Halacaridae

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]

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]