Polycladida
Encyclopedia
The Polycladida represents a highly diverse clade
of free-living marine turbellarian flatworm
s. They are known from the littoral
to the sublittoral zone (extending to the deep hot vents), and many species are common from coral reef
s. Only a few species are found in freshwater habitats.
Often polyclads are associated with other invertebrate
s, such as bivalve mollusks, sponge
s, coral
s, or ascidians. In such associations, the worms may use the invertebrates as a source of food, or they may find protection from predators inside the structural framework of these "hosts". Most polyclads hide away from direct light. However, some of the brightly colored species often are active during the day. With their flamboyant coloring they advertise their potential toxicity to visual predators such as fish.
Polyclads range from 3 to 20 mm (0.118110236220472 to 0.78740157480315 in) in length with a flattened, roughly oval, body shape and, in many cases, a pair of short tentacles on the head. They are distinguished from other related animals by the presence of a folded pharynx
, an elongated intestine with numerous complex diverticula
, and multiple ocelli
.
The order Polycladida is divided into the two suborders Cotylea and Acotylea, based on the character "presence/absence of a cotyl or sucker". Of the two, the Acotylea is the larger group with over 26 families worldwide. Acotyleans are major predators of sessile marine invertebrates such as all commercial bivalves species (including pearl and rock oysters), mussels, scallops and giant clams. Many acotyleans are dull in coloration, and cryptic in their behavior, hiding in crevices and under coral during the day. Cotyleans, on the other hand, with as many as 16 families, are prominent members of tropical coral reef communities. Although cotylean flatworms are conspicuous predators in subtropical and tropical ecosystems, they are difficult to study. These worms are very fragile and when disturbed can break apart
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of free-living marine turbellarian flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s. They are known from the littoral
Littoral
The littoral zone is that part of a sea, lake or river that is close to the shore. In coastal environments the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas that are permanently submerged. It always includes this intertidal zone and is often used to...
to the sublittoral zone (extending to the deep hot vents), and many species are common from coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s. Only a few species are found in freshwater habitats.
Often polyclads are associated with other invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s, such as bivalve mollusks, sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...
s, coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, or ascidians. In such associations, the worms may use the invertebrates as a source of food, or they may find protection from predators inside the structural framework of these "hosts". Most polyclads hide away from direct light. However, some of the brightly colored species often are active during the day. With their flamboyant coloring they advertise their potential toxicity to visual predators such as fish.
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, an elongated intestine with numerous complex diverticula
Diverticulum
A diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....
, and multiple ocelli
Ocellus
A simple eye refers to a type of eye design or optical arrangement that contains a single lens which detect light. A "simple eye" is so-called in distinction from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word...
.
The order Polycladida is divided into the two suborders Cotylea and Acotylea, based on the character "presence/absence of a cotyl or sucker". Of the two, the Acotylea is the larger group with over 26 families worldwide. Acotyleans are major predators of sessile marine invertebrates such as all commercial bivalves species (including pearl and rock oysters), mussels, scallops and giant clams. Many acotyleans are dull in coloration, and cryptic in their behavior, hiding in crevices and under coral during the day. Cotyleans, on the other hand, with as many as 16 families, are prominent members of tropical coral reef communities. Although cotylean flatworms are conspicuous predators in subtropical and tropical ecosystems, they are difficult to study. These worms are very fragile and when disturbed can break apart