Class Prosobranchia

largest and most diverse

mostly marine; about 60,000 species

there are freshwater species in this order

(i.e., Pleuroceridae: Pleurocera, Campeloma, Viviparus, etc.)

 

Class Opisthobranchia:

exclusively marine, 11,000 species

 

Class Pulmonata

primarily freshwater and terrestrial, and marine limpets

the mantle cavity has been modified for air-breathing; have no gills

about 20,000 species

aquatic species have secondarily invaded water and must come to the surface periodically to breathe; no operculum

gill have been lost; the roof of mantle cavity is highly vascularized and acts as a functional lung

air gets in and out by the pneumostome

divided into 2 superorders:

Suborder Basommatophora

mainly freshwater species; some marine

one pair of tentacles with eyes at base of tentacles

 

Suborder Stylommatophora

terrestrial species

2 pairs of tentacles; eyes at end of posterior tentacles

some species have great reduction or loss of shell; these are the slugs

 

In both the Opistobranchs and pulmonates there is tendency for loss of larval forms, and a reduction of torsion; reduced to about 90 degrees

 

Foot - movement by the foot; various wave-like peristaltic movements cause gastropods to creep along due to muscular contraction

Ciliary Action - cilia at bottom of foot; especially in small species that live on sand and mud bottoms

Operculum - those that have operculum use it by extending and anchoring it and foot contracts and pulls animal along; also used to dig

Swimming - by the use of extensions of the foot (fins) also called parapodia; best developed in opisthobranchs such as sea hares, sea butterflies

 

Shell- structures and terms

1. shell orientation; types of spires

conical - as in limpets

planospiral - spiral along a horizontal plane; Helisoma

spiral - along a vertical plane; Pleurocera

2. Operculum - trap door to aperature

In Prosobranchia three types

concentric- rings in concentric circles; Viviparidae

paucispiral- rings few in number, usually a single loop; Pleuroceridae

multispiral- rings few in number, usually one, but many loops; Pleuroceridae

No operculum in Pulmonata

3. Columella - central "post" around which shell spirals

4. shell aperature orientation

dextral- opening on right side

sinestral- opening on left side; Physidae

5. Protoconch- "first shell"; shell that is laid down upon hatching, newly hatched snail is called a spat; first whorl

6.  whorls

individual spirals of shell

body whorl- last whorl, where largest portion of body is located

morphology of whorls

rounded; Physa

carinate- with an edge, or two; Valvata

spines; Io

nodules; Lithasia

smooth; Campeloma

Umbilicus

perforate- with umbilicus

imperforate- without

Teeth- refers to structures along lip of body whorl of shell; not common in freshwater species- in some Planorbidae

6.  Shell layers; laid down by mantle

Periostracum- outer horny layer, high concentration of organic matter

Prismatic- middle layer of calcium carbonate; molecules laid down vertically

Nacre- inner layer, lies against the body

 

NUTRITION- All gastropods have a radula and it is used for feeding; it is modified from one group to the next depending on the type of feeding that is involved

more primitive condition is that the radula is composed of several transverse rows of teeth which is used for rasping

in more advance gastropods, especially neogastropods, the radula is modified for predation

have radula consists of long sharp, barb-like teeth

In some cones radula developed into a harpoon-like structure used to inject venom into prey

gastropods are found in all parts of the food chain; freshwater gastropods tend to be herbivores or detritivores

Herbivores - found in both classes; snails are part of a small group of animals that produce cellulases (ie. enables them to digest cullulose)

feed on algae; rasping it off the substrate or may feed on the softer tissues of plants, using the radula to shred the food; mostly prosobranchs such as Pleuroceridae

some are able to feed on macrophytes, ie. Lymnaeidae; most graze (ie. scrape) on periphyton- especially green algae and diatoms

 

Carnivores - no freshwater species specifically carnivores; mostly marine species such as cones

 

Scavengers and Deposit Feeders- feed primarily on dead and decaying organisms or on detritus; found primarily in slow moving, silty habitats; primarily pulmonates, and some prosobranchs such as Camperloma

may have a proboscis that enables them to suck up detritus off the bottom

 

Suspension Feeders (Filter Feeders)- primarily feed on plankton; example is Bithynia tentaculata (Bithyniidae) that uses its ctenidium (gill) to filter phytoplankton from water; found in eutrophic lakes in New York

food is carried in by inhalant current over gills

gills secrete a sticky substance trapping food

in these the radula is usually very reduced; in a few species it seems to be lost

there is usually a well developed ciliary system in the gills and proboscis to transport trapped food to mouth

