be narcotized before fixing by keeping them alive in a clean container using the water of origin and adding a few crystals of menthol. After several hours, when the animals do not respond to touching, most of the water is carefully removed and replaced with Kahle's or Bouin's fluid. These acidic fluids will damage or destroy the shells, which allows better viewing of the soft parts. The soft parts are stored in 70% ethanol. The shells are best preserved dry as they slowly dissolve in alcohol. The soft parts of large snails can be removed after killing the animal by freezing and then thawing. The soft parts so removed can also be preserved, but they are suitable only for radula extraction. The operculum of prosobranchs should be retained with the shell. The soft parts of small snails need not be removed, but should be soaked in 70-80% ethanol for a few hours. In either case, the shells are next cleaned with ammonia water, ethanol, and finally rinsed with clean water before drying. The operculum, if removed from the animal should be placed within the shell and held in place with a cotton plug.

Taxonomy

A once popular group. Many groups suffer from an overabundance of names based on small variations in the shells. Many groups have not had much recent attention. Characters of the soft parts define the higher taxa and shell characteristics define the lower taxa. Hydrobiidae are identified primarily by the form of the male verge. Species identification for many groups in many areas is difficult.

Some key references

• Burch, J.B. 1989. North American freshwater snails. Malacological Publications, P.O. Box 4115, Ann Arbor, Michigan. 365pp.
• Turgeon, D.D. et al. 1988. Common and scientific names of aquatic invertebrates from the United States and Canada: mollusks. American Fisheries Society Special Publication 16.

Synopsis Of Nearctic Families

• Subclass Prosobranchia- operculum present, mostly dioecious, shell usually solid, often sculptured and marked with contrasting colors, always dextral, rarely discoidal, true gills (ctenidia) present in dorsal mantle cavity, mantle opens anteriorly.
  • Order Archaeogastropoda- (Aspidobranchia) primitive forms.
    • Family Neritinidae- shell globose and solid, up to 20 mm long, shape distinctive (neritiform), mostly marine in tropical waters.
  • Order Mesogastropoda- (Taenioglossa) intermediate forms.
    • Family Valvatidae- shell less than 8 mm, depressed, operculum circular and multispiral, holarctic, hermaphroditic, mostly in lakes.
    • Family Ampullaridae- (Pilidae) operculum concentric, very large, tropical, amphibious snails, oviparous.
    • Family Viviparidae- operculum concentric, ovoviviparous, male right tentacle used for copulation.
    • Family Bithyniidae- (Bulimidae) operculum calcareous and concentric, shell 10 - 15 mm long, introduced.
    • Family Hydrobiidae- shells usually less than 5 mm and always less than 10 mm, males with a verge.
    • Family Micromelaniidae- shell sculptured with numerous ridges, male with a verge, one eastern species in cave streams.
    • Family Pomatiopsidae- shell tall, smooth and 5-10 mm long, males with a verge, amphibious, East, Southeast, and California.
    • Family Pleuroceridae- (Paludomidae) river snails, shells usually over 15 mm and always over 6 mm in length, solid, males lack a verge, oviparous.
    • Family Thiaridae- similar to Pleuroceridae, except mantle edge papillate, all parthenogenetic, egg brooders, introduced in warm springs and warm streams of the southern states.
• Subclass Pulmonata- operculum absent, monoecious, freshwater shells usually fragile and uniformly horn colored and without much sculpturing, dextral, sinistral, discoidal, or cap-shaped (limpets), respiration by lungs, which open by a pneumostome and may breath water or air. Respiration in some is aided by pseudobranchs, or false gills that are located outside the mantle cavity. The mantle opens to the right or left.
  • Order Basommatophora- (Limnophila) lower Pulmonates, one pair of tentacles with the eyes at the base of the tentacles.
    • Family Acroloxidae- limpets, animal dextral, shell with apex posterior, acute, and inclined to the left, 5 mm long, pseudobranch present.
    • Family Ancylidae- limpets, animal sinistral, shell with apex posterior, blunt, and inclined to the right, pseudobranch present.
    • Family Planorbidae- orb snails, shell depressed (discoidal), animal sinistral, tentacles filiform, pseudobranch present, blood with hemoglobin (rest have hemocyanin).
    • Family Lymnaeidae- pond snails, shell raised and dextral, animal dextral, tentacles broad, flat and triangular, no pseudobranch, widespread and common.
    • Family Lancidae- limpets, animal dextral and lymnaeid line, shell with apex anterior and central, up to 12 mm long, distributed by drainage along the Pacific coast.
    • Family Physidae- shell raised and sinistral, tentacles filiform, no pseudobranch.
  • Order Stylommatophora- higher pulmonates, terrestrial snails and slugs, two pairs of tentacles with the eyes at the tips of posterior pair, many lack a shell, some occur near water.

Montana Species list can be found here.