Pathogenesis of a Tissue Loss Disease Affecting Multiple Species of Corals Along the Florida Reef Tract

Abstract

An outbreak of stony coral tissue loss disease (SCTLD), emerged on reefs off the coast of southeast Florida in 2014 and continues to spread throughout Florida's Reef Tract. SCTLD is causing extensive mortality of multiple coral species and disease signs vary among affected coral species with differences in rates of tissue loss (acute and subacute), lesion morphology (adjacent bleached zone or not) and lesion occurrence (focal and multi-focal). We examined the virulence, transmission dynamics and response to antibiotic treatment of coral species exhibiting different types of tissue loss lesions from two regions in Florida. Montastraea cavernosa with subacute tissue loss lesions in the southeast Florida region near Fort Lauderdale was compared to corals (multiple species) with acute tissue loss lesions in the Middle Keys. Corals from both regions showed progressive tissue loss but the in situ rate of mortality was significantly higher in tagged colonies in the Keys. Aquaria studies showed disease transmission occurred through direct contact and through the water column for corals from both regions. However, transmission success was higher for corals with acute vs. subacute lesions. There was 100% transmission for both test species, M. cavernosa and Meandrina meandrites, touching acute lesions. Among the three species touching subacute lesions, the disease transmitted readily to Orbicella faveolata (100%) followed by M. cavernosa (30%) with no transmission occurring with Porites astreoides. Diseased fragments of all species tested responded to antibiotic treatment with a cessation or slowing of the disease lesions suggesting that bacteria are involved in disease progression. Mortality was higher for in situ corals with acute lesions and transmission was higher in M. cavernosa exposed to acute lesions compared to subacute lesions, suggesting that different microbes may be involved with the two lesion types. However, since in situ mortality of M. cavernosa was not measured in the Middle Keys, we cannot completely rule out that a common pathogen is involved but is less virulent within M. cavernosa.

Rhodobacterales and Rhizobiales Are Associated With Stony Coral Tissue Loss Disease and Its Suspected Sources of Transmission

“…It is possible that the unique microbiome of MMEA, which is dominated by Rhodospirillales (Supplementary Figure S1C), may respond differently to SCTLD than other corals. MMEA is one of the species most susceptible to SCTLD, with up to a 98% disease prevalence in reefs, and it is one of the first species to show signs of SCTLD during outbreaks (Precht et al, 2016;Stony Coral Tissue Loss Disease [SCTLD], 2018;Aeby et al, 2019). A disturbance of the dominant Rhodospirillales may cause more severe changes in the microbiome of MMEA than in coral species with a low prevalence of this order, or with more than one dominant taxa (Supplementary Figure S1C).…”

“…Corals affected by SCTLD display focal or multifocal lesions, some of which may be preceded by a band of bleached tissue. Rates of tissue loss are acute to chronic, depending on the coral species or specific colony (Stony Coral Tissue Loss Disease [SCTLD], 2018; Aeby et al, 2019). Corals in the Meandrinidae family, such as Dichocoenia stokesii and Meandrina meandrites, are especially susceptible to SCTLD and are often the first to become affected by SCTLD (Precht et al, 2016;Stony Coral Tissue Loss Disease [SCTLD], 2018;Aeby et al, 2019).…”

“…Rates of tissue loss are acute to chronic, depending on the coral species or specific colony (Stony Coral Tissue Loss Disease [SCTLD], 2018; Aeby et al, 2019). Corals in the Meandrinidae family, such as Dichocoenia stokesii and Meandrina meandrites, are especially susceptible to SCTLD and are often the first to become affected by SCTLD (Precht et al, 2016;Stony Coral Tissue Loss Disease [SCTLD], 2018;Aeby et al, 2019). Other species, including many brain corals (e.g., Diploria labyrinthiformis), are also highly susceptible, and other species, such as Stephanocoenia intersepta, appear to have an intermediate susceptibility (Precht et al, 2016).…”

“…This has led to a controversial hypothesis that sediment is a potential source of transmission of the SCTLD pathogen(s) (Miller et al, 2016;Cunning et al, 2019;Gintert et al, 2019). Initial studies also have indicated that SCTLD is likely contagious and waterborne (Precht et al, 2016;Aeby et al, 2019;Muller et al, 2020). Thus, water may be a vector through which sediment-associated SCTLD pathogen(s) may spread via the Florida current (Precht et al, 2016).…”

“…A total of five tissue types were collected: (1) vulnerable zone apparently healthy colony (VuAH), (2) endemic zone apparently healthy colony (EnAH), (3) epidemic zone apparently healthy colony (EpAH), (4) epidemic zone diseased colony unaffected tissue (EpDU), and (5) epidemic zone diseased colony lesion tissue (EpDL ; Table 1). Additionally, tissue samples from M. meandrites, another species with high susceptibility to SCTLD (Precht et al, 2016;Stony Coral Tissue Loss Disease [SCTLD], 2018;Aeby et al, 2019), were collected opportunistically during the onset of SCTLD at Looe Key, an offshore fore reef in the Lower Keys (Table 1 and Figures 1, 2). These samples were included to determine whether the SCTLD-associated microbiomes from this species were consistent with those examined in our original design.…”