Branchiobdellida is an order of freshwater

leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...

-like

clitellates The Clitellata are a class (biology), class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class o ...

that are obligate ectosymbionts or ectoparasites, mostly of

astacoidea Astacoidea is superfamily of freshwater crayfish that live in the Northern Hemisphere. The other superfamily of crayfish, Parastacoidea, lives in the Southern Hemisphere. Astacoidea consists of three families: Astacidae (from Europe and weste ...

n crayfish. They are found in the Northern Hemisphere and have a

holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical reg ...

distribution in East Asia, the Euro-Mediterranean region and North and Central America, with the greatest species diversity being in North and Central America.

Taxonomy

The exact relationship of this group of clitellates with other groups has been the subject of debate for a century or more. The American zoologist Perry C. Holt wrote his doctoral thesis on branchiobdellids and devoted his 45 years of research to the taxon. With his colleague, the American zoologist

Richard L. Hoffman Richard Lawrence Hoffman (September 25, 1927 – June 10, 2012) was an American zoologist known as an international expert on millipedes, and a leading authority on the natural history of Virginia and the Appalachian Mountains. He was a biology ...

, he described 8 new genera and 75 new

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

, and re-described various other species. In 1965, he came to the conclusion that the group should be raised from a

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

to an order, a sister group to the

oligochaetes Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworms ...

and

es. Forty years later, molecular data from rDNA and mitochondrial DNA studies has shown that he was correct, and that Oligochaeta, Branchiobdellida, Acanthobdellida and

Hirudinea Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bod ...

form a

monophyletic group A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

and that each should be considered an order. Branchiobdelida is now acknowledged to be sister to

leeches Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bo ...

and acanthobdellidans.

Description

Branchiobdellida vary in length between and . The body is formed from fifteen segments. There is no

prostomium The prostomium (From Ancient Greek, meaning "before the mouth"; plural: prostomia; sometimes also called the "acron") is the cephalized first body segment in an annelid worm's body at the anterior end. It is in front of (but does not include) t ...

, and the

peristomium The peristomium is the first true body segment in an annelid worm's body in the anterior end. It is directly behind the prostomium and contains the mouth, tentacular cirri, and sometimes feeding palps, which may instead occur on the prostomium. If ...

and next three segments are modified into a sucker, surrounded by small tentacles, with the mouth at its centre. The buccal cavity has a single dorsal and a single ventral tooth. Segment 14 has the anus on its dorsal surface, and segment 15 forms a sucker.

Diversity

The majority of branchiobdellida use crayfish as hosts, usually living on their heads, carapaces and

chelae A chela ()also called a claw, nipper, or pinceris a pincer (biology), pincer-like organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through New Latin '. The plural form is chelae. Legs bearing a chela are ...

(claws), but in some instances living inside their gill cavities. In East Asia, some species live on freshwater shrimp, and in Northern and Central America, freshwater crabs, shrimps and

isopods Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and ...

host branchiobdellids, and some have even been found on

Chesapeake blue crab Chesapeake often refers to: *Chesapeake people, a Native American tribe also known as the Chesepian * The Chesapeake, a.k.a. Chesapeake Bay *Delmarva Peninsula, also known as the Chesapeake Peninsula Chesapeake may also refer to: Populated plac ...

s. In the Euro/Mediterranean region, however, crayfish are exclusively used as hosts. Some branchiobdellids are generalists, but a few are limited to association with a single host species. Several different species of branchiobdellid are sometimes found on a single crayfish. Their hosts include open-water crustaceans, cave-dwellers and burrowers, but many branchiobdellida have a very limited range, occurring in a single river system or a single cave. An Appalachian brook crayfish was found to have one species of branchiobdellid in the gill chambers, one on the oral and ventral surfaces of the body and one on the chelae.

Ecology

Branchiobdellida feed on the micro-organisms and detritus that accumulate on their host; they will also feed on any soft tissues exposed where the host's exoskeleton is damaged. The relationship is generally symbiotic in that the host benefits from the cleaning activities of the branchiobdellids, and the latter benefit from a constant supply of food and a surface on which to deposit their cocoons; the worms have been maintained for months in the laboratory in the absence of a host, but the cocoons must be attached to a living host in order for normal development of the embryos to occur.

References

{{Taxonbar, from=Q1810556 Clitellata Protostome orders