Spatial Variability in a Symbiont-Diverse Marine Host and the Use of Observational Data to Assess Ecological Interactions

Abstract

Despite a rich taxonomic literature on the symbionts of ascidians, the nature of these symbioses remains poorly understood. In the Egyptian Red Sea, the solitary ascidian Phallusia nigra hosted a symbiotic amphipod and four copepod species, with densities as high as 68 mixed symbionts per host. Correlation analyses suggested no competition or antagonism between symbionts. Ascidian mass, ash-free dry mass per wet mass (AFDM/WM), and both symbiont density and diversity per host, differed significantly among three reefs from El Gouna, Egypt. However, there was no correlation between amphipod, total copepod, or total symbiont densities and host mass or AFDM/WM. A host condition index based on body to tunic mass ratio was significantly related to symbiont density overall, but this positive pattern was only strong at a single site studied. Despite assumptions based on the habit of some of the symbiont groups, our analyses detected little effect of symbionts on host health, suggesting a commensal relationship.

The Ascidian–Amphipod Association between Phallusia mammillata (Cuvier, 1815) and Leucothoe richiardii (Lessona, 1865) in the Mar Grande of Taranto (Mediterranean Sea, Italy)

“…Indeed, spatial and temporal variations in host abundance may lead to variations in the density of Leucothoe spp. in their hosts [23] and may explain the results of Cantor et al [36], who found a positive correlation between P. nigra biomass and the number of amphipods hosted. In addition, the results of Cantor et al [36] were based on a single sampling, which increased the random component in the study of the population of L. angraensis (see [12]) and its relationship with P. nigra.…”

“…Given the high morphological similarity, the occurrence in the same host, and the relatively short distance between the bays, it was assumed that the first two cases were also specimens of the new species and the P. nigra/L. angraensis association is currently considered the correct symbiotic match [23]. In some cases, L. richiardii may also have been misidentified in the past in the Mediterranean region, where it is reported to be a generalist commensal of several hosts, especially ascidians.…”

“…Tropical ascidians have been particularly studied in this regard, and Phallusia nigra (Savigny, 1816) alone is known to host at least six Leucothoe species, numerous copepods, and even a polychaete as symbiotic partners worldwide [23]. In contrast, Phallusia mammillata (Cuvier, 1815) is one of the largest and most abundant ascidians in the Mediterranean [19], commonly known to host several parasitic/commensal copepods [24,25].…”

Distinct microbial communities in an ascidian–crustacean symbiosis

“…In other cases, the ascidian can become infected by parasitic copepods that feed on its cellulose tunic (Hirose, 2000 ), suppressing the sexual reproduction of the ascidian host (Hirose et al, 2005 ). Multiple species and individuals of crustaceans can be present in an ascidian host simultaneously (Cruz‐Rivera et al, 2022 ; Kim et al, 2016 ), and residence in hosts is reportedly seasonal (Thiel, 2000 ) and dependent on the host's physiological state (White & Reimer, 2012 ).…”