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, 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 with appropriate and specific direction to the original content.

“You live 6 weeks and I live 6 months”

Says the winter bee to the summer bee

I suppose the Nobel prize winner Bob Dylan did not think of the honey bee, when he penned the lyrics to his song “Forever Young”, but when we compare the longevities between summer and winter honey bees, it almost seems as if the long-lived winter bees stay forever young: they stay around from about October until at least March while the short-lived summer bees are lucky to reach an age of 6 weeks. The reason is not that during the summer, bees are more susceptible to disease, preyed upon more heavily or work so much harder than the winter bees, for even though winter bees spend the entire winter in the hive, they too work hard. They shiver to create heat and make sure that the cluster of bees surviving the cold season with their queen does not become colder than 15°C. But how can they possibly stay active (and alive) for so many months longer than the summer bees?

First of all, the members of a bee hive need to sense that the cold season is approaching. This may involve the increasingly difficult task of finding pollen in August to feed the brood. It follows that brood rearing becomes reduced and eventually stops until it resumes in January. With no foraging summer bees needed, they all die in the autumn and are replaced by the morphologically identical winter bees. The latter begin to store large amounts of ‘vitellogenin’, (a glycolipoprotein known to be a zinc carrier) in their fat body, the equivalent of our liver. In the winter this substance also increases in the haemolymph, the equivalent of our blood.  At the same time, the so-called ‘juvenile hormone’, important during the nursing of the brood, decreases to almost undetectable levels. The amount of a neuromodulator molecule from the brain, known as octopamine, also falls when foraging is no more possible in winter.

The nutrients stored by the winter bees in the form of vitellogenin are not used by them for themselves, but are kept to be utilised when new bee brood is produced again by the queen bee in anticipation of the coming spring with its flowers and sources of protein-rich pollen. How the queen bee ‘knows’ that spring will approach when she starts to lay eggs anew in the middle of winter, the coldest and darkest season of the year, is still a mystery, but how the winter bees keep themselves and their queen warm is much better understood. In the ball-like cluster that the winter bees form, those at the periphery every now and then exchange places with bees from the warmer centre and then also to replenish their food intake. After all, to create heat by shivering, means that they are expending a lot of energy.

The ‘fuel’ the bees use to warm themselves and the hive up is sugar!  It’s the main or perhaps the only reason why they collected nectar and turned it into honey during the summer: the carbohydrates keep them alive throughout the winter, so that in spring they can feed the new brood with the vitellogenin they stored in their fat body. Actually, rearing new brood already starts in late winter when pollen are not yet available and the trigger for the transition from winter to short-lived summer bees is, therefore, not at all understood. Perhaps the decrease in vitellogenin, needed to feed the new brood, causes a concomitant rise in juvenile hormone and octopamine. The fact is that all winter bees, irrespective as to when they left their pupal cases, whether that was in October and the early months in winter or as late as January, they all die within a very short time. Forever young? Well maybe not forever, but applied to humans it would be a life of 400 years! And if that’s not long enough, how about the rotifer, a tiny wheel animalcule, an extreme “Rip Van Winkle” of the animal world? Dormant in a frozen state for 24,000 years, it was revived recently by the Russian soil scientists Lyubov Shmakova and colleagues of Pushchino in Russia, and it even multiplied (it did not need a male). A multicellular organism with a gut, a nervous system, sense organs, frozen stiff, for 24,000 years: what a life –  if you can call that a life at all.

© Dr V.B. Meyer-Rochow and, 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 with appropriate and specific direction to the original content.

zoology biology benno meyer rochow science blog spermatozoa

Some Scientists’ Favourites – Spermatozoa

Spermatozoa – a scientist’s favourite cell

People have preferences: favourite colours, favourite dishes, favourite authors, favourite this and that. Scientists, in addition, may have “favourite cells” as I have discovered early in my career. The American researcher Charles Brokaw in an article titled “My favourite Cell” had revealed in it that his was the sea-urchin spermatozoon. But he is not the only one who finds spermatozoans fascinating. For decades the Italian Baccio Baccetti and co-workers had been “at it” and were the first to report an in-depth study of backward swimming spermatozoans. To be precise, the sperm cells of the two species of fruit fly that were looked at did not exclusively swim backward (they use the forward gear when it comes to penetration of the egg cell), but the ability to reverse had only been reported once before. —>