Rotifers inhabiting shells of testate amoebae
De Smet W. H.
Laboratory of Polar Biology, Limnology and Palaeobiology, University of Antwerp
The Phylum Rotifera are a group of microscopic pseudocoelomate metazoans. They have a widespread distribution in water's and limno-terrestrial habitats of all kinds. Many of them are known to live in close association, whether synoecious, commensal or parasitic, with a large and diverse number of animal taxa, including testate amoebae of which both empty shells and shells of live amoebae have been reported to be inhabited.
The first report on rotifers dwelling in empty tests was by Gosse (1886) who created the rotifer genus Eretmia and described two species, which judging from his description were bdelloid rotifers inhabiting tests of Euglypha acanthophora (Ehrenberg ) var. flexuosa Penard. Empty tests of Nebela, Dffflugia and other genera are known to be inhabited by the bdelloids Habrotrocha annulata (Murf) and Hangusticolfis (Murr). Habrotrocha angusticolfis is a case builder, and secretes its own case even when occupying amoebal tests, whereas Hannulata is unable to secrete a case for itself. Habrotrocha incola Bartos has so far only been reported from empty tests of Centropyxis aerophila Deflandre.
Records on rotifers inhabiting tests of live amoebae are scarce. Habrotrocha angusticollis has been observed in shells of live Difflugia, Nebela and others as well. A single monogonont, Dicranophorus diffluglarum Penard, was described as parasitizing Difflugia acuminata Ehrbg.. Two new, apparently parasitic species of monogononts are reported here. A new family, Asciaporrectidae, and genus, Asciaporrecta, was created to accomodate the new species A. arceflicola sp. nov. and A. diffflugicola sp. nov., and A. hyalina (Wulfert) comb. nov. (Zootaxa, in press). The presumed parasite-host relationship seems quite specific, despite the diverse and numerous co-occurring testate amoebae. This suggests that both rotifers find and select their appropriate host by chemotaxis and/or size and shape of the amoebal test. Some testate amoebae have been recognized as predators of rotifers or as feeding on immobile senescent and dead animals (Gilbert et al 2000, 2003). In the present study Lesquereusia spiralis (Ehrbg ) and Difflugia labiosa were found feeding on Mytilma and on Testudinella and bdelloids respectively. It thus is puzzling why Asciaporrecta spp. nov. and their eggs are not consumed. Are foreign bodies inside the shell not recognized as such by the amoebae, or have rotifers and eggs become unattractive or inedible by some chemical adaptation?
Sexual reproduction in monogonont rotifers is induced by endogenous clues or specific environmental factors, e.g. periodic drying, which results in an encysted diapausing embryo, called resting egg, that can withstand harsh conditions. Depending on the species, several resting eggs can be produced or the resting egg is retained within the female, which dies after its production. lt thus is possible to imagine, that the shell-dwelling behavior of parasitic monogonont rotifers eventually began, with undigested resting eggs remaining inside the shell of testate amoebae behaving as predators or scavengers The evolution of the amoeba shells, and in particular the region of the pseudostome as well as the development of and internal diaphragm, have been explained as adaptive processes related to the availability of water and protection against predators. In some cases it also may be an adaption avoiding penetration into the test by parasitic rotifers.
(Text copied with permission of Prof. Dr. Willem De Smet, from a poster on the International Symposium on Testate Amoebae, Antwerp 2006.)