Halacaridae

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Halacaridae
Agauopsis brevipalpus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Trombidiformes
Superfamily: Halacaroidea
Family: Halacaridae
Murray, 1877

Halacaridae is a family of meiobenthic

mites found in marine, brackish, and freshwater habitats around the world. It includes more than 1100 described species belonging to 64 genera [1][2] It is the largest marine radiation of arachnids.[3]

Description and life cycle

Halacarids have four pairs of legs (as adults and nymphs; see below), of which the first two pairs point forwards and the last two pairs point backwards. This is a

synapomorphy of the group. Another synapomorphy are four plates on the dorsal surface of the body, except for several genera with reduction of certain plates.[4]

The life cycle of halacarids consists of egg, larva, 1-3 nymphal stages (protonymph->deutonymph->tritonymph) and adult. Additionally, between each of the free-living stages (i.e. except for the egg) is a quiescent pupal stage.[5]

  • Eggs are usually deposited by an adult female in a substrate, with the help of an ovipositor.
  • Larvae have three pairs of legs, with each leg five-segmented, and lack a genital plate.
  • Protonymphs have four pairs of legs (as do all following stages), of which the fourth pair is five-segmented, and they usually have a distinct genital plate.
  • Deutonymphs and tritonymphs have each leg six-segmented.
  • Adults are often similar to the last nymphal stage, but they have an ovipositor (if female) or spermatopositor (if male).

Like mites in general, halacarids have a pair of palps. The palps usually have four segments each, but they are three-segmented in Simognathus and just two-segmented in Acaromantis.[3]

Halacarids of subfamily Copidognathinae have just a single nymphal stage. Additionally, the number of genital papillae is reduced to a single pair.[3]

The subfamily Rhombognathinae, which is algivorous, can be recognised by the dark green or almost black pigment inside their digestive system. This pigment is partially digested chlorophyll from algae.[3]

Ecology

Halacaridae occur in various habitats including sandy beaches, tidal sediment, interstitial spaces, hydrothermal vents, mangroves, salt marshes and on larger animals.[2][6] They spend their entire lives on a substrate such as attached algae or sand.[3]

Most species and genera are predators, though Rhombognathinae are instead algivores.[3] In freshwater halacarids, some species are restricted to crayfish gill chambers, implying a parasitic lifestyle, while Lobohalacarus weberi is a scavenger that feeds on dead nematodes and oligochaetes but not on live ones.[7]

Phylogeny

Recent analyses place Halacaridae as the sister group to Parasitengona.[8][9] Within the group, algivorous Rhombognathinae consists of two lineages (Rhombognathus+Isobactrus and Rhombognathides+Metarhombognathus), meaning the habit of algivory has evolved two independent times.[3]

Genera

References

  1. doi:10.3906/zoo-1803-6
    .
  2. ^
    doi:10.11646/zootaxa.1998.1.1
    .
  3. ^
    PMID 30172010
    .
  4. ISSN 1445-5226
    .
  5. PMID 25781126
    .
  6. S2CID 85063373
    .
  7. ISBN 978-0-12-370626-3
    , retrieved 2022-10-14
  8. PMID 26330076
    .
  9. PMID 27150348
    .

Further reading

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