Limnocnida tanganyicae medusae (Cnidaria: Hydrozoa): a semiautonomous microcosm in the food web of Lake Tanganyika

Abstract

Medusae are important members of marine food webs, but are rare in lakes. In one of the largest lakes in the world, Lake Tanganyika, a small medusa (Limnocnida tanganyicae) is a prominent component of zooplankton. We used field and laboratory methods to study the ecological role of Lake Tanganyika medusae, which occasionally reached high local densities in the whole epilimnion. The largest individuals showed low amplitude, diel vertical migration which minimized their exposure to harmful UV radiation and also may be important for picocyanobacteria regularly present in the medusae. The endosymbiotic picocyanobacteria differed morphologically among medusae and were predominantly one Lake Biwa type Cyanobium sp. that typically was abundant in the water column. Under light, some medusae were net primary producers. Although nitrogen stable isotopic ratios indicated that the free-living cyanobacteria were nitrogen-fixers, the picocyanobacteria in medusae obtained nitrogen predominantly from their host. Stable isotopic ratios of carbon and nitrogen further suggested that copepods were the most likely prey for the medusae. Lake Tanganyika medusae apparently base their metabolism both on animal and plant sources, with possible internal cycling of nutrients; however, the role of picocyanobacteria gardening in the Lake Tanganyika ecosystem and its medusae requires quantification.

Surviving but not thriving: inconsistent responses of zooxanthellate jellyfish polyps to ocean warming and future UV-B scenarios

“…Our observations that Cassiopea sp. polyps were negatively affected by elevated UV-B are consistent with those of the only other study of the effects of UV radiation on jellyfish 28 . The freshwater hydrozoan, L. tanganyicae died after only 1 h exposure to high UV conditions, but were unaffected when exposed to conditions where UV wavelengths were filtered out.…”

“…Recent research, however, suggests jellyfish may not be immune to environmental stressors 26 27 and changes to UV radiation may adversely affect jellyfish populations. Indeed the freshwater hydrozoan, Limnocnida tanganyicae , died when exposed to elevated UV for only one hour and avoidance of UV was postulated as the reason why this species migrates to deeper waters during the day 28 . Species of jellyfish that host zooxanthellae, however, must occur in shallow waters to expose their zooxanthellae to light for photosynthesis.…”

Freshwater hydrozoan blooms alter activity and behaviour of territorial cichlids in Lake Tanganyika

“…Limnocnida tanganjicae (Hydrozoa, Cnidaria) is the only documented species of jellyfish present in Lake Tanganyika [19] and is typically found throughout the oxygenated water column in the upper 100 m [18]. While not a ‘true’ scyphozoan jellyfish, L. tanganjicae is a major part of the zooplankton community in the lake [20] though it has received very little research attention relative to other fauna in Lake Tanganyika [21].…”

“…Variabilichromis moorii is a socially monogamous and biparental species of cichlid fish, and like many Tanganyikan cichlids, V. moorii is highly territorial and ecologically constrained to rocky littoral habitats. Thus, it is unable to actively relocate to avoid jellyfish blooms, which can sometimes span areas covering several hundreds of square kilometres [18]; instead, male–female social pairs, as well as solitary territory-owners, remain on their territories during the blooms despite the large numbers of jellyfish that they encounter. Variabilichromis moorii pairs hold territories (1–4 m 2 ) containing several rocky crevices where they can take shelter, graze on algae and raise offspring.…”

Areas of endemism in the Southwestern Atlantic Ocean based on the distribution of benthic hydroids (Cnidaria: Hydrozoa)

“…Therefore, our search is not limited on the exclusive use of distribution of endemic species of hydroids for the SWAO. Indeed, species of hydroids can have restricted and punctual geographic distribution, living at specific and specialized habitats, such as freshwater, and in interstitial sediments and ice (e.g., Bouillon & Grohmann 1994;Jankowski et al 2008;Piraino et al 2008;Siebert 2009;Salonen et al 2012). However, many taxa have wide geographic distribution and low incidence of endemism (e.g., families Bougainvilliidae, Campanulariidae, Eudendriidae, Hydractiniidae, Sertulariidae, Tubulariidae ;Millard 1975;Cornelius 1995;Bouillon et al 2006;Schuchert 2012).…”