Scanning electron microscopic observation of Acarothrix grandocularis (Acari, Halacaridae) and notes on the species of the genus Acarothrix

Scanning electron microscopic observations of some characters of Acarothrix grandocularis Chatterjee, Marshall, Guru, Ingole, Pešić 2012 is presented. Distribution and characters of spe- cies belonging to the genus Acarothrix are also discussed.


Introduction
Acarothrix grandocularis Chatterjee, Marshall, Guru, Ingole and Pešić (2012) was first described from India and Brunei Darussalam among algal turf growing on pneumatophores of mangroves in estuarine habitats . Bartsch (2015) reported this species from Kranji, Singapore among green and red algae on trunks of mangrove trees. In the present paper some morphological characters of A. grandocularis is studied in detail, based on scanning electron microscopic observation of specimens from Goa, India. Distribution and characters of species belonging to the genus Acarothrix are also provided.    Rostrum triangular, tip of rostrum not surpassing the anterior end of P 2 . P 1 and P 3 without any setae; P 2 with 1 dorsal seta; P 4 with 3 long proximal setae and 1minute distal seta. Proto and deuto-rostral setae situated at the tip of rostrum, long maxillary setae of rostrum anterior to middle of rostrum, gnathosomal base with a pair of setae (3B, C). Rostral sulcus is extending posteriorly to just beyond the tritorostral setae ( Figure 3B, D).
Remarks. In India. A. granocularis is found from both pneumatophores turf and mud flat also. The salinity of mangrove area ranges from 3-7‰ . A detailed study based on SEM and molecular analysis for specimens from different localities is necessary to reveal variations in this species between the localities.

Notes on species of the genus Acarothrix
Acarothrix is a genus of halacarid mites that was proposed by Bartsch (1990) and has A. palustris Bartsch 1990 as the type species. There are five species viz. A. palustris Bartsch (1990), A. longiunguis Bartsch (1997), A. umgenica Procheş (2002), A. ampliata Bartsch (2004) and A. grandocularis  so far recorded under this genus.
The genus has been recorded from Southern China, Singapore, northern Australia, southeastern Africa, Florida and India. All species are known from tropical or warm temperate regions. Table 1 shows detail distribution of the species in Acarothrix along with the habitats and references.  Bartsch (1997) Acarothrix palustris Bartsch 1990 Hong  Bartsch (1990) Bartsch (2006) Bartsch (2006) Chatterjee et al (2013) Acarothrix umgenica Procheş 2002 Procheş (2002) All species live in intertidal muddy environments characterized by fluctuating salinities. Four species of Acarothrix are found to be associated with mangroves (Chatterjee, Pfingstl, Pešić, 2018a).
Sexual dimorphism is common in many groups of Arthropoda. Sexual dimorphism is also found in some species of halacarid mites . A pair of external genital acetabula is present on genital sclerites in males of the genus Acarothrix, while the external genital acetabula on genital sclerites are absent in the female.