"Imagination is everything.
It is the preview of life's upcoming attractions."

    Albert Einstein (1879-1955)

     German-born American physicist, Nobel Prize for Physics in 1921

Poteriid land snail, Cyclojamaica sp. Windsor, north-central Jamaica.

The Silent Sliders

    The "invertebrates" are an artificial aggregation of animals that encompasses widely different phyla and comprises the bulk - in terms of both number of species and of individuals -  of the animal biomass in every ecosystem on Earth. Insects, arachnids, crustaceans, onycophorans, mollusks, earthworms, rotipherans, nematodes, centipedes, millipedes, and others inhabit the tropics of the World, perhaps by the millions of species. The largest higher group alone, Insecta, includes more species than all others - plant and animal - put together.

    The soils of the tropics are rife with small invertebrates of all sorts. While most of them are tiny, even microscopic, their lives are carried out in a world that is very complex in its own scale. Hordes of pillbugs, collembolla, earthworms, slugs, insect larvae, and others are the herbivores and carrion-eaters of this scaled-down universe. Its predators and parasites come in the shape of flatworms, nematodes, leeches, pseudoscorpions, mites, ants, tiny wasps and flies and their larvae, and many others.

    Even more complex are the invertebrate faunas that live above ground, on the vegetation and to the highest canopies of the rain forests. Immense numbers of species which shapes, colors, and lifestyles defy description await discovery high above human eyes, and are beyond the reach of the average person.

PHYLUM PLATYHELMINTHES: FLATWORMS

    The term "worm" has no systematic value whatsoever. Basically, any elongated invertebrate can be called a "worm", regardless if it is or not related to similar creatures. However, two of the most primitive types of organisms receive that common name: flatworms (Phylum Platyhelminthes) and roundworms (nematodes, Phylum Nematoda).

    These two groups are among the simplest among the Bilateria, animals which bodies can be longitudinally and dorsoventrally divided into halves that are mirror images of one another (as is our own case).

    Flat- and roundworms lack a "coeloma", an internal body cavity (other than the gut itself) where the viscera are located. They lack respiratory, circulatory, and digestive systems. Oxygen is absorbed through the external epithelial cells, food is absorbed directly through their internal bodily tissues, and nutrient molecules are spread straight through cells through extracellular ("interstitial") fluids. Another trait peculiar to both, and which differentiates them from every other wormlike group, is that they lack true segmentation in their bodies, which are mainly solid, uninterrupted rods of muscular tissue.

Flatworms (phylum Platyhelminthes) are the most primitive of bilaterally symmetrical organisms.

Most members of the group are aquatic denizens of freshwater or marine habitats.

This terrestrial planarian (class Turbellaria)  is Bipalium kewense. Photographed in Carolina, north-eastern Puerto Rico.

This species is Cosmopolitan, after being shipped around Earth on plant pots.

Its place of origin is not known for certain, but it could be eastern Asia.

Turbellarian flatworm, species undetermined. El Verde Biological Station, north-eastern Puerto Rico.

    Among platyhelmynthes, and unlike the many parasitic cestodes ("tapeworms") and trematodes (flukes), most turbellarians (planarians), are aquatic, living in freshwater ponds and streams, or at sea. Also, most are carnivores. Terrestrial species feed on earthworms and other soft-bodied invertebrates. A predatory flatworm envelop its prey with their its body, immobilizes them with mucus, and then protracts what resembles a pliable tube from its underside. This organ dissolves its victims' tissues with the use of digestive enzymes. The resulting "soup" is suctioned through the tube and then absorbed directly through the inner bodily cells in an amoeboid fashion. The mouth through which this protractible pharynx is extruded also serves as the anus.

    While most flatworms are small to tiny one species of cestode, a parasite of whales, is 25 meters in length.

Turbellarian flatworm, species undetermined. Crown Mountain, central Saint Thomas, United States Virgin Islands.

Turbellarian flatworm, species undetermined. Cambalache State Forest, northern Puerto Rico.
(Photograph courtesy of Mr. Alfredo Colon Archilla).

