Turbellaria (Free-Living Flatworms)

views updated

Turbellaria

(Free-living flatworms)

Phylum Platyhelminthes

Class Turbellaria

Number of families 102

Thumbnail description
Mostly free-living flatworms with a cellular epidermis that is usually ciliated; the mouth leads to a stomodeal pharynx and incomplete gut

Evolution and systematics

The phylogeny of the platyhelminth classes is not clear. Recent morphological and molecular studies have generated numerous hypotheses as to their relation to each other and to other phyla. Traditionally, the class Turbellaria was thought to be the basal ancestor of the parasitic classes (Trematoda, Cestoda, Monogenea) within the phylum. However, some researchers believe that the parasitic classes should be separated into a separate phylum (Neodermata) based on their unique tegument, the neodermis that may be adaptive to a parasitic existence. Both morphological and molecular studies also suggest that the Turbellaria are paraphyletic and that the orders Acoela and Nemertodermatida should be placed into a separate phylum. The Acoela have a primitive nerve net (no brain as in other flatworms), a simple pharynx when present, and a syncytial cellular gut without a cavity, entolecithal ova, and a lack of protonephridia. They may be the closest relatives to the acoeloid ancestor that gave rise to bilateral metazoa. It also has been postulated that the acoelomate condition (no body cavity) of the other platyhelminths may be secondarily derived from more advanced protostomes. The Acoela and Nemertodermatida are considered a separate, distinct taxon.

The ten recognized orders constitute the remaining members of the class Turbellaria. There are more than 4,500 described species within the Acoela and Turbellaria combined; however, many species have yet to be discovered and described. The characteristics of each order is the following:

Order Catenulida has a simple pharynx and sac-like gut; the mesenchyme is poorly differentiated; ova are enotlecithal. They are elongate forms that occur in freshwater and marine habitats.
Order Haplopharyngida are small worms with a simple pharynx, the proboscis is simple and ventral to the anterior tip of body (reminiscent of nemereans), anal pore is weakly developed; the brain is encapsulated with two ventral-lateral nerve cords; the ovary is simple without accessory organs; the male pore has a circle of hard straight stylets, anterior to female pore; the are free living and marine. They were once considered to be macrostomids. They contain two species.
Order Lecithoepitheliata's ordinal status is questionable; the pharynx is complex and somewhat variable in the anterior of the body, the gut is simple; the ova are not entolecithal and surrounded by vitelline cells. There are about 30 species.
Order Macrostomida have a simple pharynx, gut is a simple sac; the posterior end of the body may be broadened into an adhesive disc; there is no asexual zooid formation; the ovaries are often paired, the eggs entolecithal; the male pore is usually separate. They are mostly small interstitial marine and freshwater forms.

Order Polycladida have ruffled plicate pharynx, the gut is multibranched with diverticula that may be anastomosing; the ovaries are scattered, with entolecithal ova; testes are scattered follicular, the male pore is usually anterior to the female pore. They are mostly large free-living marine flatworms that may be brightly colored.
Order Prolecithophora's ordinal status is questionable. They have a plicate or bulbous pharynx, the gut is simple; the ovaries, and testes are follicular or compact, vitellaria is diffuse, eggs are ectolecithal. They are small, free-living or commensal, freshwater and marine forms.
Order Proplicastomata is similar to the Acoela based on a few specimens; they have an elongate plicate pharynx; no statocysts; entolecithal ova. They are free-living marine forms.
Order Proseriata is closely related to Tricladida. They have a cylindrical plicate pharynx, a simple gut, small, compact, paired ovaries at the end of the vitelline duct, the vitellaria is arranged along duct, ectolecithal ova. They are free-living marine forms.
Order Rhabdocoela is a large diverse group with four suborders (Dalyellioida, Typhloplanoida, Kalptorhynchia, and Temnocephalida). They have a bulbous pharynx, a simple sac-like gut; the mesenchyme is fairly open; protonephridea are paired when present; the anterior brain and ventral nerve trunks are usually with cross connections, no statocyst; testes are compact; ovaries separate or joined with vitellaria, ova ectolecithal, a uterus is sometimes present. They have marine, freshwater, and terrestrial forms, many of which are symbiotic.
Order Tricladida have a cylindrical plicate pharynx posteriorly directed, gut has one anterior and two posterior branches and numerous diverticula; mesenchyme is thick; no statocyst; male and female copulatory structures are complex, posterior to pharynx; follicular testes; one pair of small ovaries is usually anterior, vitellaria is extensive over most of lateral body, ectolecithal ova. They are usually large, flattened, and sometimes elongate worms with marine, freshwater, and terrestrial forms.

