Slugs that glide, use “smoke screens” - and can swim

Few people like slugs. While I consider them to be pests for eating too many of our garden plants, I rather like the forms that live in water and, especially Aplysia, the “sea hare”. 

Philip Henry Gosse, the great Nineteenth Century Natural Historian and populariser, encouraged readers of his books to discover the animals of the coast and he wrote, in Tenby, ¹ that specimens of Aplysia..

..crawled, or rather glided, along over the stones upon a fat fleshy disk or foot, and up the slender stems of Sea-weeds by bringing the edges of the same muscular foot to meet around the stem, grasping it thus, as if enclosed in a tube. The fore part, as the animal progressed, was poked forward as a narrow neck, furnished with two pairs of tentacles; one pair of which, standing erect, and being formed of thin laminae, bent round so as to bring the edges nearly into contact, looked like the long ears of a beast; whence this creature is called by the vulgar the Sea-hare (by naturalists, Aplysia punctata). On each side of the body, which is large and semi-oval, rises up a great fold of flesh (the mantle), which, arching over the back, is itself overlapped by its fellow on the opposite side. These wings are sometimes carried apart, exposing the back, and are said to be used as swimming fins; but this I cannot confirm from my own observation. 

Nine years later, in A Year at the Shore, ² Gosse is more dismissive about descriptions of swimming by Aplysia, writing:

It is reported that these mantle-lobes are capable of being used as swimming-fins, by their undulations; but I doubt the correctness of the observation.

 

As can be seen from the clip above, he was mistaken.  

Another feature of this slug, described by Gosse, ¹ is its ability to exude clouds of purple “fluid”:

Sometimes on being disturbed, and sometimes quite spontaneously, or at least without any visible cause, they would pour out from beneath the mantle-lobes a copious fluid of the richest purple hue, which stained the stones, the sands, and the water, of the same gorgeous tinge.. ..a few hours sufficed to remove all trace of it.

In both Tenby and A Year at the Shore, Gosse also describes the complex gut found in Aplysia, with chambers modified for holding food, milling it and then digesting the milled contents. He concludes the descriptions of the mollusc by saying:

..the whole organization affords us an instructive example of the Divine resources, and of the adaptation of organs to their requirements which an enlightened research is continually finding in Creation.

The sense of wonder is no less in those of us who consider that all the adaptations result from the selection of genetic mutations. Whether we believe in The Creation or in evolution, we can all agree that these slugs crawl by means of muscular waves passing over the foot; they can swim; they produce a dye to act as a diversion when disturbed; and they have a gut structure that is well adapted to their diet of strengthened seaweeds. The interesting question for an evolutionist is how did all these adaptations come about? What were the precursors, if precursors were present, or were there sudden mutations producing changes in the form and function we observe? It is easy to see how the gut structure enabled more efficient digestion of an abundant source of food, but how important was predation pressure in selecting for genetic changes that resulted in swimming and dye production? We do not know the answers, but pondering the possibilities is fascinating, although such thoughts would have been inconceivable to Gosse the Creationist.

I am an old-fashioned Zoologist, a Natural Historian, so looking at the wide diversity of animals and considering their adaptations and way of life comes as second nature to me. This approach, driven by a child-like curiosity, is now much less common in learned circles, as we move to a strongly anthropocentric and more “useful” Zoology. A contemporary student is unlikely to learn about the Natural History of Aplysia as described by Gosse, but be more familiar with the role the slug plays in applied research:

The marine mollusk Aplysia californica is an important animal for experiments in cellular, molecular and behavioral neuroscience because of the distinctive organization of its nervous system, which makes it appropriate for cellular analysis of a variety of behaviors, learning and memory. Aplysia presents a one-of-a-kind model for developmental and neurophysiological studies. ³

It is interesting that this quote is given under the title “Scientific Importance” and it represents a growing trend in using organisms as models. ⁴

I am sure that these investigations are of value but they say little about the animal in its natural environment and that is the main interest of a Natural Historian. It could be suggested that contemporary Biologists spend too much time dwelling on humans, and models for humans, without looking at the extraordinary natural world of which we are a part. That’s a bit preachy, but I’m sure that Henry Gosse would feel the same way. No doubt, critics of this view would suggest that I am looking backward, not forward, in suggesting that all organisms should be viewed in the context of their natural environment. However, we wreck environments in the cause of human advancement, and Modern Biology does little to address that issue. Rather, it encourages us to be concerned with all aspects of human health and wellbeing without being involved in the wider context of the health of the Earth. Is that wise?

