Some Animals Can Do It

Wouldn’t it be nice if we could do it too and close our ears

The daughter of one of my friend’s has recently learned to swim  –  and she loves it. But what she does not love, is having to shake out the water in her ear after a swim. So, she asked me why we can’t close our ears. Nice question, and I answered I don’t think dogs and cats can close their ears either when they swim and whales and harbour seals (so-called phocids) don’t have any ears at all  -well, at least those parts of the ear that you can see, i.e. the outer part, the so-called “pinna”. But it made me think, because in Antarctica I was surrounded by fur seals (so-called otariid seals) and they all had small, but very prominent ears. Besides, even though whales and phocids lack the pinna, they can certainly hear quite well under water and possess all the inner ear structures (like ear canal, middle ear and cochlea ) that are similar to even our own ears.

All diving animals, whether they be whales, phocids and even walruses (none of which possessing external ears) or whether they be those with ears such as otariid fur seals and sea lions or otters and hippopotamuses or the aquatic insectivores known as desmans or egg-laying platypuses: they all can hear under as well as above water. However those with visible ears have means to prevent water from entering their ears and hippopotamuses, for example, angulate their small ears backward and close the ear canal by contraction when they dive; desmans achieve the same result by glandular swellings to seal up their ears temporarily and eared seals like fur seals are capable of controlling the state of their ear canals by muscles when diving or when in air. Rising pressure expels the air that happens to be trapped in the outer ear when the fur seal dives and improves underwater hearing. 

In the aquatic mammals that lack an outer and visible ear like whales, harbour or hooded seals and walruses, adaptations to hearing under water are a little different. There is no need to close the outer ear, because there isn’t one. However, hollow structures like ear canals and the air-filled middle ear (connected via the Eustachian tube to the respiratory tract) need to be protected against a collapse during a dive owing to the increasing pressure.  During dives the tissues around the external ear canal and the middle ear fill with blood to occupy any air spaces and as the air spaces get smaller and smaller with depth, hearing under water improves. In phocid seals the middle ear bones are less separated from the skull than those of the fur seals, and that enhances sound amplification, but fur seals are better in determining the direction of a sound under water. The underwater sounds of the latter range from about 1000 to 4000 Hz, but those of whales and phocids cover ranges from 40 to 8 kHz and 100 to 15 kHz, respectively. Some species of dolphins (with very long ear canals) can even hear frequencies across the enormous range of about 100 to 150 kHz.

Other adaptations include copious amounts of earwax (especially in the ears of walruses) and elastic fibres in the walls of the Eustachian tube. An interesting adaptation to strengthen the ear canal lateral to the eardrum (the well known tympanic membrane that causes the problem in our ears when it’s blocked and we are in an aeroplane or are diving) are the so-called exostoses and the latter are thought to facilitate deep dives into very cold water. Exostoses do sometimes also occur in humans, where they are often referred to as “surfer’s ear” and involve benign, non-tumorous, firm and sessile, often bilaterally symmetric, nodular bony growths within the ear canal. Their occurrence seems to be closely connected with the amount of time a person spends surfing or diving in cold seawater. Can this similarity to phocid seals and other diving animals be used by people who champion the idea that during the evolution of humans there was an aquatic period? It’s a long shot and I doubt it very much. But what this blog has shown once again is that responding to a child’s question is something not to dodge, because it makes you think. And that can only be something positive.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

You Don’t See Them, but They Sure Are There

The Ears that Fish Have

That fish can hear was documented nearly 100 years ago by Nobel Prize winner Karl v. Frisch, famous for his work on honeybee communication. Freshwater fish of the carp family, like most bony fishes, not only possess swim bladders that allow them to stay buoyant at different depths, but unlike others also possess small bony connections between the fish’s inner ear and the swim bladder. The gas in the latter gets compressed in response to sound pressure in the water, starts vibrating, and then transmits the signal via the aforementioned small bones, known as Weberian ossicles, to sensitive hair cells of the inner ear. Sound amplitude (loudness) and frequency (pitch) are important, but even in fish with the best hearing, sounds above 6,000 Hz would be ultrasound to them and in swim bladder-lessfish like the fast mackerel and the tuna but even in the more sluggish Antarctic icefishes only lower frequencies can be detected.

Sounds are longitudinally transmitted waves, whose frequencies and amplitudes may vary. Because of the water’s greater density than that of air, sounds are propagated 4.8 times faster in water than in air. In a broad sense sounds are generated by movements or vibrations and in water can be the results of an animal’s vocalizations or activities, of sounds created by ice-floes or logs rubbing or bumping against each other, breaking waves, anthropogenically-produced noise like explosions and disturbances created by ships. In order to sense the sounds, fish use ear stones, i.e. so-called otoliths. Bathed in endolymphatic fluid and resting on a pad of receptor cells with sensory hairs, the two major otoliths are located just below the brain in two bony sacs of the inner ear known as the saccule and the utricle. The otoliths are part of the bony vestibule’s two regions, i.e. the cochlear portion (for hearing) and the vestibular portion with its semicircular canals (for balance and angular change). Thus, otoliths can be said to be involved in the detection of gravity and linear accelerations and serve as a structure of hearing in fish, so well explained in a recent review by Dr. Tanja Schulz-Mirbach of Munich.

