Diving in darkness: whiskers as sense organs of the ringed seal (Phoca hispida saimensis)

Abstract

Underwater vocalization and the functional structure of different vibrissae of the ringed seal (Phoca hispida saimensis) of Lake Saimaa, Eastern Finland, were studied. These seals live in darkness under the ice cover for several months during the year. It is known that blind seals are managing well in the lake. Visibility under water in some parts of the area where the seals live is only 2 m. It is suggested that echolocation is used in orientation and feeding. The Saimaa seal has click and click trial underwater vocalizations. However, both the frequency and intensity of the vocalization are low compared with, for example, those of dolphins. The structural adaptations for underwater sound localization are also not well developed. The ringed seal has, however, extremely well‐developed vibrissae. The innervation of one vibrissa is more than 10 times greater than normally found in mammals. The main structural deviations from normal mammalian vibrissae are: (1) an upper cavernous sinus, (2) a groove in the wall of the capsule at the level of the lower cavernous sinus, (3) elasticity of the connective tissue bands fixing the hair root to the capsule in the lower cavernous sinus and especially (4) the structure and innervation of the ring sinus area. Sensory elements are situated upon the glassy membrane on the surface of the outer rootsheath and in the basal cell layer of the outer rootsheath which is like a sensory epithelium. Below this epithelium a layer of liquid or gelatinous material and large amounts of glycogen are found. This sensory epithelium is especially well developed in the superciliary vibrissae. These vibrissae are protruded some millimetres when the seals are attentive. It is suggested that the vibrissae also sense sounds, which are transmitted to the sensory elements by tissue conduction through the capsule wall and via the blood sinuses. The seals may possibly detect compressional waves with the vibrissae.

Microstructure and innervation of the mystacial vibrissal follicle‐sinus complex in bearded seals, Erignathus barbatus (Pinnipedia: Phocidae)

“…Other than differences in the length of the UCS, the vibrissal F-SC microstructure of bearded seals was generally similar to that of other pinnipeds, for which data are available (Ling, 1968(Ling, , 1972Stephens et al, 1973;Hyvä rinen and Katajisto, 1984;Hyvä rinen, 1989). The mean length of the UCS of bearded seals is similar to ringed seals (60% of the total F-SC length) (Hyvä rinen and Katajisto, 1984;Hyvä rinen, 1989Hyvä rinen, , 1995 and the UCS length of caudal F-SCs (62%) exceeded that of ringed seals.…”

“…The number of mystacial F-SCs of bearded seals is the highest of any phocid and second only to walruses among all pinnipeds (Mohr, 1950;Yablokov and Klevezal, 1964;Ling, 1968Ling, , 1972Ling, , 1977Stephens et al, 1973;Fay, 1982;Hyvä rinen and Katajisto, 1984;Hyvä rinen, 1989). The number of rows containing F-SCs in bearded seals is comparable to walruses (13-18 rows) (Fay, 1982).…”

“…Such abilities could explain cases of blind seals successfully foraging for fish, despite their lack of vision. It is hypothesized that the pinniped mystacial sensory system is an aquatic adaptation that allows this taxa to navigate and forage in conditions where vision is limited (Hyvä rinen, 1989;Dehnhardt, 1994). However, it is clear that there is a lot of diversity in the structure and function of vibrissal F-SC among pinnipeds.…”

“…This is particularly true between aquatic and terrestrial taxa. However, only a few F-SCs of pinniped species have been investigated at the light and electron microscopy level [southern elephant seal, Mirounga leonina, Ross seal, Ommatophoca rossii (Ling, 1968(Ling, , 1972, California sea lion (Stephens et al, 1973), ringed seal, Pusa hispida (Hyvä rinen and Katajisto, 1984;Hyvä rinen, 1989)]. Although pinniped vibrissae are similar to terrestrial taxa in many ways, distinct differences in the structure of pinniped F-SCs are immediately evident, even at a cursorial level.…”

“…First, the blood sinus complex of pinnipeds is tripartite; a ring sinus (RS) is juxtaposed between a lower cavernosal sinus (LCS; as in terrestrial species) and a novel upper cavernosal sinus (UCS) (Ling, 1968(Ling, , 1972Stephens et al, 1973;Hyvä rinen and Katajisto, 1984;Hyvä rinen, 1989). In ringed seals, this UCS comprises 60% of the total follicular length (Hyvä rinen, 1995).…”