Physical characteristics

The class Turbellaria share the following characteristics with other classes within the Platyhelminthes:

triblobastic (three tissue layers)
acoelomate (no fluid filled body cavity or coelom)
bilaterally symmetrical
dorsoventrally flattened
spiral cleavage and mesoderm derived from the 4d cell
complex, incomplete gut (no anus)
cephalized, with cerebral ganglion (brain) and longitudinal nerve cords that form a ladder-like nervous system
numerous sense organs at the anterior end of the body, and tactile receptors distributed over the body, especially around the pharynx
protonephridia that function in excretion and osmoregulation
no circulatory system, which restricts the size and shape of these animals
hermaphroditic with complex reproductive system

Turbellarians also are free-living or commensal (not usually parasitic), usually aquatic, and have a stomodeal pharynx. Their cellular epidermis is usually ciliated and contains mucous secreting cells and structures called rhabdoids that can produce copious mucus to prevent desiccation. Most turbellarians also have pigment-cup occelli for detecting light; some have an anterior pair where larger species may posses numerous pairs along the body.

Distribution

Turbellarian species are distributed worldwide, mostly in freshwater and marine environments with a few taxa occurring on land.

Habitat

Many of the minute species occur interstitially between grains of sand in aquatic habitats. Larger species are pelagic (marine) or live among submerged substrates such as rocks, coral, and algae. Many species, especially of the order Rhabodocoela, are symbiotic with various invertebrates and fishes. A few genera of the order Tricladida are terrestrial, living in damp leaf litter and soil.

Behavior

Turbellarians display a number of behaviors that prevent them from straying beyond their normal habitats and allows to them to maintain orientation within those habitats. For instance, most turbellarians are positively thigmotactic (touch) ventrally and negatively thigmotactic dorsally. This allows them to maintain their ventral side against the substrate in benthic forms. In other species where touch may not be the best way to orient to a substrate (such as interstitial and pelagic

forms), they have statocysts so that they can orient to gravity (geotaxis). Most species are also negatively phototactic, which prevents worms from coming out in the daylight where they may get eaten or dry out in the case of terrestrial forms.

All turbellarians have a strong sense of smell that can be used to find food or mates (chemotaxis). Chemosensors are concentrated on each side of the head to help them determine the direction that the chemical trail is coming from. The heads of freshwater species are often expanded into auricles that have sensors. Some species have tentacles and ciliated pits to assist in chemotaxis. Dugesia swings its head back and forth to help determine the proper direction of the food source. Other species use trial and error to determine the proper direction to find food. They move in one direction until the signal gets weaker, and then continue switching direction until the signal is strongest. Some species also have been shown to orient to currents in order to find food (rheotaxis).

Feeding ecology and diet

Most turbellarians are carnivorous predators or scavengers. Carnivores feed on organisms that they can fit into their mouths, such as protozoans, copepods, small worms, and minute mollusks. Some species use mucus that may have poisonous or narcotic chemicals to slow or entangle prey. Some have specific diets and feed on sponges, ectoprocts, barnacles, and tunicates. Several species have commensal relationships with various invertebrates and few actually border on being parasitic because they graze on their live hosts. Terrestrial species feed on earthworms and land snails. A few species feed on microalgae that may be incorporated into the body, forming a symbiotic relationship in which the algae supply the worm with carbohydrates and fats and the worm supplies the algae with nitrogen waste products and a safe haven.

The pharynx and gut cells produce digestive enzymes that breakdown food extracellularly. Nutritive cells in the gastrodermis then phagotize partially digested material that is distributed throughout the body. Because these worm lack a circulatory system, larger species have extensive anastomosing guts to aid in distribution. Since these worms have incomplete guts, all waste must pass back out of the mouth.

