The Ecology of Sponges at Palmyra Atoll: Variability, Introduced Species and their Potential Direct and Indirect Impacts

Abstract

Anthropogenic pressures, direct and indirect, have left no coral reef untouched. Those that remain in a near-pristine condition are remote islands and atolls removed from the majority of direct impacts, but even these are still subjected to the pressures of global climate change to which they are demonstrating a higher resilience than those which are already severely compromised. These near-pristine systems should be protected, managed and studied to better understand how they function and hopefully ensure the future of coral reefs. Unfortunately a number of the remote atolls and islands in the Pacific were modified or used by the US military during WWII, which altered these systems in unknown ways and threatened their surrounding reefs. Palmyra Atoll in the Central Pacific for example had its lagoons dredged and blocked to create more landmass for building, along with the creation of a channel through the reef to allow boat access into the lagoons making them susceptible to introduced species. Fortunately the surrounding reefs at Palmyra are still in a near-pristine condition with high densities of scleractinian corals, however the lagoon fauna is now predominantly sponges. Sponges in high enough densities can have considerable impacts on semi-enclosed bodies of water through their high filtering capabilities and could even threaten native species through competition if they were to extend onto non-lagoon reefs. Therefore, the broad aim of this thesis was to understand the ecology of the sponges in the lagoons at Palmyra and determine their potential impacts on the atoll directly and indirectly. To answer these questions I collected sponge assemblage data across the lagoons at both shallow and deep depths and sampled those species found on the reefs, which were surprisingly entirely different from the lagoon species. I then modelled a suite of environmental predictors to ascertain whether environmental conditions might be maintaining the sponges in the lagoon. To further examine whether the sponges were capable of extending onto the reef I also looked at larval recruitment patterns and assessed the temporal stability with semi-permanent quadrats. The initial surveys revealed the presence of at least two introduced species: Haliclona caerulea (Hechtel 1965) and Gelliodes fibrosa (Wilson 1925). Molecular tools were then employed to confirm the identification and attempt to ascertain the introduction pathway of H. caerulea. Finally, to assess the potential impact of the sponges on the water column I calculated the filtration rates of all the morphologies in the lagoon and extrapolated to the time required to clear all the available water in the lagoon as well as the removal rates of dissolved organic carbon and oxygen.

Overall the lagoons appear to have undergone a phase-shift from a coral to sponge dominated system. However, the direct threat of the sponges extending onto the adjacent near-pristine reef currently seems negligible as they appear to be relatively “stable” (sponge mortality and recruitment are in equilibrium) and maintained in the lagoons by the environmental conditions, despite larval production. Sponge diversity changed over depths but the total number of species was consistent with other atoll systems despite the military modifications, with the most prominent sponge being a Hawaiian endemic species: Iotrochota protea (de Laubenfels 1950). The introduction of H. caerulea, a Caribbean sponge is thought to have occurred to Palmyra via Hawaiʻi; however, the molecular data also revealed further cryptic speciation at both the species and order levels, suggesting greater species diversity at Palmyra than previously believed. Finally, the indirect impacts of the sponges on the water column also appear to be small and with limited future risks to the reef organisms, as recruitment and mortality are currently in equilibrium and therefore unlikely to increase dramatically in percentage cover. Globally sponges can play important functional roles in semi-enclosed bodies of water and in summary, despite the sponges being the most dominant fauna on the hard substrate in the lagoons, they appear to pose little threat to the atoll and the adjacent reefs either directly or indirectly despite the confirmed introduction of non-endemic species. However, it must be noted that the time since the modification (70 years) is not that long in an evolutionary sense, so the sponges may still have the potential to extend onto the reef, particularly if there are any dramatic changes to the environmental conditions on the outer reefs. Therefore, to detect whether the sponges are extending onto the reef I propose, as a future management tool, the use of I. protea as an indicator species. The use of multidisciplinary approaches to answer important ecological questions with respect to the potential for sponges to have negative impacts on the nonlagoon reefs proved to be essential in understanding whether the modifications to the lagoons and the subsequent dominance of sponges could be threatening one of the last few remaining near-pristine reef systems in the world.