Food and Feeding
Most common, polyclads are active carnivorous predators and scavengers
and can be found feeding on various sessile invertebrates. Some species
are herbivores and have specialized on green algae or benthic diatoms.
In a few flatworm species of the order Acoela
(an old taxonomic order which is distinct
from the order Polycladida), ingested microalgae are not degraded
but become endosymbionts (Zoochlorella). In
this symbiontic relationship algae remain photosynthetically active in
gut and parenchyma cells and significantly contribute to the energy intake
of the worms. In some species of the genus Convoluta (Convoluta
roscoffensis - PHOTO to the right © Arthur Hauck),
juvenile worms acquire a high number of green algae (Tetraselmis convolutae,
about 25000 per individuum). After entering the adult phase, they completely
rely on their endosymbionts being reflected in crucial anatomical changes.
Their pharynx and mouth loose function. For feeding, C. roscoffensis
comes to the surface in the bright light of low tide, where the symbiotic
algae distributed throughout the epidermis of the body, actively photosynthesize
(Holligan et al., 1977). Food produced by the algae (mannit sugar) is used
by the flatworms. This phenomenon can be observed
on sheltered sandy beaches of France and parts of England when under optimal
environmental conditions these worms form great green streaks across the
sand at low tide. Many species of the Pseudocerotidae family are
thought to prefer colonial and solitary ascidians (see photos below), sponges,
and bryozoans showing no regular specificity in their diet. For feeding,
the highly ruffled pharynx (pharynx plicatus) which, when not in use is
retracted in a pocket, protrudes and can be expanded into the individual
zooids of colonial ascidians. While discharging proteolytic secretions
by accessory glands the muscular pharynx is used like a pestle to macerate
the prey's tissue. Partially digested tissue is then drawn into the intestine
which acts as a reservoir while further digestion takes place in the highly
branched gut. The gastrovascular cavity also transports food particles
to all parts of the body.
solitary ascidian Corella willmeriana. Several juveniles
are inside of each tunicate. After they have eaten
the entire contents, they crawl around for another
victim (PHOTO © Sandra Millen).
length of up to 3 cm. It is distributed throughout
the Mediterranean Sea and the temperate
eastern Atlantic and apparently feeds on
tunicates of the genus Clavelina
(PHOTO © Peter Wirtz).
of a polyclad (longitudinal section)
with highly branched diverticles (ventral view)
Observations on Pseudobiceros species suggest that prey can also
be engulfed by the muscular pharynx (see left image below), which can expand
to the same size as the whole animal. Digestions is then started outside
of the body allowing the pharyngeal muscles to break up the prey which
is then sucked, whole, into the intestine. Yet another technique has been
observed for Eurylepta leoparda. This
species penetrates the mantle of the solitary ascidian
and by using these drill holes, they suck its complete content within several
hours. Juveniles even can be found inside the tunicate. After they have
eaten the entire contents, they crawl across the rocks to another tunicate.
When occuring in masses flatworms can have disastrous impact on human aquaculture.
Tropical polyclads are known as pests of oysters and giant clams (Stylochus
Aquaplana/Paraplanocera sp. extends throughout the
entire body (PHOTO © Robert F. Bolland).
After further enzymatic degradation of food particles in the gastrovascular cavity nutrients are transported into the intestinal branches resembling a highly absorptive surface. Most food particles are engulfed by phagocytosis of the gastrodermal cell layer and further enzymatic breakdown occurs intracellularly. Undigested material is egested through the pharynx, the same opening through which food enters, because flatworms have a blind digestive system. In some species it has been observed, that after complete digestion the gut was cleaned by flushing it with water.
| main | back | photo galleries! |