N° 7 2021 
3ottlenose dolphins off the Spanish coast. 
In general, marine mammals hear in a relatively wide bandwidth, 
much beyond what humans can hear, which is between 20 Hz and 
20 kHz. Whales can likely hear infrasound (below 20 Hz) and dolphins 
and other toothed whales can hear ultrasound up to around 200 
kHz. In 2008, the degree of uncertainty on marine mammal hearing 
was ‘moderate’ (Boyd et al., 2008), presumably because several 
studies had been undertaken with captive animals. Hearing tests 
had been conducted on toothed whales (e.g. porpoises, dolphins) 
and as well as sea lions and seals, but not on baleen whales such as 
blue, fin or humpback whales, due to their size and the associated 
challenge of keeping them in captivity or experimental settings. 
Recently, anatomical studies using CT scans have been used to 
predict hearing sensitivities in baleen whales (Cranford & Krysl, 
2015). Numerous hearing studies have been carried out since 2008 
an toothed whales and pinnipeds, including new species and by 
increased sample sizes in species already studied (Erbe et al., 2016; 
Southall et al., 2019). One important finding was that some toothed 
whales have the ability to adjust their hearing sensitivity depending 
on incoming sounds; a phenomenon known as ‘auditory gain 
control’ (Nachtigall & Supin, 2014; Southall et al., 2019). 
Hearing in fishes has continued to be studied during the past ten 
years, however substantial gaps remain. This is partly because much 
ofthe data focuses on sound pressure and not particle motion, which 
as we have pointed out already is more important for most fishes. 
Another source of uncertainty is that most audiogram** studies 
use electrophysiological approaches to measure the response 
of the ear to lower levels of the central auditory system, whilst 
behavioural studies are thought to be the more valid measures of 
hearing sensitivity in fishes. Thus, our understanding of fish hearing 
is still limited (Popper et al., 2014; Popper & Hawkins, 2021). One 
Aeld of progress was that anatomical ‘hearing types’ were better 
specified in the past 10 years. Accordingly, fish hearing types can be 
arranged on a graded scale depending on anatomical adaptations. 
Fish species such as flatfish or elasmobranchs (e.g. sharks and rays) 
"hat lack a swim bladder are only sensitive to particle motion over a 
small bandwidth of a few hundred Hz. Species with a swim bladder, 
such as cod, that are sensitive to sound pressure in addition to 
particle motion, still exhibit a rather limited bandwidth of hearing. 
Finally, some fishes have special anatomical adaptations connecting 
the swim bladder with the inner ear. Herring, for example, can hear 
sound pressure over a wide bandwidth and are relatively sensitive 
to sound. Compared to marine mammals, fishes hear over a much 
smaller bandwidth and are more or less restricted to hearing 
sounds with frequencies of up to a few kHz at most (ie. in the lower 
frequencies; see Popper & Fay, 2011; Popper et al., 2014). 
Recent research on crustaceans and cephalopods shows that they 
can sense particle motion (and perhaps also pressure) via sensory 
hairs (inside small sack-like structures called statocysts) on their 
body and in body cavities. Their hearing is limited to relatively low 
requencies (i.e. up to a few kHz; see Popper et al., 2001; Sole et al, 
2013; Hughes et al., 2014; Radford et al., 2016). Jellyfish are also able 
to detect low-frequency sound (Sole et al., 2016). 
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