Female cusk-eel captured on Cape Cod, MA. Photo Credit: Rodney Roundtree.
Cusk eels (family Ophidiidae) are demersal fishes often found in temperate and tropical waters worldwide. They comprise one of the most widespread families of deep-sea fishes. Despite the name, cusk eels are not Anguilliformes (true eels). Unlike true eels, cusk eels have modified, forked fins along the throat area that contain sensory receptors used to detect potential prey along the ocean bottom. Solitary in nature, cusk eels often remain burrowed in the sediment or hidden in bottom features, emerging after sunset to feed. As cusk eels are nocturnally active, direct observation of their behavior is challenging. Monitoring their sounds via passive acoustic monitoring has been important in understanding species presence, distribution, and diversity.
Cusk eels produce pulsed underwater sounds in association with courtship and spawning. Diel, monthly, and seasonal patterns in calling behavior have been observed (Piccuilin et al. 2019, Mooney et al. 2016, Kever et al. 2016, Parmentier et al. 2010, Mann et al. 1997). Sounds are produced by contraction of swim bladder-associated sonic muscles. These muscles originate at the skull and insert onto modified vertebrae and rib structures that contact the swim bladder (Courtenay, 1971).
Image used with permission from: Fine, M. L. (2022). Fish Sound Production: The Swim Bladder. Acoustics Today, 18(3), 13. https://doi.org/10.1121/AT.2022.18.3.13
Male cusk eels form choruses during the reproductive season (Mooney et al. 2016). Characteristics of cusk eel sounds, including peak frequency and pulse period, appear to be influenced by environmental factors such as water temperature, lunar cycle, and seasonal changes in sunrise and sunset (Mooney et al. 2016, Kéver et al. 2016, Mann & Grothues 2009).
The striped cusk eel and the Roche’s snake blenny are two species of cusk eel for which acoustic behaviors are well described. The striped cusk eel, found in the western Atlantic, produces highly stereotyped, broadband pulses. These “chatter sounds” are produced in different bouts, grouped as 1-5 pulses, 6-14 pulses, and 16-27 pulses (Rountree and Bowers-Altman, 2002, Mann et al. 1997). Most striped cusk eel calling occurs at or just after dusk (Mann & Grothues 2009, Mann 1997). The onset of calling tracks the timing of sunset throughout the spawning season. In laboratory studies, male calling started just before sunset while the cusk eels were still in their burrows. Calling continued after the fish emerged from the sediment, peaking around one hour after sunset. Females often appeared to increase their calling activity in response to male sound production (Rountree and Bowers-Altman 2002, Mann et al. 1997).
Similar to the striped cusk eel, the Roche’s snake blenny, endemic to the Mediterranean, produces long trains of pulses with calling activity also beginning after dusk (Kever et al. 2016). Through a combination of field and laboratory studies, male, female, and juvenile sounds have been recorded (Parmentier et al. 2010, Kever et al. 2012, Kever et al. 2016). Male and female calls are sexually dimorphic. Males produce low frequency, non-harmonic, multiple-pulsed calls that last for several seconds and consist of two parts. The first part of the call contains a single train of pulses with increasing amplitude. The second part of the call has a longer duration and a pulse period that alternates between short and long periods. Two frequency peaks have been noted, one around 225-230 Hz, and the other around 410-435 Hz (Parmentier et al. 2010, Kever et al. 2012, Piccuilin et al. 2019).
Females produce pulsed, harmonic sounds with a shorter pulse period (±3.7 ms) and a fundamental frequency around 250 Hz. Juvenile sounds are weaker in intensity but are otherwise similar to female sounds (Kever et al. 2012). Structures involved in sound production (swim bladder, associated musculature, and associated skeletal structures) are significantly different among males, females, and juveniles (Kever et al. 2012). Female and juvenile structures were less complex than in adult males. Moreover, the anterior part of an adult male swim bladder thickens into a hard structure, the “rocker bone.” Sonic muscles insert onto this structure. The rocker bone was found only in adult male Roche’s snake blennies and may reflect a specialization for sound production during the reproductive season (Kever et al. 2016, Kever et al. 2012).