Eutardigrada from the central Pacific: ecdysis, morphology, taxonomy, and zoogeography

Abstract

Fourteen species of Eutardigrada were found on the islands of the central Pacific: Caroline Islands - Ponape, Truk, Yap, and Palau: Hawaiian Islands - Maui; Mariana Islands- Guam, and Marshall Islands - Eniwetok Atoll. They represented four genera: Hypsibius, Itaquascon, Macrobiotus, and Milnesium. The prime zoogeographic pattern was that of faunal dominance by two species, Macrobiotus harmsworthi and Milnesium tardigradum, that are highly adaptable and cosmopolitan. The other species that were present included: Hypsibius augusti, H. conjungens, H. convergens, H. truncatus, Itaquascon umbellinae, Macrobiotus allani, M. ambiguus, M, evelinae, M. hufelandii hufelandii, M. h. recens, M. hufelandioides, and M. richtersi. These were marked by limited distribution and paucity of numbers, in spite of their being species generally known from diverse localities throughout the world. Neither endemic nor new eutardigrade species were found. Aside from the lichens and mosses, an unusual habitat was sampled, the thalassosupralittoral fringe at Eniwetok Atoll. Three species were present there: Hypsibius augusti, H. truncatus, and Macrobiotus harmsworthi. The two species of Hypsibius are of interest in that they never were found together on the same islet, though M. harmsworthi was occasionally present with either species but was never abundant. Information regarding ecdysis was obtained for three species. The curve resulting from plotting the body length versus the 303 Individuals of H. truncatus indicated six ecdyses and seven instars, while that for M. harmsworthi indicated four ecdyses and five definite instars and a possible sixth; all ecdyses for each species were confirmed by the presence of specimens in the simplex state. The curve for H. tardigradum revealed that the Eniwetok population consisted of five Instars, but only two ecdyses were confirmed by appropriate specimens.