 

Effects of Plant Production by Snails

Gastropods are known to play an important role in plant, especially periphyton, production. Numerous experiments have shown a distinct difference in production in the presence vs the absence of snails

 

EXCRETION AND WATER BALANCE- Many archeogastropoda have 2 nephridia, but in all others the right one has disappeared or is very reduced and functions in reproduction only

because of torsion nephridia are located anteriorly in visceral mass

nephridia are sac-like structures that usually open into mantle cavity and wastes are removed by water currents

in pulmonates this is not possible because mantle cavity forms a lung so ureter is elongate and opens to front of mantle near the anus and pneumostome

freshwater species have to get rid of excess water so nephridia transport lots of water to outside

 

CIRCULATION

because of torsion heart is anterior in the visceral mass

in primitive archaeogastropods there are two auricles,

in all other gastropods the right auricle has become vestigial or lost (fig. 11-40, page 382 gives a diagram of typical circulation of blood in snail)

 

 

NERVOUS SYSTEM

in advance gastropods it is twisted (see Barnes Fig. 11-41, page 383) into a figure eight

as with other systems this twisting results in the loss or reduction of structures on the right side in this case it is affects the number of sense organs

sense organs include:

eyes- used to detect changes in light; not form images

tentacles

osphradia- used to detect food; located in mantle cavity near gills

statocysts- located near foot used for orientation

REPRODUCTION

Prosobranchia

most prosobranchs are dioecious with a single ovary or testis in visceral mass

males use the enlarged right tentacle as a copulatory organ, or may have a specialized penis (= verge) as in Valvatidae, Hydrobiidae, and Pomatiopsidae; Some have no special organ as in Pleuroceridae

Many lay egg masses; Pleuroceridae

Some are ovoviviparous such as Viviparidae- give birth to live young that developed in anterior fold of mantle

Some viviparids (ex. Campeloma) are parthenogenetic, which has adaptive advantages in certain environmental conditions; colonization of new habitats or in unpredictable habitats

The Valvatidae are hermaphroditic, but usually cross fertilization is the rule

 

Pulmonata

Pulmonates are hermaphrodites (monoecious), where copulation is the rule

sperm and eggs produced in an ovitestis that leave via an hermaphroditic duct (except in ancylids which have separate ducts; cross fertilization is required)

most are simultaneous hermaphrodites - produce eggs and sperm at the same time

eggs fertilized in hermaphroditic duct; internal fertilization

self fertilization

cross fertilization through copulation, most common when possible

eggs laid in gelatinous masses and attached to some substrate

 

Development:

in terrestrial and freshwater snails there is direct development with no larval stages

 

 

Prosobranchia

Pleuroceridae

widespread in Africa and Asia; greatest diversity in streams of SE U.S.

shell anatomy is used for genus level identification

operculum is corneus and paucispiral

 

Viviparidae

worldwide distribution

Campeloma (widespread), Lioplax (widespread), Tulotoma (restricted to Alabama) are endemic to North America

Viviparus is widespread in eastern North America, probably more common in North East

 

Ampullaridae

restricted to Florida, most common is kite snail, Pomacea

 

Valvatidae

11 species in North America

egg laying hermaphrodites

small shells, often carinated, dextral, multispiral operculum

 

Hydrobiidae

28 genera in North America; 152 species

dextral, paucispiral operculum, very small shells

 

Pomatiopsidae

one genus, Pomatiopsis, 6 species

amphibious, often found along shoreline

similar in appearance to hyrobiids

widespread

 

Pulmonata

Ancylidae

worldwide distribution

freshwater limpets, simple cone-shaped, sinestral shell

apex inclined to right

have modified gill (pseudobranch); secondarily derived; unique to this family in Pulmonates

 

Lymnaeidae

Worldwide distribution, most diverse pulmonate group

9 genera and 57 species

most have very thin shells, some get very large

mostly found in lentic waters

 

Physidae

worldwide distribution; ubiquitious in North America

Often referred to as having a single genus, Physa; in recent years it has been divided into 4 genera with 37 species

all are sinestral, most with very thin shells

 

Planorbidae

widespread

11 genera and 44 species

sometimes aligned with ancylids into a single family, Ancyloplanorbidae (See Branson reprint)

have hemoglobin

shells planospiral; range from 1mm (Neoplanorbis) to 30mm (Helisoma)