[en] Carapus acus and Carapus mourlani are able to live inside sea cucumbers and sea stars respectively. Unlike other carapids whose sounds have been recorded (C. boraborensis, C. homei and Encheliophis gracilis), these two species have a central constriction in their swimbladder and are unlikely to encounter heterospecific carapids within their hosts. We evoked sound production in Carapus acus and Carapus mourlani by adding several individuals to a tank with a single host and found that their sounds differ substantially from the sounds emitted by other carapids in pulse length, peak frequency and sharpness of tuning (Q(3 dB)). Unlike the other carapids, C. mourlani and C. acus produce shorter and less repetitive sounds and do not produce sounds when they enter their host. Since sounds produced within a sea cucumber have the potential to be heard by distant carapids and are typically recorded outside the sea cucumber, we examined the effect of the sea cucumber tegument on acoustic transmission. Attenuation by the tegument was negligible at the frequencies within carapid sounds. Therefore, carapids have the potential to call from the relative safety of a sea cucumber without sacrificing the distance over which their transmissions are heard.