Trematode flatworm, species undetermined, parasitic under the skin of a xenodontine snake, Borikenophis portoricensis portoricensis.

Fort Buchanan, Guaynabo, north-eastern Puerto Rico.

    Because, unlike higher metazoans, it lacks any complex organs located in specific parts of the body sometimes a flatworm severed into two or more pieces will result into as many complete individuals, after each piece regenerates any missing parts.

PHYLUM NEMATODA: ROUNDWORMS

    Only slightly more advanced than flatworms, roundworms are among the most abundant organisms on Earth, both in terms of species and in the sheer number of individuals. Astronomical hordes of them swarm in most habitats that have a minimum of available water, from the deepest oceanic abysses to alpine environments, and especially in the soils and leaf litters of the tropics. In humid terrestrial habitats, their combined biomass may be staggering, far outweighing that of all macro-organisms combined. This is remarkable given the fact that, by far, most species are microscopic. A rather few species are large enough to be noticeable to the naked eye, and one (another parasite of whales) is more than 10 meters long.

    Some nematodes are important parasites of plants and animals, as well as of man. Others feed on dead, decaying tissues, or on bacteria and protozoans. Still other species are commensal symbionts that live in the guts of higher animals.

A nematode on leaf litter, species undetermined.

Neiba Mountains, south-central Dominican Republic, Hispaniola.

PHYLUM MOLLUSCA: MOLLUSKS

    All mollusks have a "mantle", a dorsal body wall that encircles the entire visceral cavity. Interestingly, the members of this group do not have a segmented bodily pattern, which is the rule among most other metazoan phyla. In spite of being a very ancient group of animals, as a group molluscs possess the most complex nervous systems among invertebrates.

    This phylum is second only to arthropods in the number of species it contains. The great majority of mollusks are aquatic and, of such, most are marine. Its classes include, among others, snails and slugs (Gastropoda), clams and oysters (Bivalvia), chitons (Polyplacophora), and octopi, squid, and nautili (Cephalopoda). Among these last is the largest known invertebrate, Mesonychoteuthis hamiltoni, a squid perhaps longer than 20 meters, including its tentacles.

CLASS GASTROPODA: SNAILS

    The one thing that all gastropods (both terrestrial and aquatic) have in common is their means of locomotion. They possess a muscular "foot" on their undersides that propel them through undulations as they glide over a film of mucus. Indeed, thence comes their name, "gaster-pous": "it that has a foot on its stomach".

The underside of a land snail, Austroselenites alticola, (family Haplotrematidae), shows its "foot".

Florida, central Puerto Rico.

Most land snails possess two pairs of head tentacles. The upper ones bear eyes, while the lower ones are chemical receptors.
Camaenid snail, Pleurodonte caracolla. El Verde Field Station, north-eastern Puerto Rico.

    Only bivalves and gastropods have colonized freshwater, and both of these groups are found in West Indian rivers, streams, and lakes. However, only gastropods of the subclass Pulmonata have also invaded land. Snails and slugs (these last are simply snails that evolved to loose their shells) are especially common in the damp environment of humid and rain forests. Many beautiful species are found especially in Cuba and Hispaniola (especially those of the genera Liguus and Polymita, as well as some Pleurodonte), but all the Antillean insular banks have endemic representatives of this group.

    Snails have secondarily and partially lost their bilateral symmetry. The body of the animal is encased in a spiral shell (secreted, as in all shelled mollusks, by the mantle) and, as the animal matures from the larval stage to adulthood, the whole body twists about 180 degrees. Hence, the anus ends up facing forward, above and behind the head. Slugs better preserve bilateral symmetry since, lacking a shell, their bodies are not twisted into a spiral.

    Most species of terrestrial snails are hermaphrodites, and mutually inseminate each other during their sexual encounters. During mating, some species insert a sort of calcareous "dart" into each other. It is believed that the object contains hormones that "activate" the female gonads, making them receptive to sperm.

Pleurodontid snails, probably Zachrysia provisoria, during mating. Notice the dart made of calcium carbonate that one individual crawls away with.

