Halacaridae is a family of meiobenthic
mites
Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...
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 Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...
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 (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
, 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
The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...
.
* 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 secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicera ...
. 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 papilla The genital papilla is an anatomical feature of the external genitalia of some animals. In mammals
In mammals, the genital papilla is a part of the vulva not present in humans, which appears as a small, fleshy flab of tissue. The papilla covers the ...
e 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
Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
from algae.
Ecology
Halacaridae occur in various habitats including sandy beaches, tidal sediment, interstitial spaces,
hydrothermal vent
Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hot ...
s,
mangrove
A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...
s,
salt marsh
A salt marsh, saltmarsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. I ...
es and on larger animals.
They spend their entire lives on a substrate such as attached
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
or sand.
Most species and genera are
predators
Predation is a biological interaction in which 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 ...
, though Rhombognathinae are instead algivores.
In freshwater halacarids, some species are restricted to
crayfish
Crayfish are freshwater crustaceans belonging to the infraorder Astacidea, which also contains lobsters. Taxonomically, they are members of the superfamilies Astacoidea and Parastacoidea. They breathe through feather-like gills. Some spe ...
gill chambers, implying a
parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...
lifestyle, while ''Lobohalacarus weberi'' is a
scavenger
Scavengers are animals that consume Corpse decomposition, dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a he ...
that feeds on dead
nematode
The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
s and
oligochaetes
Oligochaeta () is a subclass of soft-bodied 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 megadril ...
but not on live ones.
Phylogeny
Recent analyses place Halacaridae as the
sister group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and ...
to
Parasitengona
Parasitengona is a group of mites, variously ranked as a hyporder or a cohort, between the taxonomic ranks of order (biology), order and family (biology), family.
They are divided into the aquatic Hydrachnidia (water mites) and the terrestrial ...
.
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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{{Authority control
Trombidiformes
Acari families