Ecological aspects of the ascidian community along the Israeli coasts

Ascidians (Phylum: Chordata, Class: Ascidiacea), or sea squirts, are the largest and most diverse class of the sub-phylum Tunicata (also known as Urochordata). They comprise approximately 3000 species found in all marine habitats from shallow water to the deep sea. Despite the enormous progress that has been achieved in the field of ascidian research worldwide, only a few studies have focused on the ascidians of the Red Sea and the Eastern Mediterranean. This study is the first to investigate this unique group of invertebrates from the Mediterranean and Red Sea coasts of Israel, from ecological, taxonomical and biological aspects. I first ascertained the dominant species found in the region, and determined whether these include any non-indigenous species. I then assessed the annual appearance and spatial distribution of these ascidians and how this appearance may be related to different a-biotic factors. Finally, I studied the possible impact of anthropogenic activities on ascidian ecology in both coral reef and Mediterranean shore environments. In section I, I review the available data regarding the ascidian fauna along the Israeli coasts and describe the most dominant species found along the Red Sea coast of Israel. This part of the study led to the discovery and description of a new species, Botryllus eilatensis (Shenkar & Monniot 2006). In addition, two solitary species (Rhopalaea sp. and Boltenia sp.), found in Eilat, are also suspected to be new species. Six non-indigenous species (Phallusia nigra, Herdmania momus, Microcosmus exasperatus, Rhodosoma turcicum, Ascidia canellata, and Symplegma brakenhielmi) were recorded from the Mediterranean shores of Israel. These species probably reached the Mediterranean via the Suez Canal, since they have an extra-tropical Indo-Pacific distribution and a restricted distribution in the Eastern Mediterranean. In section II, in order to obtain a broader view of the ecology of non-indigenous species along the Mediterranean coast of Israel, I have chosen to focus solely on the study of comparative aspects of the ecology of the solitary ascidian H. momus, in its native (Red Sea) environment versus its introduced (Mediterranean) environment. During 2005 monthly samples of H. momus were collected along the Mediterranean and Red Sea coasts of Israel, in order to investigate possible differences in life history strategies of the two populations. The samples were preserved, dissected, and measurements were taken of length, total weight, gonad weight and oocyte diameter, and the occurrence of symbionts was recorded. In the Mediterranean H. momus exclusively inhabits artificial substrates and is common at greater depths than in Eilat (Red Sea). Individuals of H. momus in Eilat reproduced year round. Although individuals from the Mediterranean were significantly larger than individuals collected in Eilat, their gonad indices and oocyte diameter measurements indicate that they have a short reproductive season. Copepods were found in 50% of the samples from both sites, while a pontoniine shrimp was found in 14% of the samples from Eilat only. The marked differences between the native vs. non-indigenous populations of H. momus are attributed to differential food availability, water temperature, currents and wave exposure. In section III, I report on the ascidian recruitment patterns on an artificial reef in Eilat. The location and timing of one species’ appearance may be the result of seasonality of reproduction, larval preferences, and effects of different a-biotic factors such as food availability, light intensity, water temperature and more. By identifying the ascidians to species level, we were able to distinguish between the specific recruitment patterns of the different species, rather than generalizing patterns among groups as done in most studies. The major contributions of the data presented in this section are: 1) the finding that two dominant species, Herdmania momus and Didemnum granulatum, established the pattern for the entire group; and 2) the finding that the epifaunal assemblage developing on an artificial substrate in a tropical environment is influenced by the season of deployment. These results should be taken into consideration when investigating community development on such structures, especially since in coral reefs, possibly due to the less dramatic changes between seasons, this aspect has not been considered to date. One of the most striking seasonal phenomenona observed in the coral reefs of Eilat during the study was that of the seasonal appearance of a new colonial ascidian, Botryllus eilatensis. In order to study overgrowth dynamics and the relationship between coral morphology, size, and orientation and ascidian overgrowth, I conducted monthly monitoring of 50 70 tagged coral colonies interacting with B. eilatensis. This study is the first to investigate coral-ascidian competitive interactions and of major importance since the environmental conditions that are deleterious for corals (e.g. eutrophication) may create a favorable environment for filter-feeding organisms, such as ascidians, accelerating their growth and spread. 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. This seasonality was linked to the vertical mixing phenomenon that occurs in Eilat every winter and results in higher nutrient levels in the water column, and thus in higher food availability. I further demonstrate that B. eilatensis fragments translocated to a nutrient-rich site increased three-fold in size and exhibited higher survivorship rates in comparison to intact fragments in a nutrient-poor area. The data collected in the above studies demonstrate the significant impact of human activities on ascidian ecology. Since ascidians are able to filter even minute particulate matter, any rise in nutrient levels and organic material in coastal waters will have a direct influence on their appearance. This was evident in the large size of H. momus found in the Mediterranean and the seasonal outbreaks of B. eilatensis in the coral reefs of Eilat. In addition, being a major component of the fouling community, ascidians play an important role in marine bioinvasion, as demonstrated by the appearance of six non-indigenous ascidian species with an Indo-Pacific origin. These species were particularly abundant on artificial marine structures, which are known to facilitate the spread of non-indigenous species. Thus, another human activity that may contribute to the spread of non-indigenous ascidians is that of the proliferation of artificial structures along the coast line. Ascidians can be strong spatial competitors, and once they become established they often experience population explosions that can develop into dense mats that overgrow and cover available surfaces. However, although there is a growing awareness that ascidians can become of economic and ecological concern, relatively little is known at present about the ecology of many of these ascidians, unlike those taxa with a direct economic value. It is my hope that the current thesis will create a renewed interest in the field of ascidian ecology and that it will encourage future studies in this field and help in the development of tools for controlling the negative impacts of human activities on coastal regions.