Singing whales generate high levels of particle motion: implications for acoustic communication and hearing?

Acoustic signals are fundamental to animal communication, and cetaceans are often considered bioacoustic specialists. Nearly all studies of their acoustic communication focus on sound pressure measurements, overlooking the particle motion components of their communication signals. Here we characterized the levels of acoustic particle velocity (and pressure) of song produced by humpback whales. We demonstrate that whales generate acoustic fields that include significant particle velocity components that are detectable over relatively long distances sufficient to play a role in acoustic communication. We show that these signals attenuate predictably in a manner similar to pressure and that direct particle velocity measurements can provide bearings to singing whales. Whales could potentially use such information to determine the distance of signalling animals. Additionally, the vibratory nature of particle velocity may stimulate bone conduction, a hearing modality found in other low-frequency specialized mammals, offering a parsimonious mechanism of acoustic energy transduction into the massive ossicles of whale ears. With substantial concerns regarding the effects of increasing anthropogenic ocean noise and major uncertainties surrounding mysticete hearing, these results highlight both an unexplored pathway that may be available for whale acoustic communication and the need to better understand the biological role of acoustic particle motion.

Coral reef soundscapes may not be detectable far from the reef.

Biological sounds produced on coral reefs may provide settlement cues to marine larvae. Sound fields are composed of pressure and particle motion, which is the back and forth movement of acoustic particles. Particle motion (i.e., not pressure) is the relevant acoustic stimulus for many, if not most, marine animals. However, there have been no field measurements of reef particle motion. To address this deficiency, both pressure and particle motion were recorded at a range of distances from one Hawaiian coral reef at dawn and mid-morning on three separate days. Sound pressure attenuated with distance from the reef at dawn. Similar trends were apparent for particle velocity but with considerable variability. In general, average sound levels were low and perhaps too faint to be used as an orientation cue except very close to the reef. However, individual transient sounds that exceeded the mean values, sometimes by up to an order of magnitude, might be detectable far from the reef, depending on the hearing abilities of the larva. If sound is not being used as a long-range cue, it might still be useful for habitat selection or other biological activities within a reef.

Ambient noise and temporal patterns of boat activity in the US Virgin Islands National Park.

Human activity is contributing increasing noise to marine ecosystems. Recent studies have examined the effects of boat noise on marine fishes, but there is limited understanding of the prevalence of this type of sound source. This investigation tracks vessel noise on three reefs in the US Virgin Islands National Park over four months in 2013. Ambient noise levels ranged from 106 to 129dBrms re 1µPa (100Hz-20kHz). Boat noise occurred in 6-12% of samples. In the presence of boat noise, ambient noise in a low-frequency band (100-1000Hz) increased by >7dB above baseline levels and sound levels were significantly higher. The frequency with the most acoustic energy shifted to a significantly lower frequency when boat noise was present during the day. These results indicate the abundance of boat noise and its overlap with reef organism sound production, raising concern for the communication abilities of these animals.

Repeated call types in Hawaiian melon-headed whales (Peponocephala electra).

Melon-headed whales are pantropical odontocetes that are often found near oceanic islands. While considered sound-sensitive, their bioacoustic characteristics are relatively poorly studied. The goal of this study was to characterize the vocal repertoire of melon-headed whales to determine whether they produce repeated calls that could assist in recognition of conspecifics. The first tag-based acoustic recordings of three melon-headed whales were analyzed. Tag records were visually and aurally inspected and all calls were individually extracted. Non-overlapping calls with sufficient signal-to-noise were then parameterized and visually grouped into categories of repeated call types. Thirty-six call categories emerged. Categories differed significantly in duration, peak and centroid frequency, and -3 dB bandwidth. Calls of a given type were more likely to follow each other than expected. These data suggest that repeated calls may function in individual, subgroup, or group recognition. Repeated call production could also serve to enhance signal detection in large groups with many individuals producing simultaneous calls. Results suggest that caution should be used in developing automatic classification algorithms for this species based on small sample sizes, as they may be dominated by repeated calls from a few individuals, and thus not representative of species- or population-specific acoustic parameters.

Adverse effects of ocean acidification on early development of squid (Doryteuthis pealeii).

Anthropogenic carbon dioxide (CO2) is being absorbed into the ocean, altering seawater chemistry, with potentially negative impacts on a wide range of marine organisms. The early life stages of invertebrates with internal and external aragonite structures may be particularly vulnerable to this ocean acidification. Impacts to cephalopods, which form aragonite cuttlebones and statoliths, are of concern because of the central role they play in many ocean ecosystems and because of their importance to global fisheries. Atlantic longfin squid (Doryteuthis pealeii), an ecologically and economically valuable taxon, were reared from eggs to hatchlings (paralarvae) under ambient and elevated CO2 concentrations in replicated experimental trials. Animals raised under elevated pCO2 demonstrated significant developmental changes including increased time to hatching and shorter mantle lengths, although differences were small. Aragonite statoliths, critical for balance and detecting movement, had significantly reduced surface area and were abnormally shaped with increased porosity and altered crystal structure in elevated pCO2-reared paralarvae. These developmental and physiological effects could alter squid paralarvae behavior and survival in the wild, directly and indirectly impacting marine food webs and commercial fisheries.