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
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.
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.
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Eggs are usually deposited by an adult female in a substrate, with the help of an ovipositor.
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Larvae have three pairs of legs, with each leg five-segmented, and lack a genital plate.
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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.
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Deutonymphs and tritonymphs have each leg six-segmented.
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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 Pedipalp. The palps usually have four segments each, but they are three-segmented in Simognathus and just two-segmented in Acaromantis.
Halacarids of subfamily Copidognathinae have just a single nymphal stage. Additionally, the number of is reduced to a single pair.
The subfamily Rhombognathinae, which is Algae eater, can be recognised by the dark green or almost black pigment inside their digestive system. This pigment is partially digested chlorophyll from algae.
Ecology
Halacaridae occur in various habitats including sandy beaches, tidal sediment, interstitial spaces, hydrothermal vents,
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and on larger animals.
They spend their entire lives on a substrate such as attached
algae or sand.
Most species and genera are Predation, though Rhombognathinae are instead algivores.
Phylogeny
Recent analyses place Halacaridae as the
sister group to
Parasitengona.
Within the group, algivorous Rhombognathinae consists of two lineages (
Rhombognathus+
Isobactrus and
Rhombognathides+
Metarhombognathus), meaning the habit of algivory has evolved two independent times.
Genera
Further reading