Reproductive biology

Asexual reproduction is a common method of reproduction in freshwater and terrestrial turbellarians. Many of the triclads divide by transverse fission: the body splits transversely behind the pharynx and each part generates the rest of the body. The posterior portion attaches to the substrate and the anterior portion crawls away until it tears in two. In species such as Dugesia, the cells tend to vary in their ability to regenerate. The cells in the middle portion of the body have the strongest ability to regenerate. Experiments have shown that if just the tail is cut off, it will not grow a new body, whereas the main portion of the body will regenerate a new tail. The ability of the fissioned portions of these worms to regenerate the proper half has interested scientists for years in investigating why the head portion grows a tail and why the tail portion regenerates a new head. In the genera Catenula, Microstomum, and Stenostomum (orders Catenulida and Macrostomida), multiple transverse planes develop that lead to a train of individuals called zooids that do not detach until they reach a certain stage. Other species (e.g., Phagocata and some terrestrial species) detach fragments that become encysted and eventually develop into new individuals.

Turbellarians are hermaphroditic and their sexual reproductive systems are quite complicated. The male system may have one, two, or multiple testes that drain via sperm ducts that may lead to a storage area called the seminal vesicle. Prostate glands may be present that produce seminal fluid that mixes with the sperm in the seminal vesicle. The sperm then exits the worm via the protrusible penis or eversible cirrus with help from a muscular ejaculatory duct. The female system is more variable among the Turbellaria, depending on whether they produce entolecithal (Macrostomida and Polycladida) or ectolecithal (Rhabdocoela, Prolecithophora, and Tricladida) ova. A germovitellarium, which may be single or paired, produces entolecithal ova (yolk reserves within ova). A germarium or ovary, which is separate from the vitellaria, produces yolk-free ova that eventually are surrounded by separate yolk cells in a tanned protein capsule to form the ectolecithal egg. Sperm also are included within the egg capsule to insure fertilization. Eggs pass through the oviduct that may be differentiated into a seminal receptacle or uterus before deposition.

Cross fertilization (mating) usually occurs when worms align themselves with each other, and the cirrus or penis of each worm is inserted into the female gonopore or atrium of the other and deposits sperm. The worms then go their own way with the sperm stored in their seminal receptacles. In some species, mating occurs by hypodermic impregnation in which the male copulatory organ penetrates the body wall of the mate and deposits sperm in the mesenchyme. The sperm then make their way to the ova.

Turbellarians have either direct development or produce a pelagic larva. Polyclads often produce a pelagic Muller's larva that settles to the bottom and goes through metamorphosis in a few days. This larva has eight ventrally directed ciliated lobes, which it uses to swim. Stylochus, a parasitic polyclad, produces the Gotte's larva, which has only four ciliated lobes.

Conservation status

No turbellarians are considered threatened by the IUCN Red Book.

Significance to humans

The regenerative abilities of Dugesia have been studied extensively by scientists to better understand the healing and cell regeneration processes in humans. Several species parasitize commercially important species such as oysters and a few species cause pathological problems in marine ornamental fishes kept in aquaria.

Species accounts

List of Species

Notoplana acticola
Pseudoceros ferrugineus
Oyster leech
Kronborgia amphipodicola
Paravortex scrobiculariae
Temnocephala chilensis
Bdelloura candida
Land planarian
Freshwater planarian

No common name

Notoplana acticola

order

Polycladida

family

Leptoplanidae

taxonomy

Notoplana acticola (Boone, 1929), Monterey Bay, California, United States.

other common names

None known.

physical characteristics

Adults are 1–2.4 in (25–60 mm) long when extended, body usually widest anteriorly, tapering posteriorly; tentacular eyes in rounded clusters with scattered eyes lying anterior, posterior, and sometimes lateral to them; about 25 cerebral eyes occur in an elongate band; eyes consist of single, cup-shaped pigment cell covering 6–10 retinal cells; color tan or pale gray with darker markings along midline.

distribution

Pacific coast of United States.

