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
It is the largest marine radiation of arachnids.
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
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...
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
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.
...
, 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.
* 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
Pedipalps (commonly shortened to palps or palpi) are the second pair of appendages of chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") a ...
. 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
genital papillae is reduced to a single pair.
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.
Ecology
Halacaridae occur in various habitats including sandy beaches, tidal sediment, interstitial spaces,
hydrothermal vents,
mangroves
A mangrove is a shrub or tree that grows in coastal saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evolution in severa ...
,
salt marshes and on larger animals.
They spend their entire lives on a substrate such as attached
algae or sand.
Most species and genera are
predators
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
, though Rhombognathinae are instead algivores.
In freshwater halacarids, some species are restricted to
crayfish gill chambers, implying a
parasitic
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 ha ...
lifestyle, while ''Lobohalacarus weberi'' is a
scavenger that feeds on dead
nematodes
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
and
oligochaetes
Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworms ...
but not on live ones.
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
References
Further reading
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{{Taxonbar, from=Q1104692
Trombidiformes
Acari families