Population dynamics of a coral reef ascidian in a deteriorating environment

Abstract

In the coral reefs of Eilat, a newly described colonial ascidian, Botryllus eilatensis, has been observed overgrowing dead coral skeletons and rapidly colonizing artificial substrates. To study overgrowth dynamics and the relationship between coral morphology, size, and orientation and ascidian overgrowth we monitored (monthly) the interaction of 50 to 70 tagged coral colonies with B. eilatensis. The rate of B. eilatensis overgrowth on corals was influenced by the coral's morphology and orientation combined with a seasonal appearance of the ascidian. During spring, the average ascidian cover was 27 ± 4.6 % (mean ± SE), decreasing rapidly during the summer months when sexual reproduction took place. This seasonality is linked to the vertical mixing phenomenon that occurs in Eilat every winter. Moreover, fragments of B. eilatensis translocated to a nutrient-rich site increased 3-fold in size and exhibited higher survivorship rates in comparison to the fragments at the original site. Increased anthropogenic activity, particularly the eutrophication occurring along the Israeli coasts, creates favorable conditions for filter-feeding organisms such as ascidians, providing them with an advantage in competition for space over reef-building corals and other organisms.

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

“…Concordant, Chadwick and Morrow (2011) showed that ascidians can successfully compete with stony corals under eutrophic conditions. Growth rates in the degraded reefs reported here outnumber those reported by Rius et al (2014) by a factor of seven and may be explained by a lack of predators (Osman and Whitlatch 2007;Roth et al 2017) and an excessive supply of food (Shenkar et al 2008;Sawall et al 2013;Roth et al 2017). Illegal fishing activities (von Heland and Clifton 2015) and other unsustainable activities endangering the WMNP are well known in the studied area (Szuster and Albasri 2010).…”

“…found in degraded reefs occupied twice the space of the largest colony found in non-degraded reefs. Eutrophic conditions resulting from untreated sewage and land-based pollution (Riisgård et al 1995;Shenkar et al 2008) from adjacent villages are likely favourable for ascidian growth (Rodríguez-Martínez et al 2012) due to an increase in food resources resulting from an increase in phytoplankton and subsequent organic matter (Herbert 1999). Such conditions may also lead to an earlier fusion of colonies in degraded waters Shenkar et al 2008).…”

“…Eutrophic conditions resulting from untreated sewage and land-based pollution (Riisgård et al 1995;Shenkar et al 2008) from adjacent villages are likely favourable for ascidian growth (Rodríguez-Martínez et al 2012) due to an increase in food resources resulting from an increase in phytoplankton and subsequent organic matter (Herbert 1999). Such conditions may also lead to an earlier fusion of colonies in degraded waters Shenkar et al 2008). Despite constant recruitment in non-degraded waters, colonies may be restrained from fusion by environmental factors such as resource availability or competing invertebrate species (Lambert 2005;Dias et al 2008;López-Legentil et al 2013).…”

“…Illegal fishing activities (von Heland and Clifton 2015) and other unsustainable activities endangering the WMNP are well known in the studied area (Szuster and Albasri 2010). Additionally, a higher food supply for filter feeders may be provided by unfiltered sewage discharge and nutrients (Shenkar et al 2008;Sawall et al 2013) from villages at Kaledupa island, undermining the reef's resilience against opportunistic species.…”

Coral diversity indices along the Gulf of Aqaba and Ras Mohammed, Red Sea, Egypt

“…Although the two sites Temple and Katy suffered also old breakage, diversity was not improved because of the sponge and ascidian domination that represent potent competitors with corals beside the high amount of nutrients (nitrates) in those sites. Sponges and ascidians dominate in areas of high particulate organic nitrogen (Ribes et al 2005;Yahel et al 2005;Ribes et al 2005;Shenkar et al 2008) and have negative effects on developing coral embryos and larvae (Sammarco, 1996). Lack of significant predators makes ascidians very successful competitors (Lambert 2002).…”