Internal Mechanism of Rorqual Feeding

This study elucidated the internal mechanism that enables rorquals (Balaenopteridae) to engulf a large volume of water prior to filtering it for prey. Anatomic experiments and observations were made on specimens of Balaenoptera acutorostrata, B. borealis , and B. physalus to assess the dynamic relationships of the tongue, nonlingual intermandibular lining, and cavum ventrale (a tissue cleft beneath the grooved, ventral body wall) as they would occur during engulfment. With the mouth empty, a distinct, centrally furrowed tongue was present in all three species. The nonlingual intermandibular lining ran outwards from the root of the tongue to attach along the ventral border of the mandibles. The cavum ventrale extended anteriorly beneath the tongue and nonlingual intermandibular lining from the cervico-thoracic region. By simple manipulation, the tongue could be transformed into a hollow, sac-like structure invaginating the cervical portion of the cavum ventrale. During this transformation, the nonlingual intermandibular lining was stretched posteriorly to completely cover the floor of the mouth. When the mouth of a suspended B. acutorostrata specimen (head only) was filled with water to simulate engulfment the tongue and nonlingual intermandibular lining expanded through the transected space of the cavum ventrale to form a capacious “oral sac.” The posteriodorsal wall of this sac was formed by what had been the tongue. Adaptations that enhance the expansibility of the oral sac include a highly elastic connective fiber matrix and a redundant surface created by epithelial corrugations and the tongue's central furrow. It is concluded that 1) during engulfment the tongue would initiate distension of the cavum ventrale by everting through the floor of the mouth between the cavum's inner and outer walls, 2) the everted, elastic tongue would act to enlarge the capacity of the mouth, and 3) the sac lining of the cavum ventrale at the end of engulfment would be created by the tongue and nonlingual intermandibular lining.