Aurelia solida, Browne, 1905

Gueroun, Sonia K. M., Molinero, Juan Carlos, Piraino, Stefano & Yahia, Mohamed Nejib Daly, 2020, Population dynamics and predatory impact of the alien jellyfish Aurelia solida (Cnidaria, Scyphozoa) in the Bizerte Lagoon (southwestern Mediterranean Sea) Abstract, Mediterranean Marine Science 21 (1), pp. 22-35 : 24

Aurelia solida predation impact

Since no light/dark difference in feeding rate was found for A. aurita (Bailey & Batty, 1983) , and the homogeneity of the shallow water column (Sakka Hlaili et al., 2008) suggested the absence of vertical migration for A. solida , we considered that diet, feeding rate, and predation impact were homogeneous over 24 h.

Since March 2013, 5–20 A. solida were sampled in the first 1 m depth layer with a hand net and immediately preserved individually in 4% formalin for gut content analysis. In the laboratory, the formalin solution was filtered to collect any possible egested material, and the jellyfish was dissected to examine canals, stomach, and gastric pouches for prey organisms, which were identified to major taxonomic groups, and to Order for copepods. The relative importance of A. solida prey was expressed as the percentage of each prey taxon relative to all prey items in the gut contents, the percentage of numerical abundance of prey items in the gut contents (N;%), the index of frequency of occurrence in the gut ( FO;%) and the index of relative importance ( IRI; %) ( Laroche, 1982).

Pearre’s selectivity index (C) (Pearre, 1982) was applied to estimate prey taxon selectivity of A. solida . C ranges between -1 and +1 and depicts the magnitude of negative and positive selection of prey. This method is based on the Chi-square (X 2) analysis, comprising 2 × 2 configured comparisons between the average abundance of each taxon in the medusa gut contents and the corresponding abundance in the ambient environment (Pearre, 1982). The selectivity C is given by the equation:

C = ± [(| a d b e − abde b a a e | − 2 n)](2 1 2)

where a is the number of individuals in a particular species and b is the number of individuals of all other taxa in the diet (subscript d) and in the environment (subscript e), respectively; a = a d + a e; b = b d + b e; c = a e + b e; and d = a d + b d; and n = a + b + c + d. This was repeated for each taxon of interest. A Chi-square test was applied to test the significance of the values.

Jellyfish digestion time for zooplankton was determined at three temperatures (13, 18, and 23°C) using the protocol established by Purcell (2003). To avoid a thermal shock caused by a difference between in situ water and incubation temperature, the experiments were conducted when the in situ water temperature was close to one of the three experimental temperatures. The experiments were conducted during March and May (2014) when the SST values were close to the experimental temperatures (∆ = 0.8 ± 0.1 °C). Sixty medusa individuals were collected by

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hand net from the Bizerte Lagoon and maintained in an 80 L tank with fresh zooplankton collected with a WP2 net from the lagoon to assure continuous feeding until return to the laboratory (30 min after sampling). Five medusae were preserved immediately (t 0). In the laboratory, after 1h of acclimation, the medusae were transferred from the 80 L tank to 11 20 L tanks (i.e. five medusae per tank) filled with 20 µm-filtered seawater maintained at a constant temperature (average salinity = 37 ± 0.5). Five medusae were preserved at 30 min or 1 h intervals for up to 8 h for gut content analyses. Digestion time was determined by solving (prey = 0) the linear regressions evaluating the relation between the time and the number of available prey in the gut contents (Purcell, 2003). Medusa bell diameter was measured to the nearest 1.0 mm .

Individual rates of feeding on mesozooplankton were expressed as the number of prey items consumed per medusa per day: F = C m / D × 24 h where F is the number of prey items consumed per medusa per day, C m is the number of prey items in medusa gut, and D is the digestion time (h). Digestion time was estimated by the previous experiment, and SSTs recorded in the sampling campaigns were used as a reference to choose the right digestion time estimated from the three experimental temperatures. Since bivalve larvae survive their transit through the jellyfish gut and are egested alive, only 1% of the bivalve larvae found in the gut content were used to estimate the feeding rate in order to avoid an overestimation (Purcell et al., 1991).

Predation impact was expressed as the percentage of prey standing stock consumed per day: P = F × M / C × 100 where P is the percentage of prey standing stock consumed per day, F is the feeding rate, M is the abundance of medusae per cubic metre, and C is the abundance of prey per cubic metre.