Barrenspot, central Saint Croix, United States Virgin Islands. Introduced from Cuba.

ORDER MESOGASTROPODA

    This group contains a number of species whose closest affinities lie in Earth's oceans. They can be told apart from other land snails by the presence of an operculum, a horny plate attached to the back of the foot that closes the entrance to the shell when the animals hides within it. Unlike the more advanced stylommatophores, their eyes are not borne at the ends of long, tentacle-like stalks, but on the sides of their heads.

Family Cyclophoridae

Cyclophorid snails, Megalomastoma croceum, (family Cyclophoridae). Florida, central Puerto Rico.

The first photograph shows an adult and a juvenile.

Cyclophorid snail, Farcimoides orbignyi. Ebano Verde Nature Reserve, central Dominican Republic, Hispaniola.

(Photograph courtesy of Dr. Jozef Grego).

ORDER NEOTAENIOGLOSSA

    This also an order whose majority of members are marine. A number of species have invaded land and, like those of the previous order, have opercula to close their shells when under threat or while resting.

Family Annulariidae

    This family is best represented in Central America and the Caribbean region. Several similar genera are found in the islands of the Caribbean, particularly in the Greater Antilles. They are usually small and have conical but apically truncated shells.

ORDER NERITOIDA: NERITES

    This is the third group of mainly marine and freshwater snails some of which members have secondarily adapted to life in humid terrestrial habitats. Only the next family is represented in this region.

Family Helicinidae

    This group has an odd distribution in both the Caribbean region and some islands in the Indo-Pacific oceans. They are small tropical snails, some with gaudy colors on their shells. They seem to represent a secondary evolutionary invasion of a terrestrial habitat, after that of more typical and "ancient" land snails. Being more tied to water than other snails, they frequently inhabit damp montane forests.

Helicinid snail, Helicina gabbi, (family Helicinidae).  Ebano Verde Scientific Reserve, central Dominican Republic, Hispaniola.

(Photograph courtesy of Dr. Jozef Grego).

Helicinid snail, Helicina phasianella. Florida, central Puerto Rico.

Helicinid snail, probably, Helicina sp. Kenscoff, south-eastern Haiti, Hispaniola.

(Photograph courtesy of Mr. Eladio M. Fernandez).

Helicinid snail, probably, Helicina sp. Macaya National Pak, south-western Haiti, Hispaniola.

(Photograph courtesy of Mr. Eladio M. Fernandez).

Helicinid snail, probably, Helicina sp. Macaya National Pak, south-western Haiti, Hispaniola.

(Photograph courtesy of Mr. Eladio M. Fernandez).

Helicinid snail, Alcadia alta. El Yunque National Forest, north-eastern Puerto Rico.

(Photograph courtesy of Mr. Alfredo Colon Archilla).

Helicinid snail, Alcadia sp. Windsor, north-central Jamaica.

Helicinid snail. Carite State Forest, east-central Puerto Rico.

Helicinid snails, species undetermined. Mount Scenery, central Saba, Lesser Antilles.

Mating pair of helicinid, snails. Mount Scenery, central Saba, Lesser Antilles.

ORDER STYLOMMATOPHORA: PULMONATES

    This order comprises the vast majority of terrestrial mollusks on planet Earth. They are the "pulmonates", snails and slugs adapted to habitually breath gaseous oxygen by means of gills adapted to become primitive but functional lungs. They all present the morphological character of having their eyes at the end of retractable stalks.

Family Amphibulimidae

    This family is an Antillean endemic, represented in the region by Amphibulima and Amphibulina, from the Lesser Antilles, and Gaeotis from Puerto Rico. They are mostly snails of wet tropical forests, many with reduced shells. Indeed, some seem to be evolving into slugs.

Amphibulimid snail, Amphibulina patula dominicensis. Eggleston, south-central Dominica, Lesser Antilles.

Amphibulimid snails, Gaeotis flavolineata. Carite State Forest, east-central Puerto Rico.

 

A helicidid snail preys on the eggs of Gaeotis flavolineata. Carite State Forest, east-central Puerto Rico.

