Testing sea urchin and green sea turtle consumption of the allelopathic macroalga Galaxaura divaricata.

Galaxaura divaricata is a partially calcified macroalga that hampers coral recruitment, growth, and recovery via the excretion of allelopathic secondary metabolites. Herbivorous fishes are not major consumers of Galaxaura spp. and there is a need to understand feeding preferences for Galaxaura divaricata in other macroherbivores, like sea urchins and green sea turtles that could act as potential controlling agents. Under certain environmental conditions, G. divaricata can proliferate and overgrow degraded reefs for several years, as documented for several coral patch reefs in the lagoon of Dongsha Atoll, South China Sea. This study aimed to experimentally test the feeding preferences of five species of sea urchin and two individual green sea turtles, Chelonia mydas, for G. divaricata. Specifically, we quantified and compared the consumption rates of the allelopathic G. divaricata with Gracilaria edulis, a nonallelopathic, fleshy red alga, known to be highly favored by herbivores. Results showed that the five urchin species fed on both G. edulis and G. divaricata. However, urchins consumed 2-8 times less wet weight of G. divaricata (range 0.3-3.1 g urchin-1 24 h-1) compared to G. edulis (range 0.6-18 g urchin-1 24 h-1), suggesting that urchin grazing may exert some control on G. divaricata abundance but is likely ineffective for a large-scale removal of the alga. Further, both green sea turtles avoided G. divaricata and selectively fed on G. edulis. More experiments are needed to test the potential role of herbivores in controlling the overgrowth of coral competitive and allelopathic macroalgae, like Galaxaura on coral reefs.

Comparison of acoustic particle acceleration detection capabilities in three shark species.

Behavioural studies have shown that sharks are capable of directional orientation to sound. However, only one previous experiment addresses the physiological mechanisms of directional hearing in sharks. Here, we used a directional shaker table in combination with the auditory evoked potential (AEP) technique to understand the broadscale directional hearing capabilities in the New Zealand carpet shark (Cephaloscyllium isabellum), rig shark (Mustelus lenticulatus) and school shark (Galeorhinus galeus). The aim of this experiment was to test if sharks are more sensitive to vertical (z-axis) or head-to-tail (x-axis) accelerations, and whether there are any differences between species. Our results support previous findings, suggesting that shark ears can receive sounds from all directions. Acceleration detection bandwidth was narrowest for the carpet shark (40-200 Hz), and broader for rig and school sharks (40-800 Hz). Greatest sensitivity bands were 40-80 Hz for the carpet shark, 100-200 Hz for the rig and 80-100 Hz for the school shark. Our results indicate that there may be differences in directional hearing abilities among sharks. The bottom-dwelling carpet shark was equally sensitive to vertical and head-to-tail particle accelerations. In contrast, both benthopelagic rig and school sharks appeared to be more sensitive to vertical accelerations at frequencies up to 200 Hz. This is the first study to provide physiological evidence that sharks may differ in their directional hearing and sound localisation abilities. Further comparative physiological and behavioural studies in more species with different lifestyles, habitats and feeding strategies are needed to further explore the drivers for increased sensitivity to vertical accelerations among elasmobranchs.

Comparison of auditory evoked potential thresholds in three shark species.

Auditory sensitivity measurements have been published for only 12 of the more than 1150 extant species of elasmobranchs (sharks, skates and rays). Thus, there is a need to further understand sound perception in more species from different ecological niches. In this study, the auditory evoked potential (AEP) technique was used to compare hearing abilities of the bottom-dwelling New Zealand carpet shark (Cephaloscyllium isabellum) and two benthopelagic houndsharks (Triakidae), the rig (Mustelus lenticulatus) and the school shark (Galeorhinus galeus). AEPs were measured in response to tone bursts (frequencies: 80, 100, 150, 200, 300, 450, 600, 800 and 1200 Hz) from an underwater speaker positioned 55 cm in front of the shark in an experimental tank. AEP detection thresholds were derived visually and statistically, with statistical measures slightly more sensitive (∼4 dB) than visual methodology. Hearing abilities differed between species, mainly with respect to bandwidth rather than sensitivity. Hearing was least developed in the benthic C. isabellum [upper limit: 300 Hz, highest sensitivity: 100 Hz (82.3±1.5 dB re. 1 µm s-2)] and had a wider range in the benthopelagic rig and school sharks [upper limit: 800 Hz; highest sensitivity: 100 Hz (79.2±1.6 dB re. 1 µm s-2) for G. galeus and 150 Hz (74.8±1.8 dB re. 1 µm s-2) for M. lenticulatus]. The data are consistent with those known for 'hearing non-specialist' teleost fishes that detect only particle motion, not pressure. Furthermore, our results provide evidence that benthopelagic sharks exploit higher frequencies (max. 800 Hz) than some of the bottom-dwelling sharks (max. 300 Hz). Further behavioural and morphological studies are needed to identify what ecological factors drive differences in upper frequency limits of hearing in elasmobranchs.

Operant conditioning as a tool to assess hearing abilities in sharks.

Sharks (elasmobranchs) are an ancient, diverse group of fishes, representing a basal stage in the evolution of vertebrate hearing. Yet, our understanding of behavioural measures of hearing abilities in sharks is limited. To address this, an operant conditioning paradigm was designed, and scalloped hammerhead Sphyrna lewini and rig (spotted estuary smooth hound) Mustelus lenticulatus were successfully trained to respond to pure-tone acoustic stimuli from an underwater speaker. After 2-3 weeks of training, both species showed distinctive responses to these acoustic stimuli and retained this behaviour when reinforced. S. lewini responded to a 400 Hz pulsed tone with an abrupt increase in tailbeat frequency (97 beats per 30 s vs. 69 beats for a 2 kHz control and 70 beats for no signal) and sustained vigorous swimming (arousal response) for at least 30 s. In response to a 200 Hz pulsed tone, M. lenticulatus visited a target area under the speaker significantly more frequently (13.4 ± 4.3 times per minute vs. 1.4 ± 1.5 times for a 1.2 kHz control and 0.9 ± 0.01 times for no signal) and swam circles under the speaker to search for food. The authors used S. lewini arousal responses to pure-tone stimuli of 40, 80, 200, 400, 600 and 800 Hz to generate a provisional hearing-threshold curve. The results show that S. lewini adapts to low-frequency hearing (greatest sensitivity at 200 Hz, upper limit 800 Hz), which is like other coastal pelagic sharks that have been investigated so far. Despite challenges operant acoustic conditioning studies are a viable method for revealing auditory capabilities of sharks.

Novel field observations of coral reef fishes feeding on epiphytic and epizoic organisms associated with the allelopathic seaweed Galaxaura divaricata.

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.

Morphology and unbiased stereology of the lateral superior olive in the short-beaked common dolphin, Delphinus delphis (Cetacea, Delphinidae).

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 » 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.

Filamentous calcareous alga provides substrate for coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll, Taiwan.

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.