Limnohalacarus, Walter, 1917

Bartsch, Ilse, 2018, Freshwater halacarid mites (Acari: Halacaridae) from Madagascar - new records, keys and notes on distribution and biology, Bonn zoological Bulletin 67 (2), pp. 79-99 : 96-97

Limnohalacarus : Characters not or rarely observed in other halacarid genera

Species of the genus Limnohalacarus demonstrate two characters which are either restricted to this genus or only rarely found in other halacarid genera. Unique is that the eggs are fixed to the basal part of the tibiae IV ( Viets 1940; Ramazotti & Nocentini 1960; Petrova 1966; Bartsch 1999) but the oviposition has not been observed. Up to 9 eggs, or their remnants, can be found per leg ( Bartsch 1999). The embryos in the eggs are often in different states of development. In a population of L. wackeri from northern Italy, the development of the eggs took about one month ( Ramazotti & Nocentini 1960). In general, halacarids deposit their eggs via the ovipositor into a substratum ( Teschner 1963; Kirchner 1969; Bartsch 1972), either singly or in clusters. When at rest, the ovipositor is retracted and in most halacarid females it extends internally slightly or distinctly beyond the anterior margin of the genital foramen. The outline of the ovipositor and its genital spines are at least partly visible through the genital plate. The extruded ovipositor ends with genital spines. In some genera these spines are more or less spiniform, in others palmate, either faintly or distinctly sclerotized (cf. Bartsch 2015). In contrast to the majority of halacarid species, the ovipositor of Limnohalacarus novus is short and narrow (LxW 74 x 10 µm), the 1–2 µm-long genital spines at the end of the ovipositor are very faintly sclerotized, they arise from minute papillae.

Another detail often found in Limnohalacarus species is the combination of an ovoid, somewhat flattened body with accumulated excretory material, and small anal sclerites. Such an excretory body is already present in larvae but is most conspicuous in adults ( Walter 1914; Petrova 1966; Ramazotti & Nocentini 1960; Pepato & Dos Santos 2015). In most of the other halacarid species, in larvae, nymphs and adults, the waste products are concentrated within a dorsomedian bar (cf. Soldanellonyx monardi — Fig. 10G ; Isobatrus uniscutatus ( Viets, 1939) — Bartsch 2014b: fig. 1). The length of the bar, if present, can correspond to about half or almost twothirds of the idiosomal length, it is filled with white or light brown granulated material. The ‘light mesial line’, mentioned by Johnston (1836) in the description of Thalassarachna basteri ( Johnston, 1836) , and the ‘dark line’ in Lohmannella falcata ( Hodge, 1863) , described by Hodge (1863), may represent such bars. Halacarids have the excretory organ dorsal to the gut ( Thomae 1926), defecation is through the anus which is guarded by the anal sclerites. Anatomical details in Thalassarachna basteri have been studied by Thomae (1926).

The presence of ovoid excretory bodies, in Limnohalacarus novus distinctly stratified, is not restricted to Limnohalacarus species but documented also from oth- er halacarid genera. Examples are Halacarus excellens Lohmann, 1907 and Rhombognathus amplus Bartsch, 2013 ( Bartsch 2010, 2013c) as well as other prostigmatid mite families, e.g. the Cunaxidae ( Kielczewski & Wisniewski 1977) . In halacarid species with such an excretory body the anal sclerites are unusual small, just as in L. novus. More species with small anal sclerites are Bathyhalacarus anomalous Bartsch, 2005 , Rhombognathus bulbosus Bartsch, 2005 , R. cyrtonotus Bartsch, 2000 , R. delicatulus Bartsch, 2000 ( Bartsch 2000 c, 2005a, b) but in individuals of these species no ovoid bodies with excretory material were found. However, the number of mites studied is very small. At present it seems that halacarid species with a large ovoid excretory body (20–50 % of idiosomal length) have small anal sclerites, though not all mites with small anal sclerites have such an ovoid body.