habitat

Rocky intertidal and subtidal zones.

behavior

Experimental severing of main nerve pathways resulted in rapid repair of nerves and pathways.

feeding ecology and diet

Can ingest prey up to half its size. Feeds on limpets (Collisella digitalis), small acorn barnacles, and captive worms, and have been observed eating the red nudibranch (Rostanga pulchra) in captivity.

reproductive biology

Functional hermaphrodites throughout the year with mature sperm and eggs present; a small number of worms were found with only ovaries in the spring and 10–50% of the population only had testes throughout the year; two-thirds of all worms had sperm in their seminal receptacles, indicating that mating had taken place; egg deposition occurs from late spring to early fall.

conservation status

Not threatened.

significance to humans

None known.

No common name

Pseudoceros ferrugineus

order

Polycladida

family

Pseudoceridae

taxonomy

Pseudoceros ferrugineus Hyman 1959 Palau, Indonesia.

other common names

None known.

physical characteristics

Elongate worm 0.7 in long by 0.4 in wide (18 mm long by 11 mm wide); may get larger. Two slightly developed nuchal tentacles. Body is deep red with white flecks or dots centrally located; yellowish band around the margin with deeper shades of purple between margin and central area with flecks. This genus is speciose on coral reefs and colors vary considerably between species ranging from bright pink and orange to blue and green.

distribution

Tropical coral reefs in South Africa, Red Sea, Indo-Pacific, and Hawaii.

habitat

Coral reefs often under rubble.

behavior

Displays aposematic coloration to prevent predation.

feeding ecology and diet

Many species of the genus Pseudoceros, including P. ferrugineus, feed on colonial tunicates.

reproductive biology

Hermaphroditic, eggs hatch as Muller's larva that will settle out in a few days and metamorphose into a juvenile resembling the adult.

conservation status

Not threatened.

significance to humans

Aesthetic value for scuba divers.

Oyster leech

Stylochus inimicus

order

Polycladida

family

Stylochidae

taxonomy

Stylochus inimicus Polombi, 1931, Florida, United States.

other common names

None known.

physical characteristics

Body is oval or discoid with retractile nuchal tentacles; marginal eyes in band, cerebral and tentacular eyes present; pharynx long in middle of body, intestinal branches not anastomosing; male and female gonopores close to each other in posterior of body.

distribution

Florida, United States.

habitat

Under rocks and algae and in oyster shells and other invertebrates.

behavior

Tends to hide under debris or in shells of oysters and barnacles.

feeding ecology and diet

Carnivorous feeding on animal matter, including oyster tissue. Hypsoblennius, a small blenny, has been shown to feed on it.

reproductive biology

Following mating, which may last nine hours and involve more than four partners, an individual can deposit 7,000–21,000 eggs (0.007 in [0.18 mm]) during a summer month. Egg masses usually attached to clean oyster shell and irregular in shape and only one layer thick. Worms observed covering eggs, protecting them. Ciliated larvae with six eyes hatch from eggs. Positively phototactic, causing them to swim up and away from the bottom helping them to disperse.

conservation status

Not threatened.

significance to humans

Observed entering, devouring, and killing oysters. It has been blamed for the large oyster kill in Florida in 1932–1933. However, it is likely that other environmental factors were also compromising the oysters. The eradication of the worms from oyster beds is difficult; it has been suggested that flooding with freshwater may control worms.

No common name

Kronborgia amphipodicola

order

Rhabdocoela

family

Fecampiidae

taxonomy

Kronborgia amphipodicola Christensen and Kanneworff, 1964, Øresund, Denmark.

other common names

None known.

physical characteristics

Parasitic in Ampelisca macrocephala; have separate sexes; males are cylindrical red worms, 0.12–0.20 in (3–5 mm) long, lack eyes, mouth, pharynx, and intestine, but possess distinctive gonopore at one end, active and able to swim using cilia; females white with no gonopore, also lack eyes and gut, 1.5 in (40 mm) long, fragile, sluggish and unable to swim.

distribution

Host occurs through out the North Atlantic Ocean. It is not known whether it infects A. macrocephala throughout its range. It has only been identified from the northeast Atlantic area.

