| Halacaridae | |
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| Agauopsis brevipalpus | |
Scientific classification  | |
| 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]
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]
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]
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]
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]
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