To Be or Not to Be a Flatworm: The Acoel Controversy

Since first described, acoels were considered members of the flatworms (Platyhelminthes). However, no clear synapomorphies among the three large flatworm taxa - the Catenulida, the Acoelomorpha and the Rhabditophora - have been characterized to date. Molecular phylogenies, on the other hand, commonly positioned acoels separate from other flatworms. Accordingly, our own multi-locus phylogenetic analysis using 43 genes and 23 animal species places the acoel flatworm Isodiametra pulchra at the base of all Bilateria, distant from other flatworms. By contrast, novel data on the distribution and proliferation of stem cells and the specific mode of epidermal replacement constitute a strong synapomorphy for the Acoela plus the major group of flatworms, the Rhabditophora. The expression of a piwi-like gene not only in gonadal, but also in adult somatic stem cells is another unique feature among bilaterians. These two independent stem-cell-related characters put the Acoela into the Platyhelminthes-Lophotrochozoa clade and account for the most parsimonious evolutionary explanation of epidermal cell renewal in the Bilateria. Most available multigene analyses produce conflicting results regarding the position of the acoels in the tree of life. Given these phylogenomic conflicts and the contradiction of developmental and morphological data with phylogenomic results, the monophyly of the phylum Platyhelminthes and the position of the Acoela remain unresolved. By these data, both the inclusion of Acoela within Platyhelminthes, and their separation from flatworms as basal bilaterians are well-supported alternatives.     

How To Be a Predator

This paper discusses what would appear to be the optimal strategyof a predator given two alternative objectives: (1) maximumyield commensurate with the long-term maintenance of the preyspecies, and (2) extinction of the prey species. The problemis examined in terms of past research on Daphnia and Hydra,and takes into account estimates of ecological efficiency, populationefficiency, and growth efficiency based on many laboratory experiments.It is shown that there may be distinct differences in the effectsof removing different kinds of individuals from a prey population.The optimal system for a predator wishing to insure the continuedavailability of its prey is to take animals which are aboutto die anyway, i.e., to alter the natural pattern of mortalityas little as possible. It is concluded, with some reservations,that predators in nature generally act in this manner. Conversely,if one's intent is to exterminate a prey species (e.g., an objectionablepest), the animals removed should be those which natural predatorsdo not select.