Editing Aspidogastrea

{{Short description|Species of fluke}}
{{Automatic taxobox
| image = Multicalyx elegans.jpg
| image_caption = ''[[Multicalyx elegans]]''
| taxon = Aspidogastrea
| authority = Faust & Tang, 1936 
| subdivision_ranks = Orders
| subdivision = *[[Aspidogastrida]]
*[[Stichocotylida]]
}}

The '''Aspidogastrea''' ([[Ancient Greek]]: {{lang|grc|ἀσπίς}} ''{{lang|grc-Latn|aspis}}'' “shield”, {{lang|grc|γαστήρ}} ''{{lang|grc-Latn|gaster}}'' “stomach/pouch”) is a small group of [[Trematoda|fluke]]s comprising about 80 [[species]]. It is a [[Subclass (biology)|subclass]] of the [[Trematode|trematoda]], and sister group to the [[Digenea]]. Species range in length from approximately one millimeter to several centimeters. They are [[parasite]]s of [[freshwater]] and [[ocean|marine]] [[mollusc]]s and [[vertebrate]]s ([[Chondrichthyes|cartilaginous]] and [[Actinopterygii|bony]] [[fish]]es and [[turtle]]s). Maturation may occur in the [[mollusc]] or [[vertebrate]] host. None of the species has any [[economic]] 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 [[Pulmonary alveolus|alveoli]] ("suckerlets") or a row of suckers and a [[Tegument (Helminth)|tegument]] with short protrusions, so-called "microtubercles". Aspidogastreans are an understudied class of parasitic flatworms that possess unique anterior attachment structures and are found exclusively in freshwater and marine environments, infecting a variety of hosts including fish, amphibians, and reptiles.

===Larval physiology===
[[Larva]]e of some species have [[cilia]]ted patches. Those of ''Multicotyle purvisi'' have four patches on the [[anterior]] side of the [[Posterior (anatomy)|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]], aspidogastreans use [[flame cell]]s as an [[excretion|excretory mechanism]]. The two excretory [[Gas bladder|bladder]]s are located [[Dorsum (biology)|dorsal]]ly, 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]] of extraordinary complexity, greater than that of related free-living forms, and a great number of [[sensory receptor]]s of many different types. The nervous system is of great complexity, consisting of a great number of longitudinal [[nerve]]s (connectives) connected by circular commissures. The [[brain]] (cerebral commissure) is located dorsally, in the anterior part of the body, the [[simple eye in invertebrates|eye]]s 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 microscope|Electron-microscopic]] studies revealed 13 types of receptors.<ref name="RohdeWatson1990a">{{cite journal |last=Rohde |first=K. |author2=Watson, N. A. |year=1990 |title=Non-ciliate sensory receptors of larval ''Multicotyle purvisi'' (Trematoda, Aspidogastrea) |journal=Parasitology Research |volume=76 |issue= 7|pages=585–590 |doi=10.1007/BF00932567 |s2cid=52835654 }}</ref><ref name="RohdeWatson1990b">{{cite journal |last=Rohde |first=K. |author2=Watson, N. A. |year=1990 |title=Uniciliate sensory receptors of larval ''Multicotyle purvisi'' (Trematoda, Aspidogastrea) |journal=Parasitology Research |volume=76 |issue=7 |pages=591–596 |doi=10.1007/BF00932568 |s2cid=42418699 }}</ref><ref name="RohdeWatson1990c">{{cite journal |last=Rohde |first=K. |author2=Watson, N. A. |year=1990 |title=Paired multiciliate receptor complexes in larval ''Multicotyle purvisi'' (Trematoda, Aspidogastrea) |journal=Parasitology Research |volume=76 |issue=7 |pages=597–601 |doi=10.1007/BF00932569 |s2cid=11307808 }}</ref>

==Life cycles==
Their [[biological life cycle|life cycle]] is much simpler than that of [[digenea]]n [[trematode]]s, including a [[mollusc]] and a facultative or compulsory [[vertebrate]] [[host (biology)|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]]'' infects many species of [[freshwater bivalve]]s belonging to several families, as well as [[freshwater snail]]s, many species of [[freshwater fish]]es of several families, and freshwater [[tortoise]]s.<ref name="Rohde1972">{{cite book|last=Rohde |first=K. |year=1972 |chapter=The Aspidogastrea, especially ''Multicotyle purvisi'' Dawes, 1941 |volume=10 |pages=77–151 |doi=10.1016/S0065-308X(08)60173-6 |title=Advances in Parasitology|editor-last=Dawes|editor-first=Ben|isbn=0-12-031710-9}}</ref>