habitat

Males occur in hemocoel and females wrap themselves around the intestine of host.

behavior

Each host only harbors either male or female worms.

feeding ecology and diet

Most likely feed on host blood and gut contents.

reproductive biology

Dioecious, separate sexes. Males and females leave host at the site of the anus by dislodgement of the intestine. Females immediately secrete cocoon longer than its body using epidermal glands; cocoon attaches to host amphipod tube. One or more males enter cocoon through trumpet-shaped opening and inseminate females most likely by hypodermal impregnation. After laying numerous capsules, weakened female crawls out of cocoon and dies. Cocoon morphology is quite elaborate and host specific. Ciliated larvae hatch in 50–60 days and are able to continue swimming searching for a host for two to three days. Larvae have two rhabdomeric eyes for detecting light. After contacting a host's carapace, larvae encyst in five minutes. They produce concentrated enzymes that create a small hole in carapace where larva enters host and migrates to the hemocoel.

conservation status

Not threatened.

significance to humans

None known.

No common name

Paravortex scrobiculariae

order

Rhabdocoela

family

Graffillidae

taxonomy

Paravortex scrobiculariae (Graff, 1903), Great Britain.

other common names

None known.

physical characteristics

Elongate worm approximately 0.08 in (2 mm) long, lacks adhesive organs; mouth anterior with subterminal opening that passes through small ciliated buccal cavity into muscular pharynx and then into simple saccate gut; testis sacciform, ovary single.

distribution

Its host, the intertidal bivalve Scrobicularia plana, occurs in northeast Atlantic, Mediterranean, and West Africa. It is unknown if it occurs throughout its host's range.

habitat

Lives inside gut and digestive gland of host.

behavior

Adult worms migrate within host based on the tidal cycle that relates to feeding cycle of host. Worms use cilia to swim forward from intestine through stomach to digestive gland during the ebb tide and return to intestine during the flood tide.

feeding ecology and diet

Feeds on semi-digested components of host food and residual cells released by digestive glands, which it ingests through muscular pharynx, as it makes its migrations from intestine to digestive gland. It appears that it is not capable of producing all the enzymes necessary to digest food and relies on ability to acquire enzymes along with food during its migrations to digestive gland. This strategy reduces the need to expend energy on producing these enzymes.

reproductive biology

Hermaphroditic and viviparous (releases juvenile worms); up to 40 capsules, each containing two embryos and numerous vitelline cells, which enter parenchyma from female atrium, fully developed embryos leave the capsules and move freely in parenchyma until they escape parent through mouth.

conservation status

Not threatened.

significance to humans

One species of Paravortex commonly called tang turbellaria or black ich are pathogenic on skin of several species of marine ornamental fishes commonly kept in aquaria.

No common name

Temnocephala chilensis

order

Rhabdocoela

family

Temnocephalidae

taxonomy

Temnocephala chilensis (Moquin-Tandon, 1846), Santiago, Chile.

other common names

None known.

physical characteristics

Body ovoid, extensible with five anterior prehensile tentacles, posterior attachment disc present, body color is uniform orange-brown; epidermis composed of four syncytial plates, epidermal cilia are absent; eyespots with distinctive red pigment; gonopore is mid-ventral in posterior third of body, two testes occur on each side in posterior of body, cirrus on left side; ovary lies on right side at level of genital pore, vitellaria are extensive.

distribution

Follows distribution of its hosts in Chile and Argentina.

habitat

Ectosymbiotes of freshwater decapod and isopod crustaceans. Some live on gills, while others occur on carapace and limbs. It associates with species of the anomuran crabs of genus Aegla and species of the crayfish Samastacus.

behavior

Can move in leech-like movement using posterior disc and tentacles.

feeding ecology and diet

Uses its prehensile tentacles to capture free-living and co-symbiotic invertebrates of suitable size such as protozoans, rotifers, oligochaetes, insect larvae, copepods, or ostrcods.

reproductive biology

After mating, it attaches eggs to exoskeleton of host. After eggs are deposited, detritus tends to collect around eggs, which attracts

protozoans and other organisms that provide food for newly hatched worms. Development is direct without larval stage.

conservation status

Not threatened.

significance to humans

None known.