Little is known about feeding, almost nothing about digestion and defecation in halacarid mites. Most halacarids, marine as well as freshwater species, are expected to be carnivorous. Limnohalacarus wackeri , for example, could be reared with a diet of small ciliates, Lobohalacarus weberi with pieces of oligochaetes ( Teschner 1963; Ramazotti & Nocentini 1960). Larvae are known to show an excretory bar or ovoid body soon after commencement of feeding ( Kirchner 1969; Rama- zotti & Nocentini 1960). Rate, periods and frequency of defecation are not known. A general idea may give the studies by Bowman (2017a, b) who examined the feeding of the mesostigmatid Pergamasus longicornis ( Berlese, 1906) , a carnivorous species as Limnohalacarus , but terrestrial and with a length of almost 1300 µm distinctly larger than Limnohalacarus . In P. longicornis up to three weeks may be needed to clear the idiosoma of excretory material ( Bowman 2017a, b).

Among halacarids most species have excretory bars, only a small number has ovoid bodies with concentrically arranged layers (e.g. Rhombognathus amplus ). Among the few specimens studied of the latter species, collected in Singapore mangroves, each following instar had an excretory body almost similar or larger in size than that of the preceding stage. The length of the idiosoma (in parentheses) and LxW data of the excretory body are in protonymphs (209–235 µm) about 37 x 25 µm, in deutonymphs (242–284 µm) 37–75 x 25–30 µm, in tritonymphs (245–354 µm) 127 x 30 µm, and in adults (364–415 µm) 110–182 x 62–77 µm. For critical examination of taxonomic details, halacarid mites have to be cleared, i.e., the body content has to be removed. In R. amplus the stiff excretory body could only with difficulty be pressed through the camerostome, though the size of the latter (LxW about 70 x 100 µm) is much larger than that of the anal slit (length about 20 µm). How can such large excreta pass through the small anal opening? Do the mites accumulate excretory material during their lives, from the larval to the adult stage? This seems to be unlikely; the amount of excreta produced in the course of the mite’s life is expected to be much larger than that enclosed in the ovoid body. Most halacarids studied have a life span of about 15 months, Limnohalacarus up to 24 months ( Ramazotti & Nocentini 1960; Bartsch 1972, 1987), the major part takes the adult stage. About one quarter of the Limnohalacarus novus females studied had no excretory bodies. This quarter included stages from recently hatched to cocoon-carrying females, and accordingly we can expect that the small anal sclerites do not prevent defecation. Since quiescent or moulting stages were not represented in the samples, we do not know if at least partly the mites can defecate during or immediately after moulting, as long as the integument is not hardened yet.

The two just mentioned species differ in their life style which in turn will influence digestion and defecation. Rhombognathus amplus is phytophagous and restricted to the upper tidal zone, several hours a day emerged. The delicate algal film inhabited quickly desiccates. The mites studied were collected during low tide, consequently they were dehydrated and that certainly had an influence on the compactness of the waste products and the defecation. In contrast, Limnohalacarus species are carnivorous and live in an at least constantly waterlogged habitat. Feeding and metabolic activities are not hampered by desiccation and hence the wastes may be elastic enough to be pressed through the small and narrow anal slit.

Acknowledgements. The halcarids have been collected by Reinhard Gerecke (Tübingen) and Tom Goldschmidt (München) during a visit to Madagascar, meant to study the aquatic water mite fauna in river sediments. This field trip was financed by Dave Cook (Paradise Valley, USA). Reinhard Gerecke forward- ed the mites to me and Almir Rogério Pepato (Belo Horizonte, Brasil) informed me about unpublished records of halacarids from Brazil. To all my sincerest thanks.

Figures

Fig. 10A–G. Soldanellonyx monardi Walter, 1919. A. idiosoma, dorsal, female; B. idiosoma, ventral, female; C. part of gnathosomal base with P-1, P-2 and P-3, female; D. gnathosoma, ventral, female; E. gnathosoma, lateral, female; F.leg I, ventromedial, female; G. idiosoma, dorsal, protonymph. (Scale = 50 µm) (aps, apical seta on second palpal segment; ascl, anal sclerite; bs, basal seta on second palpal segment; ep, epimeral pore; exb, excretory bar; eyp, eye pigment; mxs-1, basal maxillary seta; P-1, P-2, P-3, first, second and third palpal segment).