Award: OCE-1214303

Coral reefs worldwide are continuing to decline at an alarming rate. Emerging diseases of marine invertebrates have been implicated as one cause of the accelerating destruction of Caribbean coral reef systems. Diseases affecting natural populations threaten biodiversity, resilience and the ecological balance of communities, impacting coastal security, food and recreational opportunities. To date, most studies of diseases on reefs have focused on corals; however, reports of sponge diseases are also on the rise worldwide. On Caribbean reefs, sponges have become an increasingly dominant component of the reef community, particularly with the loss of corals, and thus play important roles in the ecology of these ecosystems. As a result, diseases affecting sponges have the potential to further imbalance already stressed reef ecosystems. The increasing prevalence and diversity of diseases affecting marine systems highlights the need to develop new tools to test hypotheses related to disease spread and impacts on populations and communities. With this proposal, we utilized available data, along with the addition of new observations and measurements, to develop a disease transmission model for a Caribbean sponge disease. The most well described disease affecting Caribbean sponges is Aplysina Red Band Syndrome (ARBS; Fig. 1), which particularly affects Aplysina cauliformis, the purple rope sponge, one of the most common sponge species on Caribbean reefs. ARBS was first documented in the Bahamas in 2004 and has since been observed on reefs throughout the broader Caribbean basin. Unlike most diseases of marine sponges, and even corals, studied to date, ARBS can be transmitted by direct contact, providing a tenable model system in which to assess factors that contribute to pathogenesis. For example, in transmission experiments, we found that as ARBS develops, it causes systemic changes to the host spongeÆs biochemistry and physiology and to its microbiome. Ultimately, ARBS reduces sponge growth, and causes tissue necrosis and breakage at the site of the lesion, particularly during storm events. Following Hurricane Irene, a major storm in the Bahamas, overall A. cauliformis biomass was reduced, as was disease prevalence, over the short term. However, as sponges grew large enough to come into contact, disease prevalence increased again. As part of this project, we attempted to identify the pathogen responsible for causing ARBS. The red band associated with ARBS is dominated by the filamentous cyanobacterium, Leptolyngbya sp., but cultures of this organism alone and in combination with numerous other bacteria collected from ARBS lesions (Fig. 2A) failed to cause disease in healthy sponges. In contrast, healthy sponges placed in contact with ARBS lesions develop the disease rapidly (typically in 3-7 days; Fig. 2B). These data suggest that ARBS may be a polymicrobial infection, which is difficult to resolve experimentally. In spite of increasing observations of diseases affecting marine organisms, relatively little is known about disease transmission and etiology in marine ecosystems. In this regard, marine researchers lag behind their terrestrial counterparts. This multidisciplinary project teamed a sponge ecologist, microbiologist and disease modeler to use field-generated observations on ARBS distribution on multiple Caribbean reefs to develop a disease transmission model for ARBS. This model, along with lab and field experiments, indicate that transmission by direct contact is the major (but not only) source of disease transmission among sponges in the Bahamas (Fig. 3), but that other sources of transmission are more important elsewhere. These differences may be due to multiple factors, including variability in the abiotic environment, susceptibility of the host sponge population, and/or virulence or prevalence of the pathogen(s). Understanding how diseases emerge and are transmitted within marine ecosystems is critic...