In Situ Observations of the Behavior of Mesopelagic Solmissus Narcomedusae (Cnidaria, Hydrozoa)
Abstract:Solmissus albescens is the most numerous medusa in the mesopelagic western Mediterranean. This endemic species undergoes a nocturnal vertical migration from a depth of 400–700 m to the upper 100 m. In situ observations of S. albescens reported here were made during 10 dives using the French submersible Cyana in 1985–1986 in conjunction with simultaneous conventional plankton sampling, and are combined here with published observations by earlier French submersible users for completeness. The hundreds of 2–5 cm diameter S. albescens seen from the Cyana were almost always actively swimming regardless of time of day, with pulsation frequencies between 1.3 and 2.0 Hz. Swimming becomes directional at dawn and at dusk and the speed of upward and downward migrations appears to be the same, approximately 100 m·h−1 as estimated from in situ point observations and from population depth distributions at different times of late afternoon, evening, and morning. Vertical migration appears to be conducted solely by swimming without need for changes in buoyancy. S. albescens can hold its non-extensible tentacles directly above its bell, curved laterally outward, or directed downward. Unlike many other hydromedusae, S. albescens swims and feeds simultaneously. Although all net-collected Solmissus had empty guts both during the day and at night, from the submersible we saw S. albescens feeding on nonmigratory epipelagic Cavolinia pteropods at night. One S. albescens during the day was seen capturing a lobate ctenophore, but all other daytime observations indicated empty guts. Solmissus and other narcomedusae may be limited to feeding on soft-bodied prey by the type of nematocysts on their tentacles (apotrichous isorhizas). S. marshalli has also been observed in situ, from the Sea Link II and Pisces IV submersibles. In comparison to S. albescens, this cosmopolitan mesopelagic species has slower swimming speed (0.6–0.7 Hz), frequent periods of quiescence, and no long-range diel migrations.
Document Type: Research Article
Publication date: November 1, 1988
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