The Aspidogastrea (
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
: ' “shield”, ' “stomach/pouch”) is a small group of
fluke
Fluke may refer to:
Biology
* Fluke (fish), a species of marine flatfish
* Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs, plesiosaurs, and metriorhynchids.
* Fluke (flatworm), parasiti ...
s comprising about 80
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 appropriat ...
. It is a
subclass of the
trematoda, and sister group to the
Digenea. Species range in length from approximately one millimeter to several centimeters. They are
parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...
s of
freshwater and
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military ...
molluscs and
vertebrates (
cartilaginous and
bony fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
es and
turtles). Maturation may occur in the
mollusc or
vertebrate host. None of the species has any
economic
An economy is an area of the production, distribution and trade, as well as consumption of goods and services. In general, it is defined as a social domain that emphasize the practices, discourses, and material expressions associated with th ...
importance, but the group is of very great interest to
biologists because it has several characters which appear to be archaic.
Morphology
Shared characteristics
Shared characteristics of the group are a large
ventral disc with a large number of small
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* M ...
("suckerlets") or a row of suckers and a
tegument with short protrusions, so-called "microtubercles".
Larval physiology
Larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
...
e of some species have
ciliated patches. Those of ''Multicotyle purvisi'' have four patches on the
anterior side of the
posterior sucker and six at the posterior side, those of ''Cotylogaster occidentalis'' have an anterior ring of eight and a posterior ring of six, while larvae of ''Aspidogaster conchicola'', ''Lobatostoma manteri'', ''Rugogaster hydrolagi'' lack cilia altogether. Larvae of some species hatch from eggs, others do not.
Excretory system
Like most
platyhelminthes
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...
, aspidogastreans use
flame cells as an
excretory mechanism. The two excretory
bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine en ...
s are located
dorsally, on the anterior side of the posterior sucker, connected to ducts, and three flame cell "bulbs" on each side of the body; the ducts contain
cilia to aid the flow of excreta.
Nervous system
Aspidogastreans have a
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
of extraordinary complexity, greater than that of related free-living forms, and a great number of
sensory receptors of many different types. The nervous system is of great complexity, consisting of a great number of longitudinal
nerves (connectives) connected by circular commissures. The
brain
A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...
(cerebral commissure) is located dorsally, in the anterior part of the body, the
eyes dorsally attached to it. A nerve from the main connective enters the pharynx and also supplies the intestine. Posteriorly, the main connective enters the sucker.
Sensory receptors are scattered over the ventral and dorsal surface, the largest numbers occurring on the ventral surface, at the anterior end and on the posterior sucker.
Electron-microscopic studies revealed 13 types of receptors.
Life cycles
Their
life cycle is much simpler than that of
digenean
trematode
Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive h ...
s, including a
mollusc and a facultative or compulsory
vertebrate host. There are no multiplicative
larval stages in the mollusc host, as known from all digeneans.
Host specificity of most aspidogastreans is very low, i.e., a single species of aspidogastrean can infect a wide range of host species, whereas a typical digenean trematode is restricted to few species (at least of molluscs). For example, ''
Aspidogaster conchicola
''Aspidogaster conchicola'' is a trematode parasite of the Aspidogastrea subclass that commonly infects freshwater clams. It has not been well studied since it is of little economic or medical importance, but ''A. conchicola'' and its fellow aspi ...
'' infects many species of
freshwater bivalves belonging to several families, as well as
freshwater snails, many species of
freshwater fish
Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%. These environments differ from marine conditions in many ways, especially the difference in levels o ...
es of several families, and freshwater
tortoises.
Life cycles have been elucidated for a number of species. ''
Lobatostoma manteri'' is an example of a species which has obligate vertebrate hosts. Adult
worm
Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always).
Worms vary in size from microscopic to over in length for marine polychaete wor ...
s live in the
small intestine of the snubnosed dart, ''
Trachinotus blochi'' (
Teleostei
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Te ...
,
Carangidae), on the
Great Barrier Reef
The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, A ...
. They produce large numbers of
eggs which are shed in the
faeces. If eaten by various prosobranch
snails, larvae hatch in the
stomach, and—depending on the species of snail—stay there or
migrate to the
digestive gland where they grow up to the preadult stage which has all the characteristics of the adult including a testis and ovary.
Evolutionary relationships
Digenean trematodes have been
culture
Culture () is an umbrella term which encompasses the social behavior, institutions, and norms found in human societies, as well as the knowledge, beliefs, arts, laws, customs, capabilities, and habits of the individuals in these groups ...
d in various, complex,
media. However, their parasitic stages die soon in water. Aspidogastreans may survive for many days or even weeks outside a host in simple physiological
saline solution). For example, adult ''A. conchicola'' survived in water for a fortnight, and in a mixture of water and saline solution for up to five weeks. ''L. manteri'' extracted from
fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
could be kept alive for up to 13 days in dilute sea water in which they laid eggs containing larvae infective to snails.
This has led to the suggestions that aspidogastreans are archaic trematodes, not yet well
adapted to specific hosts, which have given rise to the more "advanced" digenean trematodes, and that the complex life cycles of digenean trematodes have evolved from the simple ones of aspidogastreans.
Synapomorphies of the trematodes are presence of a ''
Laurer's Canal'', a posterior sucker (transformed to an adhesive disc in the Aspidogastrea), and life cycles involving molluscs and vertebrates. DNA studies have consistently supported this sister group relationship. The question of whether vertebrates or molluscs are the original hosts of the trematodes, has not been resolved.
This view is supported by the evolutionary relationships of the hosts which these two subclasses utilise. The hosts of aspidogastreans include
chondrichthyan fishes (
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s,
rays and
chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At ...
s), a group that is 450 million years old, whereas the digeneans, are known from
teleost fishes (210 million years old) as well as from various "higher" vertebrates; very few species have invaded chondrichthyans secondarily.
Families within the Aspidogastrea
Rohde (2001) distinguish four families of Aspidogastrea:
* The
Rugogastridae include a single genus, ''Rugogaster'', with two species from the rectal glands of
holocephalan fishes. It is characterised by a single row of rugae (transverse thickenings of the body surface), numerous
testes, and two
caeca. Species of all other families have a single caecum and either one or two testes.
* The
Stichocotylidae include the single species ''Stichocotyle nephropis'' from the intestine of
elasmobranchs. It has a single ventral row of well separated suckers.
* The
Multicalycidae include the single genus ''Multicalyx'' from the intestine of
holocephalans and elasmobranchs. It is characterised by a single ventral row of
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* M ...
.
* The
Aspidogastridae includes species infecting
molluscs,
teleosts and
turtles. The ventral adhesive disc bears either three or four rows of
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* M ...
. Rohde distinguishes three subfamilies of Aspidogastridae, the ''Rohdellinae'', ''Cotylaspidinae'' and ''Aspidogastrinae''.
Gibson further recognized two orders, the Aspidogastrida with the single family Aspidogastridae, and the Stichocotylida including the Stichocotylidae, Multicalycidae and Rugogastridae.
However, similarities between species of these two orders are so great that distinction at the level of orders does not seem justified.
References
External links
Aspidogastrea from Tree of LifeChecklist of European aspidogastreansFishdisease.net*Aspidogastrea knol
*Aspidogastrea konl I
*Aspidogastrea knol II
{{Taxonbar, from=Q576111
Aspidogastrea,
Protostome subclasses