The highly competitive ascidian Didemnum sp. threatens coral reef communities in the Wakatobi Marine National Park, Southeast Sulawesi, Indonesia

Abstract

Coral reefs in the Wakatobi Marine National Park (WMNP), Indonesia, are protected but have been degrading in several areas due to local anthropogenic stressors. In affected areas, benthic surveys revealed the occurrence of a dominant ascidian species of the genus Didemnum, which may negatively impact the benthic community composition and structure. We quantified the abundance, substrate preference, and growth rate of Didemnum sp. in non-degraded and degraded reefs to assess its impact on the benthic community. While Didemnum sp. occurred in similar high abundances in both, non-degraded (0.66%) and degraded (0.75 %) reef sites, this species showed a substantially (> 10-fold) increased growth rate in degrading reefs (2.7 ± 0.98% day⁻¹ increase in colony size, compared to 0.17 ± 0.39 % day⁻¹ in non-degraded reefs). Furthermore, Didemnum sp. colonized many different substrates and showed the ability to overgrow live corals quickly. These observations indicate that Didemnum sp. can be a severe threat to a reef community by outcompeting live corals and call for further studies on the interaction between environmental pollution and Didemnum growth patterns in coral reefs.

Expansion of a colonial ascidian following consecutive mass coral bleaching at Lizard Island, Australia

“…molle appears to be previously unreported, expansion of other ascidian species on coral reefs has been noted (e.g. Littler and Littler, 1995;Roth et al, 2018;Vargas-Ángel et al, 2009). For example, Trididemnum solidum in the Caribbean (Bak et al, 1996) and Botryllus eilatensis in the Red Sea (Shenkar et al, 2008).…”

“…As ascidians can overgrow most benthic organisms, including corals and macroalgae (Bak et al, 1996;Littler and Litter, 1995;Roth et al, 2018), this suggests 'available space' is not a limiting factor and, as such, the expansion of D. cf. molle may not be directly due to coral mortality opening up new space to occupy.…”

“…Indeed, expansions of ascidians on coral reefs have been noted previously (e.g. Bak et al, 1996;Oren and Benayahu, 1998;Roth et al, 2018;Shenkar et al, 2008), yet their successional dynamics following coral mortality on such unprecedented scales is currently unclear. The aim of this study, therefore, was to utilise a series of temporally and spatially matched photoquadrats to examine the successional dynamics of a widespread ascidian across both the 2016 and 2017 mass coral bleaching events on the GBR.…”

Patterns of Association between Marine Sponges and the Associated Organisms: Case Study, Losin Island, Pattani, Thailand

“…However, the outcomes of the competition of space in marine environments are complicated. The combination of size dependent, habitat facilitation (may be through the intraspecific competition of the higher hierarchical competitive species themselves) microhabitats, ocean acidification and anthropogenic impacts are able to modify the hierarchical competitive results [22][23][24][25]. Consequences of the above combinations of interaction can lead to phase shift alone the zonation of coral reef communities [24,26].…”

Variable thermal and biochemical stress responses of tissue balls from <i>Lithophyllon repanda, Pocillopora damicornis</i> and <i>Acropora muricata</i>

“…Biochemical stressors may arise from competition with sessile and sedentary reef organisms, for example, sea weeds (Brandt et al 2019), sponges (Wang et al 2012;Brown et al 2018) and ascidians (Roth et al 2018).…”