Bristle worms attack: benthic jellyfish are not trophic dead ends

P (open-ocean) jellyfish have often been considered trophic “dead ends” (Hansson and Norrman 1995; Lynam et al. 2006). However, various studies have indicated that some terrestrial and aquatic species do frequently consume jellyfish, despite their low nutritional quality (Doyle et al. 2007, 2014). Much less is known about the role of benthic (bottom-dwelling) jellyfish in marine and estuarine food webs, a topic that is increasingly relevant as human disturbances can stimulate benthic jellyfish blooms (Stoner et al. 2011, 2014). Such blooms could potentially be controlled by top-down predation pressure, but little information is available on the possible predators of these animals. So it was with great interest that we recently observed and recorded a benthic jellyfish, Cassiopea spp (hereafter Cassiopea), being preyed upon by bristle worms (Hermodice carunculata). Cassiopea are semi-sessile, benthic jellyfish found all over the world in a range of coastal habitat types, including seagrass beds, coral reefs, mangrove forests, and canals. They are commonly referred to as “upside-down jellyfish”, because as medusae (the free-swimming adult life stage of jellyfish) they rest on the substrate with their oral arms extending upward. Cassiopea use this orientation to acquire light, because, like certain corals, their tissues contain photosynthetic algae that provide them with carbohydrates. When disturbed, Cassiopea release mucus filled with nematocysts (stinging cells), presumably as a defense against predators. Where abundant, these jellyfish have been found to alter the structure and function of nearshore ecosystems (Stoner et al. 2014). For instance, they were shown to reduce seagrass shoot densities in a turtle grass (Thalassia testudinum) bed in The Bahamas, leading to a decline in the densities of other benthic fauna (Stoner et al. 2014). To date, the only documented predator of Cassiopea is a nudibranch – Dondice parguerensis – that occurs around Puerto Rico; this animal consumes only the oral arms, which can subsequently regenerate (Brandon and Cutress 1985). Our observations were made in a tidal creek on Abaco Island, part of the Bahamian Archipelago in the western North Atlantic Ocean. The dominant emergent vegetation in the area is red mangrove (Rhizophora mangle), with a heterogeneous substrate of seagrass (primarily T testudinum), hard bottom, and sand flats. Cassiopea are predominantly found in seagrass beds but often float into hard bottom areas on strong tidal currents. In April 2014, we documented the bristle worm H carunculata, an amphinomid polychaete, consuming a Cassiopea jellyfish. H carunculata, commonly called the bearded fireworm due to its venomous setae (bristles), is a well-known predator of anemones and coral polyps in reef ecosystems (Lizama and Blanquet 1975; Witman 1988). After a jellyfish had settled on the sediment surface, the bristle worms began preying on it, everting their buccal mass, a muscular area on the head containing the pharynx and esophagus (Figure 1). On several occasions, we observed multiple worms feeding on a single jellyfish at the same time, wrapping their bodies either partially or completely around it (Figure 2). Notably, all of the worms we saw feeding on Cassiopea were fairly large (> 30 cm long), whereas H carunculata in shallow marine ecosystems around Abaco Island are typically just a few centimeters long. In April and August of 2014, we used video cameras to record predation events. For each underwater trial, we selected one Cassiopea medusa from adjacent seagrass beds and placed it in front of the camera; we did not see any worms in the area when the trials were initiated. We ran trials on separate days during high tide in different parts of the creek. In one of these trials, we placed a dead fish (ballyhoo, Hemiramphus brasiliensis) next to the Cassiopea to test whether H carunculata would prefer an alternative, readily available prey item. We also conducted one nighttime feeding trial using infrared lighting with the video setup. In five of the six daytime trials, H carunculata moved toward the NATURAL HISTORY NOTES NATURAL HISTORY NOTES