Coral reef habitats in the Papahānaumokuākea Marine National Monument (PMNM) are characterized by abundant top-level predators such as sharks and jacks. The predator assemblage is dominated both numerically and in biomass by giant trevally (Caranx ignobilis) and Galapagos sharks (Carcharhinus galapagensis). A lower diversity of predatory teleosts, particularly groupers and snappers, distinguishes the PMNM from other remote, unfished atolls in the Pacific. Most coral reef top predators are site attached to a “home” atoll, but move extensively within these atolls. Abundances of the most common sharks and jacks are highest in atoll fore reef habitats. Top predators within the PMNM forage on a diverse range of prey and exert top-down control over shallow-water reef fish assemblages. Ecological models suggest ecosystem processes may be most impacted by top predators through indirect effects of predation. Knowledge gaps are identified to guide future studies of top predators in the PMNM.
Biogeochemical sulfur cycling in sulfidic karst systems is largely driven by abiotic and biological sulfide oxidation, but the fate of elemental sulfur (S0) that accumulates in these systems is not well understood. The Frasassi Cave system (Italy) is intersected by a sulfidic aquifer that mixes with small quantities of oxygen-rich meteoric water, creating Proterozoic-like conditions and supporting a prolific ecosystem driven by sulfur-based chemolithoautotrophy. To better understand the cycling of S0in this environment, we examined the geochemistry and microbiology of sediments underlying widespread sulfide-oxidizing mats dominated byBeggiatoa. Sediment populations were dominated by uncultivated relatives of sulfur cycling chemolithoautotrophs related toSulfurovum,Halothiobacillus,Thiofaba,Thiovirga,Thiobacillus, andDesulfocapsa, as well as diverse uncultivated anaerobic heterotrophs affiliated with Bacteroidota, Anaerolineaceae, Lentimicrobiaceae, and Prolixibacteraceae.DesulfocapsaandSulfurovumpopulations accounted for 12-26% of sediment 16S rRNA amplicon sequences and were closely related to isolates which carry out autotrophic S0disproportionation in pure culture. Gibbs energy (ΔGr) calculations revealed that S0disproportionation under in situ conditions is energy yielding. Microsensor profiles through the mat-sediment interface showed thatBeggiatoamats consume dissolved sulfide and oxygen, but a net increase in acidity was only observed in the sediments below. Together, these findings suggest that disproportionation is an important sink for S0generated by microbial sulfide oxidation in this oxygen-limited system and may contribute to the weathering of carbonate rocks and sediments in sulfur-rich environments.
Parrotfishes routinely form large schools close to the substrate while foraging, migrating or spawning (Johannes 1981). On 30th May 2010 at French Frigate Shoals atoll (N 23.63601, W 166.17235), northwestern Hawaiian Islands, we observed a school of bullethead parrotfish (Chlorurus sordidus) swimming at the surface in a depth of 40 m (Fig. 1a, b). The parrotfish were initially sighted at 1355 h and swam at the surface until 1410 h when the entire school descended to the reef at a depth of 20 m. The school was 3-4 m in diameter and consisted of approximately 800 individuals ranging in size from 15 to 25 cm total length. Most fish displayed pale, initial-phase coloration, but two terminal-phase individuals were also sighted swimming in the school (Fig. 1c). The school was easily approached by an 8-m boat and made no attempt to flee or alter course. Predators were abundant in the area of observation, with numerous large (87-121 cm Fork Length) giant trevally (Caranx ignobilis) seen within 200 m of the parrotfish, but none observed attacking the school. The only predators observed foraging on the school were six Great Frigate birds (Fregata minor palmerstoni), of which one was seen capturing a fish.The parrotfish behavior bore clear hallmarks of a spawning migration. The school was sighted 4 days after the full moon (27 May 2010 at 2307 h), and many reef fishes including other parrotfishes (Gladstone 1996) are known to aggregate for spawning around the full moon. The school was easily approachable, which is another characteristic of otherwise wary reef fishes immediately before and during spawning (Johannes 1981). Swimming at the surface in a predator-rich environment seems risky but our direct observations show predation can be minimal during such events. To our knowledge, this is the first time this phenomenon has been documented in parrotfishes. ReferencesGladstone W (1996) Unique annual aggregation of longnose parrotfish (Hipposcarus harid) at Farasan Island (Saudi Arabia, Red Sea).
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