Family Bulimulidae

    The bulimulids are a family of medium-sized to large snails, mainly tropical and sub-tropical in their distribution. They are among the most easily seen land snails in the West Indies, perhaps after the pleurodontids. Some species possess shells that are quite attractive in their coloration.

Bulimulid snail, Bulimulus guadalupensis, (family Bulimulidae). Carolina, north-eastern Puerto Rico.

Bulimulid snails, Drymaeus multilineatus.

First two photographs: Guanica State Forest, south-western Puerto Rico.

Last photograph: Lajas, south-western Puerto Rico.

Bulimulid snail, species undetermined. Gilboa Hill, northern Saint Eustatius, Lesser Antilles.

Bulimulid snails, Drymaeus sallei. Formond, south-western Haiti, Hispaniola.

(First photograph courtesy of Dr. Jozef Grego. Second photograph courtesy of Mr. Eladio M. Fernandez).

Bulimulid snail, Liguus virgineus. Puerto Escondido, south-western Dominican Republic, Hispaniola.

Bulimulid snail, Plekocheilus aurissileni. Vermont Nature Reserve, south-western Saint Vincent, Saint Vincent and the Grenadines, Lesser Antilles.

Family Cerionidae

    Due to their soft and mucous skin that looses water so easily, most snails are denizens of humid areas. The mesic and hydric forests of the Antilles have the highest number of species in the Caribbean region. However, some taxa are adapted to live in xeric areas, even deserts. Almost invariably, these species enter into aestivation during the hottest and driest months of the year. The mollusks will seal the entrance to their shells with mucus that hardens to a papery consistence and which prevents water loss. They become active only during the rather short periods of rain that occur every year in these regions. Then they feed, mate, and lay eggs as soon as possible, during the short periods when the ambient humidity stays high. Once the rainy season is over, they will again resume their inactive stage, often until the following year. This capacity to enter into a dormant stage during their life-cycle (shared by many organisms that live in such areas) is called "xerobiosis", and such organisms are called "xerobionts".

    This trait is especially noticeable in members of the genus Cerion, found in southern Florida and the Greater Antillean and Bahamian insular banks.

Family Clausiliidae

    The West Indian members of this family are small snails with elongated shells. They feed mostly on thin layers of microscopic algae and fungi growing on exposed surfaces in rain and cloud forests. The name of the family derives from the Latin "clausilium", which in their case defines a calcareous structure that serves a door with which the animal seals itself inside its shell. The structure fits into grooves in the shell, and closes tightly at the will of the animal. The clausilium is analogous to the "operculum" of some marine snails and their terrestrial relatives (like helicinids). Thus, it is a structure with a different origin but with fulfills the same function. (True opercula are unknown in pulmonate snails).

Snail, Nenia tridens, (family Clausiliidae). Carite State Forest, east-central Puerto Rico.

Snails, Nenia tridens, feeding on the film of algae on the shell of a much larger Pleurodonte caracolla snail.

Carite State Forest, east-central Puerto Rico.

Clausiliid snail, Nenisca franzi. Massif de la Hotte, south-western Haiti, Hispaniola.

(Photograph courtesy of Mr. Eladio M. Fernandez).

Family Haplotrematidae

    These is an American family. Most members are rather small and are inhabitants of forested or, at least rather humid regions. The genus Austroseletines and others in the Caribbean are mostly found under rocks and debris in mesis forests.

Haplotrematid snail, Austroselenites alticola. Florida, central Puerto Rico.

Family Helminthoglyptidae

    This group includes some of the most colorful land snails in the World, the Polymita of Cuba eastern Cuba. Highly prized by shell collectors, several populations are seriously endangered at this point.

Snail, Cepolis squamosa, (family Helminthoglyptidae). Guanica State Forest, south-western Puerto Rico.

Family Oleacinidae

    This taxon includes some predatory species. Their thin shells seem to be made of glass or porcelain, and the majority of species appear to be terrestrial.

Snail, probably Oleacina voluta, (family Oleacinidae). Masiff de la Hotte, south-western Haiti, Hispaniola.