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]]s live in the [[small intestine]] of the snubnosed dart, ''[[Trachinotus blochi]]'' ([[Teleostei]], [[Carangidae]]), on the [[Great Barrier Reef]]. They produce large numbers of [[egg (biology)|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 [[Insect migration|migrate]] to the [[digestion|digestive]] gland where they grow up to the preadult stage which has all the characteristics of the adult including a testis and ovary.<ref name="Rohde1973">{{cite journal |last=Rohde |first=K. |year=1973 |title=Structure and development of ''Lobatostoma manteri'' sp. nov. (Trematoda: Aspidogastrea) from the Great Barrier Reef, Australia |journal=Parasitology |volume=66 |issue=1 |pages=63–83 |doi= 10.1017/S0031182000044450|pmid=4786835 |s2cid=28751369 }}</ref>

==Evolutionary relationships==
Digenean trematodes have been [[culture]]d in various, complex, [[Growth medium|media]]. However, their parasitic stages die soon in water. Aspidogastreans may survive for many days or even weeks outside a host in simple physiological [[salinity|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]] could be kept alive for up to 13 days in dilute sea water in which they laid eggs containing larvae infective to snails.<ref name="Rohde1972" /> This has led to the suggestions that aspidogastreans are archaic trematodes, not yet well [[Adaptation (biology)|adapt]]ed 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.

[[Synapomorphy|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.<ref name="Rohde2001">{{cite book |chapter=The Aspidogastrea, an archaic group of Platyhelminthes |title=Interrelationships of the Platyhelminthes |last=Rohde |first=K. |editor=Littlewood, D. T. J. |editor2=Bray, R. A. |year=2001 |publisher=Taylor & Francis |location=New York |isbn=0-7484-0903-3 |pages=159–167 }}</ref>

This view is supported by the evolutionary relationships of the hosts which these two subclasses utilise. The hosts of aspidogastreans include [[Chondrichthyes|chondrichthyan fishes]] ([[shark]]s, [[Batoidea|rays]] and [[chimaera]]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 [[Chimaera (fish)|holocephalan]] fishes. It is characterised by a single row of rugae (transverse thickenings of the body surface), numerous [[testes]], and two [[cecum|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 [[elasmobranch]]s. It has a single ventral row of well separated suckers.
* The '''[[Multicalycidae]]''' include the single genus ''Multicalyx'' from the intestine of [[Holocephali|holocephalan]]s and elasmobranchs. It is characterised by a single ventral row of [[Pulmonary alveolus|alveoli]].
* The '''[[Aspidogastridae]]''' includes species infecting [[mollusc]]s, [[Teleostei|teleosts]] and [[turtle]]s. The ventral adhesive disc bears either three or four rows of [[Pulmonary alveolus|alveoli]]. 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.<ref name="Gibson1987">{{cite journal |last=Gibson |first=D. I. |year=1987 |title=Questions in digenean systematics and evolution |journal=Parasitology |volume=95 |issue= 2|pages=429–460 |pmid=3320890 |doi=10.1017/S0031182000057851 |s2cid=35886148 }}</ref> However, similarities between species of these two orders are so great that distinction at the level of orders does not seem justified.

==References==
{{Reflist|2}}

==External links==
* [http://tolweb.org/tree?group=Aspidogastrea&contgroup=Platyhelminthes Aspidogastrea from Tree of Life]
* [https://web.archive.org/web/20041012185338/http://www.erms.biol.soton.ac.uk/lists/brief/Aspidogastrea.shtml Checklist of European aspidogastreans]
* [https://web.archive.org/web/20071015033911/http://www.fishdisease.net/ Fishdisease.net]
*Aspidogastrea knol I [https://web.archive.org/web/20080927072657/http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/13]
*Aspidogastrea konl II [https://web.archive.org/web/20090209050220/http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/15]
*Aspidogastrea knol III [https://web.archive.org/web/20080927072659/http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/16]

{{Taxonbar|from=Q576111}}

[[Category:Aspidogastrea| ]]
[[Category:Protostome subclasses]]