Otoliths are hard, durable structures that consist primarily of calcium carbonate (CaCO₃) in the form of aragonite.They remain largely unchanged during the digestion in the stomach and gut of a predator. They are thus an excellent structure to estimate a fish’s age, because their size increases by periodically laid down alternating opaque and translucent bands that consist of CaCO₃ and collagen fibres. As daily increments are regularly added, researchers can correlate the number of layers with the fish’s body length and use the tabulated data to identify the fish’s age. What makes the study complicated is that the otoliths, not being translucent enough to count the layers, need to be sectioned. Furthermore, although the shapes of the otoliths are species-specific, they can vary in individuals of the same species, depending on the fish’s developmental stage and if the fish was actively swimming or passively drifting.

In Antarctic icefish my Polish colleague Ryszard Traczyk and I have recently concluded that the more spherical otoliths of larval specimens and the longish otolith shapes of the adults are the results of the inertia and friction experienced by the otoliths in their endolymphatic fluid when the fish swim: larvae swim less than adult icefish and the latter swim less than mackerel (which possess the most elongate otoliths). It is entirely possible that oscillations of the dense otoliths generate shearing forces that deflect the sensory hairs of the cells they are resting on, when the oscillations are due to disturbances in the water made by nearby prey or the approach of a predator. Responses to such disturbances in the “acoustico-lateralis” vicinity of the fish would then not only be sensed by the lateral line system, but picked up by the fish’s otoliths too and sent to the brain via the 8th cranial nerve, often referred to as the vestibulocochlear nerve. So, do fish make some noise and can they hear? Actually only a few produce sounds, but all bony fish can hear. However, there’s certainly no need to whisper when you sit in front of your aquarium and watch your colourful aquatic beauties in their 3-dimensional world.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

How did Ludwig Von Bertalanffy Prevent Fish Species being Over-Exploited?

As more and more consumers turn away from meat, especially that of mammals, they do, however, turn to fish. Consequently, there is increasing pressure on fish stocks in the wild, but a growing opportunity for fish culturists to improve fish rearing facilities. When I was still a student of Fisheries Science in 1967 and lectures I attended dealt with fish stock assessments, catch per unit effort, fish populations, age structures, longevities and survival rates of fishes, time and again Ludwig Von Bertalanffy was mentioned and equations he had developed were quoted and written with chalk on the blackboard (yes, chalk and blackboard in those days). But why were Von Bertalanffy’s calculations so useful and why are they still part of the backbone of fisheries assessments of fish stocks today?

Ludwig von Bertalanffy was born near Vienna in 1901 and although his parents got divorced when he was ten, he did enjoy a good home education until then, when he became a grammar school student. He had the famous anti-Darwinist Paul Kammerer as his neighbour and soon began to apply his mathematical interests to biology and the living world. He is now often regarded as the founder of General Systems Theory, which has inputs from thermodynamics, cybernetics and biology. At the University of Vienna his fields of expertise could be called Theoretical Biology and Philosophy and in 1937 he got a Rockefeller scholarship to work in the USA. When he failed to secure immigrant status, he returned to Vienna in 1938 and joined the Nazi-party. After the war he found living in Austria difficult, moved to the University of London in 1948 and from there two years later to appointments at various American and Canadian universities. He died in 1972 in Buffalo, New York.

In his biological research, Von Bertalanffy was interested in psychology, psychiatry, development and growth phenomena and concluded that thermodynamic principles worked well in closed systems, but not in open systems like those comprising living organisms. He came up with a simple growth equation for biological organisms that models mean length of animals in relation to age:  L(a) = L_∞ [1 – exp (-k(a – a₀))],  where a is age, k is the growth coefficient, a₀ is the value used to calculate size when age is zero and L_∞ is asymptotic size (which means the rate of growth continually decreases as an individual ages but never completely stops). The equation above is the solution of the linear differential equation:  dL/da = k(L_∞ – L) and applicable to organisms that do not cease to grow when adult (unlike, for instance humans, which actually shrink when reaching old age), but keep growing albeit at increasingly slower and smaller rates as they age. Fish are some of these animals and since it is important for fisheries biologists to know at what age (or body length) individuals of a species become reproductive and therefore should not be ‘harvested’ until old enough to have reproduced at least once, it’s obvious that much emphasis has been placed on Von Bertalanffy’s growth curve that relates age to body lengths.

To make this relationship ‘work’, it is crucial to know how old a fish is at any given length. Helping fisheries scientists in this matter are age-rings on the scales of fish (not unlike those that one uses to age trees). The problem is that not all fishes live in climatic zones in which there are distinct seasonal changes that result in age rings on the scales and secondly not all fish species even have scales. In my research with the Polish scientist R. Traczyk, we worked with Antarctic icefish that have no scales and live in constantly ice-cold water. In such cases one uses daily increments of extremely narrow CaCO₃ layers, visible in sectioned ear-stones of the fish examined under the microscope. The layers provide an accurate estimation of the fish’s age that can be correlated with the fish’s total body length. What is then still left to discover is at what age and body length these fish spawn. For that to find out, fish have to be trawled near spawning grounds and females must be measured and examined as to whether they still have mature eggs in their ovaries or had already spawned. Once all the essential data are in, one can use the Von Bertalanffy growth curve to make recommendations to the fishing industry at what size it is ‘safe’ to harvest and market a species without depleting the population of younger and still immature specimens. Although Von Bertalanffy’s work doesn’t save all fish from being ‘fished’, it does help to ascertain that there are still enough youngsters around to maintain the population.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.