The comparative osmoregulatory ability of two water beetle genera whose species span the fresh-hypersaline gradient in inland waters (Coleoptera: Dytiscidae, Hydrophilidae).

A better knowledge of the physiological basis of salinity tolerance is essential to understanding the ecology and evolutionary history of organisms that have colonized inland saline waters. Coleoptera are amongst the most diverse macroinvertebrates in inland waters, including saline habitats; however, the osmoregulatory strategies they employ to deal with osmotic stress remain unexplored. Survival and haemolymph osmotic concentration at different salinities were examined in adults of eight aquatic beetle species which inhabit different parts of the fresh-hypersaline gradient. Studied species belong to two unrelated genera which have invaded saline waters independently from freshwater ancestors; Nebrioporus (Dytiscidae) and Enochrus (Hydrophilidae). Their osmoregulatory strategy (osmoconformity or osmoregulation) was identified and osmotic capacity (the osmotic gradient between the animal's haemolymph and the external medium) was compared between species pairs co-habiting similar salinities in nature. We show that osmoregulatory capacity, rather than osmoconformity, has evolved independently in these different lineages. All species hyperegulated their haemolymph osmotic concentration in diluted waters; those living in fresh or low-salinity waters were unable to hyporegulate and survive in hyperosmotic media (> 340 mosmol kg(-1)). In contrast, the species which inhabit the hypo-hypersaline habitats were effective hyporegulators, maintaining their haemolymph osmolality within narrow limits (ca. 300 mosmol kg(-1)) across a wide range of external concentrations. The hypersaline species N. ceresyi and E. jesusarribasi tolerated conductivities up to 140 and 180 mS cm(-1), respectively, and maintained osmotic gradients over 3500 mosmol kg(-1), comparable to those of the most effective insect osmoregulators known to date. Syntopic species of both genera showed similar osmotic capacities and in general, osmotic responses correlated well with upper salinity levels occupied by individual species in nature. Therefore, osmoregulatory capacity may mediate habitat segregation amongst congeners across the salinity gradient

Cephalic anatomy and three-dimensional reconstruction of the head of Catops ventricosus (Weise, 1877) (Coleoptera: Leiodidae: Cholevinae)

Adult head structures are well known in the coleopteran suborders Archostemata and Adephaga, whereas the available information is very fragmentary in the megadiverse Polyphaga, including the successful superfamily Staphylinoidea. In the present study, the cephalic morphology of the cholevine species Catops ventricosus is described in detail and documented. The results were compared to conditions occurring in other polyphagan lineages, especially staphylinoid and scarabaeoid representatives. Specific external features documented in Catops and potential autapomorphies of Leiodidae include a five-segmented antennal club with a reduced eighth antennomere and the presence of periarticular grooves filled with sensilla on antennomeres 7, 9, and 10. The firm connection of the head and pronotum is possibly an apomorphy of Cholevinae. The monophyly of Cholevinae excluding Eucatopini and Oritocatopini is supported by the apical maxillary palpomere as long as or shorter than the subapical one, and the presence of cryptic pore plates on the surface of these palpomeres—a feature described and documented here for the first time. The internal cephalic structures of Catops are mostly plesiomorphic, as for instance the complete tentorium. The pattern of the muscles is similar to what is found in other staphylinoid taxa. The unusual maxillary muscle “Mx” is likely a groundplan apomorphy of the clade Staphyliniformia + Scarabaeoidea. M. hypopharyngomandibularis (M13) was identified in Catops and is ancestral for Coleoptera, even though it is often missing. The same applies to M. tentoriohypopharyngalis (M42).The PhD study of CAC as well as his research internship at the Friedrich-Schiller-Universität was funded by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (2012/19002-0 and 2014/22088-0). PG was also supported by FAPESP (2013/06314-7) and MY by the DAAD, which is also gratefully acknowledged. The stay of IR in the Phyletisches Museum in Jena was funded by a Salvador de Madariaga grant (PRX14/00583).Peer reviewe