(Photograph courtesy of Mr. Eladio M. Fernandez).

Oleacinid snail, possibly Oleacina sp. Near Pie Pol, Bahoruco Mountains, south-western Dominican Republic, Hispaniola.

Oleacinid snail, Varicella sp. El Yunque National Forest, north-eastern Puerto Rico.
This individual cleans its own shell with its radula.

Oleacinid snail, Varicella sp. Florida, central Puerto Rico.

Family Pleurodontidae

    Until recently, this family was considered one with Camaenidae, today treated as an Old World taxon. Pleurodontids are perhaps the most commonly encountered land snails in the Caribbean islands. Most species possess a flattened shell. Because some members of the family can be quite large they served as a source of protein for the pre-Colombian Antillean cultures. They broken shells a often found in Caribbean archaeological sites.

The calcareous eggs of the pleurodontid snail, Polydontes acutangula. El Yunque National Forest, north-eastern Puerto Rico.

    On several islands, terrestrial snails of the genera Pleurodonte and Polydontes are the most obvious land mollusks. Sometimes found by the hundreds or thousands per acre, they feed on leaves and on the film of microscopic algae growing on tree trunks and rocks. It is interesting to note that despite the abundance of these and similar species in the Antilles, very few predators specialized in the consumption of snails have evolved in this region. The only exceptions seem to be the Chondrohierax and Rhostramus kites present in some of the Greater and Lesser Antilles. These hawks possess strongly hooked bills adapted to extract the soft flesh of snails out of their shells.

Family Sagdidae

    Members of the genus Sagda are particularly abundant in the islands of Jamaica. Related genera, like the monotypic Platysuccinea usually also inhabit very humid, forested areas.

Mating pair of snails, Sagda pila, (family Sagdidae). Windsor, north-central Jamaica.

Sagdid snail, possibly Sagda sp. Barbecue Bottom, north-central Jamaica.

Sagdid snail, Platysuccinea portoricensis, feeding on a fungus. El Yunque National Forest, north-eastern, Puerto Rico.

This species covers its shell in soil and debris, probably as a disguise.

The shell is fragile and semi-transparent, and the animal lives during the day under rotten logs or holes in tree trunks, and axils of leaves in rain forests.

A pair of Platysuccinea portoricensis prepare to mate.

Family Subulinidae

    These pulmonated snails are usually greatly elongated into thin, spiral cones. Most are small creatures found under rocks and debris. Some prey on smaller invertebrates and on the eggs of other animals like frogs.

Snails, Obeliscus terebraster, (family Subulinidae). Guilarte State Forest, west-central Puerto Rico.

Subulinid snail, Obeliscus sallei. Ebano Verde Scientific Reserve, central Dominican Republic, Hispaniola.

(Photograph courtesy of Dr. Jozef Grego).

Family Urocoptidae

    This family is represented in the Caribbean islands snails like those of the genus Microceramus. They are commonly found in karstic zones or xeric areas. Most are small, and form part of the xerophilic gastropod fauna in such places.

Snails, Microceramus pontificus (family Urocoptidae; behind one of them is a Cerion striatella snail). Guanica State Forest, south-western Puerto Rico.

Snails into Slugs

    Although evolution takes time and cannot be seen happening before one's eyes, the diverse stages of it can be seen as one compares snail that have ever smaller and "disfunctional" shells. Some of those species are shown below.

    These are almost always inhabitants of very humid environs. Some slugs do live in rather xeric areas, but will retreat and aestivate under rocks and debris during dry spells. In fact, the term "slug" has no systematic value and only refers to a specific morphology, as a number of them have evolved from different families of snails and are not closely related.

ORDER SYSTELLOMMATOPHORA: LEATHERLEAF SLUGS

Family Veronicellidae

    These conspicuous and often big slugs can be seen crawling about at night. They do not possess lungs but nonetheless breathe air and are especially well adapted to moist tropical environments in the Antilles. Some species are quite large, and like their shelled relatives, live in humbid areas where they search for food at night.