Overview

Brief Summary

Introduction

Water mites, also known asthe Hydracarina, Hydrachnidia or Hydrachnellae, are a group of ninesuperfamilies and more than 40 families from the prostigmatan CohortParasitengona. Although other taxa of parasitengone mites have specieswith semi-aquatic habits (e.g. the Johnstonianidae), only the Hydracarinaare truly subaquatic. There are approximately 5000 named species of watermites (Smith & Cook 1991); however, as water mites of Africa, Asia, andmuch of South America have been poorly studied, this is likely a greatunderestimate of their true number. They live in all types of freshwaterbodies, from tree holes to hot springs to deep lakes to torrentialwaterfalls, and some have also invaded oceans (Cook 1974).

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Comprehensive Description

Life history and ecology

Water mites,like other parasitengones, have an extremely complex life cycle (Smith &Cook 1991). Eggs are laid under water either on the surface of substratessuch as stones or vegetation, or inside living plant tissue (Hydrachnidae:Hydrachna) or in the tissues of freshwater sponges andmussels (Unionicolidae: Unionicola; Pionidae:Najadicola). In the less derived groups, the larvae swimor crawl to the water's surface after hatching, and break through thesurface film. They then run about on the upper surface of the water or onemergent aquatic plants, seeking insect hosts to parasitize. In morederived taxa, the larvae remain under water where they search forpre-imaginal larvae (e.g. of odonates) or pupae (e.g. of dipterans).Larvae of the 'aerial' group typically begin to parasitize a hostimmediately upon encounter; in contrast, 'aquatic' larvae merely attendthe pre-adult host, and only begin parasitism after the adult has emergedfrom the last larval or pupal skin.

Larvae engorge on host haemolymphand dissolved tissues for a period lasting from a few days to severalmonths, depending on the taxon. Engorged larvae drop from the host backinto the water. Because hosts of water mite larvae are almost invariablywinged adult insects (exceptions are in the Hemiptera, where nymphal hostsmay be parasitized), mites are often carried far from their natal waterbodies. Thus parasitism serves not only a nutritional function forhydracarines, but it is the major means of dispersal and colonization ofnew habitats. Although parasitism is clearly a very important feature ofwater mite life cycles, the parasitic phase has been lost repeatedly indifferent lineages of mites (Smith 1998). Non-parasitic species appear tobe evolutionary 'dead ends', as there are no clades of non-parasitictaxa, merely isolated species in genera of otherwise parasitic species.

Once back in the water, the engorged larva transforms into acalyptostatic protonymph (also called a nymphochrysalis) that is apupa-like stage inside the integument of the larva. Within theprotonymphal cuticle, the next stage - the deutonymph - develops. Thedeutonymph ecloses as an active swimming or crawling predator of all stagesof aquatic insects, crustaceans, and other mites (Proctor & Pritchard1989). Deutonymphs are usually less highly sclerotized than adults, whichallows their integument to expand as they feed. A well-fattened deutonymphtransforms into a calpytostatic tritonymph (also called a teleiochrysalis).The adult that emerges is also predatory and typically feeds on preysimilar to that chosen by the deutonymph. Adults are more highlysclerotized than deutonymphs, and have functional genital openings. Theyare also often extremely sexually dimorphic, particularly those taxa inwhich the males transfer sperm directly to the female. Copulation hasapparently evolved repeatedly within the water mites from an ancestralstate in which sperm was transferred via spermatophores deposited on asubstrate (Proctor 1991). The full range of sperm-transfer modes(copulation, paired but indirect, completely dissociated) occurs within thewater mites, making their mating behaviour among the most diverse of anygroup of arthropods (Proctor 1992).

One of the most striking things aboutwater mites is their often brilliant colouration. Scarlet, orange, red andyellow mites are common, and are extremely visible against a background ofgreen water plants. These bright mites are often very unpalatable to fish(Kerfoot 1982), presumably due to noxious secretions from their skinglands. Although one might expect this aposematic connection betweenredness and distastefulness to have evolved as an anti-fish-predationmechanism, both the bright colour and foul taste are also present interrestrial parasitengones (Garga 1996).