No common name

Bdelloura candida

order

Tricladida

family

Bdellouridae

taxonomy

Bdelloura candida (Girard, 1850), Massachusetts, United States.

other common names

None known.

physical characteristics

Body lanceolate or oval shaped, lateral margins undulated, measure 0.6 × 0.2 in (15 × 4 mm) ranging up to 0.9 in (25 mm). Caudal adhesive disk wide as body and set off from the rest of the body; pharynx large about one-third of body; eye lenses absent; numerous testes distributed throughout body; whitish in color.

distribution

Same distribution as host Limulus polyphemus along eastern seaboard of United States from Maine south to the Gulf of Mexico.

habitat

Ectocommensal on the last pair of cleaning legs and gills of host.

behavior

Obligate commensal in that it does not occur off its host.

feeding ecology and diet

Most likely feeds on debris and food particles stirred up by its host.

reproductive biology

Lays cocoons from May until mid-August on inner surface of gill lamellae of host. Cocoons are 0.15 in (4 mm) in length by 0.07 in (2 mm) in diameter and located on a pedicel 0.39 in (1 mm) high. Development is direct.

conservation status

Not threatened.

significance to humans

None known.

Land planarian

Bipalium pennsylvanicum

order

Tricladida

family

Bipaliidae

taxonomy

Bipalium pennsylvanicum Ogren, 1987, Pennsylvania, United States.

other common names

None known.

physical characteristics

Long, brownish yellow in color with three dorsal stripes; head is half-rounded or lunate, body retracted and coiled on self at rest, during locomotion over a flat dry surface, the body is greatly extended, undulates, and head is raised above surface moving from side to side; head bordered by numerous, small eyes that extend posteriorly along the body; mouth with eversible pharynx located in mid-region of body; gonopore is just posterior to mouth.

distribution

Considered to be introduced into United States, it has only been found in Pennsylvania in outdoor habitats, though other species occur throughout eastern United States.

habitat

Lives in damp areas under stones and pieces of wood and are capable of over-wintering in soil where air temperatures reach freezing.

behavior

During day, it remains in damp, dark areas under rocks and wood, and in soil. Under dry conditions, it may move further into soil to find favorable conditions.

feeding ecology and diet

It captures earthworms by attaching ventral side of head and tail. Thrashing by prey results in planarian getting a better grasp on prey. Arthropods and mollusks are not preferred foods; slugs were only eaten if they were torn open first.

reproductive biology

No evidence for fragmentation, but it can regenerate if damaged. After mating, a cream-colored swelling is apparent in mid-region of body where cocoon is developing. Cocoons are yellowish when first deposited and change to light red and eventually shiny black and measure 0.13 in (3.3 mm). One to three juveniles hatch from each cocoon. Juveniles are grayish and lack stripes. It may take 100 days to reach adult size.

conservation status

Not threatened.

significance to humans

None known.

Freshwater planarian

Dugesia tigrina

order

Tricladida

family

Dugesiidae

taxonomy

Dugesia tigrina (Girard, 1847), New Jersey, United States.

other common names

None known.

physical characteristics

Body lanceolate with auricles laterally on head, light to dark brown in color with some forms having a stripe down its midline, light spots on a dark background or dark spots on a light background; pharynx large in middle of body, gut with two posterior and one anterior directed branch as others in order.

distribution

Widespread in North America and has scattered distribution in Europe, where it may have been introduced with aquatic plants.

habitat

Under rocks, plants, and debris in clear freshwater ponds, streams, and springs.

behavior

Tends to hide under rocks during day. When hunting, will swing head side to side to better sense sources of chemicals coming from food or prey.

feeding ecology and diet

Carnivorous, feeds on various invertebrates, including mosquito larvae.

reproductive biology

Asexual reproduction by transverse fission. Transverse fission may be more common method of reproducing since sexual organs have not been observed in some populations. It has strong capacity to regenerate. Following mating, several egg capsules are deposited per worm. Development is direct and juveniles hatch from eggs.