Basal Metazoan Cluster (Coordinators: Ralf Schaible and Daniel Levitis)
Research in this cluster has focused on whether death rates for basal metazoans—hydra, sponges, jellyfish and some kindred species at the root of the animal tree of life—increase with age or remain more or less constant. The main contribution of the group is a large study showing that age-specific mortality does not increase with age for asexually-reproducing hydra under protected laboratory conditions and age-specific fertility does not decline: see II.C. This painstaking research demonstrates that senescence is not inevitable for all species starting at reproductive maturity, a canonical belief in biology since Hamilton (1966).
A study in Rostock by Ph.D. student Felix Ringelhan focused on Eleutheria dichotoma, a marine hydrozoan with a stationary polyp stage (something like hydra or sea anemones) and a free-ranging medusa stage (roughly like the jellyfish that bother bathers). There was suggestive evidence that the polyps do not senesce but medusae suffer increasing deterioration with age. A somewhat parallel study in Denmark by Ph.D. student Josephine Goldstein, investigated the mortality of the jellyfish Aurelia aurita, which has a settled, benthic polyp stage in which reproduction is asexual as well as the more familiar pelagic medusa stage in which reproduction is sexual. The evidence suggests that polyps may not senesce whereas medusae deteriorate with age in the wild, perhaps because of food limitations, but not under protected laboratory conditions.
An ambitious project, which was cut short when the key investigator, Daniel Levitis, left MaxO (now DEM), established a sponge aquaculture in Kerteminde, Denmark, with several hundred breadcrumb sponges (Halichondria panicea), to determine age- and size-specific growth and vital rates (mortality, survival and reproduction) of sponge cohorts under natural conditions using monthly SCUBA surveys. The preliminary data over a three-year period before the study was halted suggests that breadcrumb sponges do not senesce with age. A review of other studies by a doctoral student, Lars Kumala, and various experiments carried out by Paul Dunn are also consistent with the hypothesis that mortality does not systematically increase with age and fertility does not systematically decrease with age for sponges.
A smaller, side project investigated the longest-lived non-colonial species, as far as is known, Arctica islandica, the quahog clam, in waters off Iceland and in the Baltic. Annual death rates were lower in the colder waters off Iceland, but in both cases evidence suggests that death rates do not increase with age and possibly decrease as clams grow in size.
The researchers associated with this Cluster attempted to develop a deeper understanding of the biological processes that enabled hydra to avoid senescence and the other species to probably avoid senescence under some circumstances. They carefully considered regulatory and cellular processes involved in the maintenance of constant regeneration capability and stress resistance throughout life. For hydra and, to a lesser extent, the other species, they conducted experiments aimed to identify ways species prevent damage and mutation accumulation to achieve non-senescence. In this line, theoretical models were developed to explore how species can control their damage accumulation. The Cluster researchers started to explore how hydra, sponges and jellyfish fared in the wild vs. in the laboratory. For sponges they experimentally investigate endogenously- and environmentally-driven dynamics of sponge filter-feeding and respiration. For the jellyfish they modeled ecological and demographic drivers of jellyfish blooms. These drivers include the survival and fertility of benthic (asexually reproducing) polyps and pelagic (sexually reproducing) medusae as a function of environmental factors such as food availability, climate change, hydrodynamic processes and interspecific competition.