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Characteristics

Members of theHydracarina are most obviously separated from other taxa in the CohortParasitengona by having nymphal and adult stages that live beneaththe surface of the water. Morphological characteristics thatdifferentiate the group are less obvious. The combination of larval charactersunique to the Hydracarina is that larvae lack the supracoxal seta 'e',the companion seta associated with phi and omega on leg I, and thecompanion seta associated with the dorsal eupathidium on tarsus I. Aswell, the palpal genu has two setae compared to the one seta found inlarvae of other Parasitengona (Welbourn 1991). Witte (1991) lists a numberof unifying postlarval characters, including: presence of glandularia (acombination of seta and gland), reduction of acrosomal filament of thesperm cell, and modifications of the posterior arms of the ejaculatorycomplex (the complex group of internal sclerites used by the male to formthe spermatophore). However, as Harvey (1998) points out, it isn't clearwhether Witte intended the Stygothrombidioidea to be included in theHydracarina.

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Ecology

Associations

Known prey organisms

Hydracarina (Hydracarina, Odonata, Ranatra, Belostoma) preys on:
Daphnia pulex

Based on studies in:
USA: Michigan (Lake or pond)

This list may not be complete but is based on published studies.
  • H. M. Wilbur, Competition, predation, and the structure of the Ambystoma-Rana sylvatica community, Ecology 53:3-21, from p. 14 (1972).
Creative Commons Attribution 3.0 (CC BY 3.0)

© SPIRE project

Source: SPIRE

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Known predators

Hydracarina (Hydracarina, Odonata, Ranatra, Belostoma) is prey of:
Dytiscus

Based on studies in:
USA: Michigan (Lake or pond)

This list may not be complete but is based on published studies.
  • H. M. Wilbur, Competition, predation, and the structure of the Ambystoma-Rana sylvatica community, Ecology 53:3-21, from p. 14 (1972).
Creative Commons Attribution 3.0 (CC BY 3.0)

© SPIRE project

Source: SPIRE

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Evolution and Systematics

Evolution

Classification

Some consider Hydrachnidia to be a suborder, or cohort division of the Acari; others consider it to be only a term of convenience
Creative Commons Attribution 3.0 (CC BY 3.0)

© WoRMS for SMEBD

Source: World Register of Marine Species

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Discussion of Phylogenetic Relationships

View Hydracarina Tree

tree based on Harvey (unpublished)

Based on current distributions of major taxa, water mites appear tohave had a Pangaean origin, or to have evolved prior to the continentalconsolidation that resulted in Pangaea. The group was originallyconsidered to be a polyphyletic assemblage of taxa that had independentlyinvaded water (discussed in Cook 1974). However, a number of setalcharacters of the larvae and genital/spermatozoal characters of the adultsappear to unite these superfamilies as a monophyletic group (Welbourn 1991,Witte 1991, Harvey 1998).

There is general agreement that the Stygothrombioidea, Hydrovolzioidea, andfamilies in the Hydryphantoidea are at the primitive end of the cladogram;however, the hydryphantoid families have no clear synapophorphies and thesuperfamily may be paraphyletic (Smith & Cook 1991). There is also generalacceptance of the Hygrobatoidea as being among the most derived of thesuperfamilies. Relative positions of the other groups are not soclear. Phylogenetic studies of the water mites are hindered by the ancientorigin of the group (thus removing biogeography as a tool at thesuperfamilial level), rarity offossils, and the apparent high degree of convergence in adult morphology.Larval morphology appears to be more conservative, and has been extremelyvaluable in understanding relationships among other members of theParasitengona (e.g. Welbourn 1991). To date, no DNA-based phylogeny hasbeen published, although several are in progress.

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