Background The chemically-rich seaweed Galaxaura is not only highly competitive with corals, but also provides substrate for other macroalgae. Its ecology and associated epiphytes remain largely unexplored. To fill this knowledge gap, we undertook an ecological assessment to explore the spatial variation, temporal dynamics, and diversity of epiphytic macroalgae of Galaxaura divaricata on patch reefs in the lagoon of Dongsha Atoll, a shallow coral reef ecosystem in the northern South China Sea that has been repeatedly impacted by mass coral bleaching events. Methods Twelve spatially independent patch reefs in the Dongsha lagoon were first surveyed to assess benthic composition in April 2016, and then revisited to determine G . divaricata cover in September 2017, with one additional Galaxaura -dominated reef (site 9). Four surveys over a period of 17 months were then carried out on a degraded patch reef site to assess the temporal variation in G . divaricata cover. Epiphytic macroalgae associated with G . divaricata were quantified and identified through the aid of DNA barcoding at this degraded site. Results Patch reefs in the Dongsha lagoon were degraded, exhibiting relatively low coral cover (5–43%), but high proportions of macroalgae (13–58%) and other substrate (rubble and dead corals; 23–69%). The distribution of G . divaricata was heterogeneous across the lagoon, with highest abundance (16–41%) in the southeast area. Temporal surveys showed consistently high covers (mean ± SD = 16.9 ± 1.21%) of G . divaricata for 17 months. Additional photographic evidence suggested that overgrowth of G . divaricata can persist for 3.5 years. Yet, G . divaricata provides substrate to other macroalgae (e.g., Lobophora sp.) that also limit the growth of corals. Conclusions Our study demonstrates that an allelopathic seaweed, such as G . divaricata , can overgrow degraded coral reefs for extended periods of time. By providing habitat for other harmful macroalgae, a prolonged Galaxaura overgrowth could further enhance the spread of macroalgae, and strengthen negative feedback loops, decreasing the recovery potential of degraded reefs.

The crustose calcareous alga, Ramicrusta textilis Pueschel and G.W. Saunders (Peyssonneliaceae, Rhodophyta), was recently observed to overgrow dead and living corals in shallow lagoon patch reefs of Dongsha Atoll, South China Sea. DNA barcoding revealed that R. textilis frequently co-occurred in a complex with other crustose algae of the family Peyssonneliaceae and the genus Lobophora, forming patchy crusts that cover 3%-29% of the benthic substrate. This is the first record R. textilis overgrowing corals in the Pacific Ocean.

In degraded coral reef ecosystems, allelopathic macroalgae have received increasing attention from marine ecologists because their secondary metabolites (also known as allelochemicals) kill corals that grow adjacent to them and weaken the recovery of degraded reefs. One well‐known coral‐killing macroalga is the calcareous red seaweed Galaxaura . However, our knowledge of how coral reef fishes interact with allelopathic algae like Galaxaura is very limited. Here, we documented novel observations of feeding interactions of 17 species of coral reef fishes (herbivorous and carnivorous) with the filamentous Galaxaura divaricata on degraded lagoon patch reefs in Dongsha Atoll (South China Sea). Video analyses showed that territorial farming damselfishes (i.e., Dischistodus perspicillatus , D. prosopotaenia , Hemiglyphidodon plagiometopon , Pomacentrus grammorhynchus , P. adelus , and Neoglyphidodon nigroris ) and juvenile parrotfishes ( Scarus schlegeli , S. ghobban , S. rivulatus , and Chlorurus spilurus ) likely used G. divaricata as a feeding substratum. Further, microscopic analyses revealed that the filamentous surface of G. divaricata harbored a wealth of epiphytic microalgae, such as filamentous cyanobacteria (i.e., Leptolyngbya , Lyngbya , Rivularia , Oscillatoria , and Stigonema ), diatoms (i.e., Synedra , Nitzschia , Mastogloia , and Pleurosigma ), and filamentous red algae (i.e., Heterosiphonia ), suggesting that these fishes targeted the nutrient‐rich microscopic epiphytes rather than the nutrient‐poor host. Juvenile benthic carnivores (i.e., Labridae, Parupeneus multifasciatus , and Meiacanthus grammistes ) form feeding assemblages with roving parrotfishes to feed on small invertebrates (i.e., amphipods, copepods, isopods, gastropods, and polychaetes) associated with G. divaricata . Given that coral reef fishes appear to target the epiphytes associated with Galaxaura rather than the alga itself, these observations thus substantiate the threat posed by the overgrowth of G. divaricata to coral recovery in degraded reef systems due to the lack of natural grazers.

In all mammals, the superior olivary complex (SOC) comprises a group of auditory brainstem nuclei that are important for sound localization. Its principal nuclei, the lateral superior olive (LSO) and the medial superior olive (MSO) process interaural time and intensity differences, which are the main cues for sound localization in the horizontal plane. Toothed whales (odontocetes) rely heavily on hearing and echolocation for foraging, orientation, and communication and localize sound with great acuity. The investigation of the SOC in odontocetes provides insight into adaptations to underwater hearing and echolocation. However, quantitative anatomical data for odontocetes are currently lacking. We quantified the volume, total neuron number, and neuron density of the LSO of six common dolphins (Delphinus delphis) using the Cavalieri principle and the unbiased stereology optical fractionator. Our results show that the LSO in D. delphis has a volume of 150 + (SD = 27) mm3, which is on average 69 (SEM = 19) times larger than the LSO in human, or 37 (SEM = 11) times larger than the human LSO and MSO combined. The LSO of D. delphis contains 20,876 ± (SD = 3300) neurons. In comparison, data reported for the human brainstem indicate the LSO has only about ¼ that number but about the same number for the LSO and MSO combined (21,100). LSO neurons range from 21 to 25 μm (minor axis) and from 44 to 61 μm (major axis) in transverse sections. The LSO neuron packing density is 1080 ± (SD = 204) neurons/mm3, roughly half of the LSO neuron density in human. SMI‐32‐immunohistochemistry was used to visualize projection neurons in the LSO and revealed the presence of principal, marginal, and multipolar neurons in transverse sections. The distinct morphology of the LSO likely reflects the common dolphin's superb sensitivity to ultra‐high frequencies and ability to detect and analyze sounds and their location as part of its underwater spatial localization and echolocation tasks.

6Background: Red calcareous seaweeds of the genus Galaxaura have been associated with coral