Elizabeth W. Stoner

Thresholds of ecosystem response to nutrient enrichment from fish aggregations

Biogeochemical hotspots can be driven by aggregations of animals, via excretion, that provide a concentrated source of limiting nutrients for primary producers. In a subtropical seagrass ecosystem, we characterized thresholds of ecological change associated with such hotspots surrounding artificial reef habitats. We deployed reefs of three sizes to aggregate fishes at different densities (and thus different levels of nutrient supply via excretion) and examined seagrass characteristics that reflect ecosystem processes. Responses varied as a function of reef size, with higher fish densities (on larger reefs) associated with more distinct ecological thresholds. For example, adjacent to larger reefs, the percentage of P content (%P) of seagrass (Thalassia testudinum) blades was significantly higher than background concentrations; fish densities on smaller reefs were insufficient to support sharp transitions in %P. Blade height was the only variable characterized by thresholds adjacent to smaller reefs, but lower fish densities (and hence, nutrient input) on smaller reefs were not sufficient for luxury nutrient storage by seagrass. Identifying such complexities in ecological thresholds is crucial for characterizing the extent to which biogeochemical hotspots may influence ecosystem function at a landscape scale.

Effects of anthropogenic disturbance on the abundance and size of epibenthic jellyfish Cassiopea spp.

Jellyfish blooms in pelagic systems appear to be increasing on a global scale because of anthropogenic impacts, but much less is known about the link between human activities and epibenthic jellyfish abundance. The aim of this study was to investigate whether the epibenthic jellyfish, Cassiopea spp., were found in greater abundance, and attained larger sizes, in coastal habitats adjacent to high human population densities compared to sites adjacent to uninhabited areas on Abaco Island, Bahamas. Cassiopea spp. were found to be significantly more dense and larger in areas with high human population densities. Ambient nutrient levels and nutrient content of seagrass were elevated in high human population density sites, and may be one mechanism driving higher abundance and size of Cassiopea spp. Cassiopea spp. may have important effects on community structure and ecosystem function in critical coastal ecosystems (e.g., seagrass beds), and their impacts warrant further study.

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

Comparison of zooxanthellae densities from upside-down jellyfish, Cassiopea xamachana, across coastal habitats of The Bahamas

Anthropogenic disturbances may drive jellyfish blooms, and previous studies have suggested this is the case for upside-down jellyfish (Cassiopea xamachana). Cassiopea were found to have higher mean zooxanthellae densities in human-impacted areas on Abaco Island, The Bahamas, suggesting that nutrient loading in impacted sites may be one factor driving zooxanthellate jellyfish blooms. Gut contents from Cassiopea medusae were positively correlated to zooxanthellae densities, indicating that heterotrophically-derived nutrition may be an important factor in facilitating increased zooxanthellae population densities. Understanding the mechanisms driving jellyfish blooms is crucial for developing effective management strategies in impacted coastal ecosystems.

Does landscape context mediate the nature of density dependence for a coral reef fish

Over-harvest and landscape change are two of the greatest threats to marine ecosystems. Over-harvest may directly affect key population regulation mechanisms (e.g., density dependence), with the magnitude of the effects being further influenced by changes in landscape structure and associated resource availability. Because resource availability and conspecific density often co-vary within the natural landscape, manipulative experiments are needed to understand how changes in these two drivers may affect density dependence in wild populations. We used a common, shoaling, coral reef fish (white grunt, Haemulon plumierii) as our model species, and manipulated fish densities and landscape context of artificial reef habitats to assess the effects of each on fish condition. We found evidence of inverse density dependence, where individual condition was positively related to conspecific density; landscape context had little effect. Mean grunt condition on natural patch reefs was similar to that for our low grunt density treatment artificial reefs, possibly due to differences in fish densities or landscape context. These findings suggest that over-harvest may have detrimental effects on wild populations that extend beyond mere reductions in population size, especially for group-living species.

Comparison of Substrates for Eastern Oyster (Crassostrea virginica) Spat Settlement in the Loxahatchee River Estuary, Florida

ABSTRACT The eastern oyster [Crassostrea virginica (Gmelin, 1791)] is an important epibenthic species in estuarine and coastal marine ecosystems, providing habitat for commercially valuable species and enhancing ecosystem function. One way to assess oyster population structure and the potential suitability of oyster restoration sites is through deployment of adult oyster shells or other substrates, and quantifying oyster spat settlement. The suitability of travertine tiles versus axenic adult oyster shells for C. virginica settlement was compared by deploying shellstrings with tiles and shells in four different locations across two seasons (fall or spring) in the subtropical, Loxahatchee River estuary, FL. There was no significant difference in spat densities on oyster shells compared with tile tops and bottoms, although there was significant spatial and temporal variation in spat settlement. Spat were slightly more abundant on the top of deployed tiles compared with the bottom, which differs from typical C. virginica settlement behavior. One possible explanation may be the presence of other fouling organisms on the bottom of tiles which decrease oyster settlement rates. Results show that oyster spat settlement was indistinguishable between travertine tiles and oyster shells and thus suggest that travertine tiles are preferable to axenic oyster shells because spat settlement can be precisely quantified per unit area.