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 : 30-32

Aurelia solida predation

A. solida in the present study, similar to A. aurita s.l. populations worldwide, showed prey heterogeneity, mostly reflecting opportunistic predation on available food within each ecosystem (Matsakis & Conover, 1991; Olsen et al., 1994; Ishii & Tanaka, 2001; Barz & Hirche, 2005). Jellyfish are opportunistic tactile predators whose prey selection depends on various predator and prey characteristics (reviewed in Purcell, 1991). With the exception of two samples, significant positive selection was found for mollusc larvae in the analysed gut contents. This positive selectivity may have been slightly overestimated due to the large mesh size (200 µm) of the plankton net used during the present study. However, mollusc larvae are easier prey to catch than active mesozooplankton taxa such as copepods and larvaceans. Although mollusc larvae are the dominant prey, the predatory impact of Aurelia spp. may be limited by bivalve recruitment, since they are not digested and survive the passage through the jellyfish gastric cavity (Purcell et al., 1991). A significant positive prey selectivity exhibited by A. solida for larvaceans in the present study diverges from results obtained by Purcell and Sturdevant (2001). In contrast to copepods, larvaceans do not actively swim, but are very sensitive to vibrations. At the slightest touch of their gelatinous casing or ‘house’, larvaceans react with a burst of swimming (Bone & Mackie, 1975). This behaviour should allow them to escape predation by Aurelia medusae. However, in the natural environment, responses of zooplankton to their predators can be reduced due to shear flow (Singarajah, 1975). Larvacean size might also explain their selection by A. solida as the contact probability with a predator is positively correlated to prey size ( Madin, 1988). The four species constituting larvacean

Medit. Mar. Sci., 21/1 2020, 22-35 Table 4. Literature review of digestion time estimated from the stomach contents of Aurelia spp. Reference: (1) Heeger & Möller, 1987; (2) Matsakis & Conover, 1991; (3) Sullivan et al.,1994; (4) Båmstedt & Martinussen, 2000; (5) Martinussen & Båmstedt, 2001; (6) Ishii & Tanaka, 2001; (7) Dawson & Martin, 2001; (8) Purcell, 2003; (9) This study; ND = no data.

populations in the Bizerte Lagoon, namely Oikopleura longicauda , Oikopleura fusiformis , Oikopleura dioica , and Fritillaria pellucida (Touzri et al., 2012) can reach up to 3 mm in length ( Brunetti et al., 1990; Scheinberg et al., 2005). While the copepods were the second main prey, they were always negatively selected by A. solida . Copepods are known to be active swimmers with a complex and variable behaviour to escape from predators, such as escaping at submaximum velocity or jumping away from predators (Suchman, 2000). Their escape behaviour, associated with their ability to detect the water movement created by jellyfish bell contraction, may explain why less than 1% of encountered copepods are ingested by these short-tentacled medusae (Suchman, 2000).

A. solida diet varied qualitatively and quantitatively depending on the medusa size. Diet of small medusae (bell diameter <5 cm) showed low prey diversity (1–5 different

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prey types), mainly copepods, compared to large medusae. Similar observations have been made in other areas ( Graham & Kroutil, 2001; Barz & Hirche, 2005). The composition of zooplankton in the field did not show a temporal variation during the studied period, indicating that dietary changes are linked to jellyfish predatory and clearance ability. Clearance rate rising with medusa bell diameter (Möller, 1980; Olsen, 1995), probability to encounter less abundant prey, and therefore increasing diet diversity, is much higher in larger medusa than in smaller ones.

The digestion time of jellyfish varies according to temperature and prey availability; high temperature reduces digestion time (Martinussen & Båmstedt, 2001; Purcell, 2009), whereas it increases with both prey size and number (Martinussen & Båmstedt, 1995; Båmstedt & Martinussen, 2000). In our study, only the influence of temperature was investigated. The observed digestion

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times were quite low compared to previous studies (Table 4). These differences may be due to the different prey availability (type, size, and abundance) and experimental conditions, as well as the different physiological responses of various Aurelia species. It is tempting to suggest that digestion time variability in Aurelia spp. might be interpreted as species-specific ecological adaptations to different environments.

Although there was a great variability in predation impact estimations, A. solida exerted a non-negligible pressure on the mesozooplankton daily standing stock that can reach up to 39.2%, mainly when the jellyfish abundances were high. The sporadic high predation on copepods (21%) and fish eggs (95%) can directly impact their recruitment in case of matching with reproduction and/or spawning events. In case of fish recruitment, the pressure can be amplified indirectly through competition with zooplanktivorous fish (Purcell, 2003; Lynam et al., 2005). Compared with other jellyfish populations, A. solida predation impact in the Bizerte Lagoon was considerably lower than the A. aurita s.l. pressure reported in Tokyo Bay (5–162%) ( Kinoshita et al., 2006), and Kertinge Nor cove ( Denmark) (351% of the daily rotifer biomass) (Olsen, 1995), and was greater than in Chesapeake Bay (0.3 ± 0.3% and 6.9 ± 3.9% of copepod and larvacean daily standing stocks, respectively) (Purcell, 2003).

While predation impact estimates jellyfish pressure during its occurrence, SEM, taking into account the whole period (with and without jellyfish), highlighted mesozooplankton control by A. solida through a prominent top-down control.

Nonetheless, A. solida pressure appears non-negligible; its low abundance and limited occurrence (6–7 months) probably restricted its predatory impact, avoiding a higher depletion on the mesozooplankton community, and might characterise its persistence in the lagoon (Boudouresque, 1999) and enable the establishment of a resident population. However, A. solida is not the only jellyfish recorded in the Bizerte Lagoon. Since 2012, two other non-indigenous species, Phyllorhiza punctata ( Gueroun et al., 2014) and Rhopilema nomadica (Balistreri et al., 2017) occur during the summer–autumn peri- od, causing increasing pressure on the zooplankton community, with wide implications for the pelagic food web dynamics in the Bizerte Lagoon.