¹ Philip Henry Gosse (1856) Tenby: a sea-side holiday. London, John Van Voorst.

² Philip Henry Gosse (1865) A Year at the Shore. London, Alexander Strahan.

³ https://cc1.rsmas.miami.edu/slugs/scientific-importance.html

⁴ http://www.dnalc.org/view/1278-Aplysia-and-Learning.html

Nature and identity: the links between a painting and a symphony

I spent the summers of 1977 through 1980 at Lammi Biological Station, on the northern shore of Pääjärvi. The tranquillity of the lake on summer evenings, and the wonderful quality of the light, left a strong impression on me. However, a ten-week stay at Lammi from January until March 1980 showed a contrasting Pääjärvi, covered with thick ice and with short days and very cold temperatures. Although having a different kind of beauty, the lake and surroundings were now sombre and threatening – quite the opposite of a few months before.

 

During my visits to Finland, I became interested in the paintings of Akseli Gallen-Kallela and one of his works, Lake Keitele (from 1905, his third version of this subject, seen above) is now in the National Gallery in London. It shows the lake in late summer, with the threat of severe conditions to come, and the surface beginning to freeze in places. The painting has a beautiful stillness, focussing on the water and the impending change from calm and warm to hostile and threatening. The mysterious island will then cease to be isolated by water and it will be possible to travel to it over the ice, as the lake changes from being a waterway into a roadway. 

While we can all appreciate the natural beauty shown in the painting, and the threat of impending hard times, there is something else in this picture for me and that is a strong sense of Finnish identity. At the time it was painted, Finland was part of Russia and only declared independence from Soviet Russia in 1917. Gallen-Kallela, like many Finns, was proud of the country’s beautiful, watery landscapes  and of the national heritage, represented by the epic Kalevala. He is perhaps best known for his illustrations of scenes from this work, based on Finnish folklore. Gallen-Kallela shared this feeling of Finnish identity, and love of the Kalevala, with another great artist, the composer Jean Sibelius. They had known each other for many years and drank together as members of the Symposion in the 1890s, enjoying long sessions of heavy drinking, while reflecting on “fundamental questions of art”. ¹ They remained friends, and drinking companions, for years after these Symposion sessions.

For me, one work by Sibelius forms a complement to Lake Keitele and that is the Fifth Symphony (the first, 1915 version can be heard on the link given below, two further versions being produced before Sibelius was satisfied with its structure). The comparison between the two works comes from my imagination rather than something that I have read, but it is interesting to read of Sibelius’ own words about his Fifth Symphony: ²

The autumn sun is shining. Nature in its farewell colours. My heart is singing sadly – the shadows grow longer..

.. I have a lovely theme. An adagio for the symphony - earth, worms and misery..

..In the evening, working on the symphony. This important task which strangely enchants me. As if God the Father had thrown down pieces of a mosaic from the floor of heaven and asked me to work out the pattern.

In addition to a consideration of autumn, and of passing time, Sibelius was also affected directly by Nature and the dominant theme of the final movement of the Fifth  Symphony was inspired by the observation, in April, of sixteen swans. You can easily imagine them flying in to Lake Keitele after the ice had broken up and with another cycle of life beginning.

Although Sibelius provides a musical accompaniment for the picture, I don’t need the music to feel the beauty, and also the sadness, in the painting. Nor do I need it when alone by northern lakes, as the near silence provides tranquility and peace, mixed with a little fear and anxiety about what is to come. I do not have the talent to describe my feelings, unlike Gallen-Kallela and Sibelius, so will not try further. Some might call these feelings part of a religious experience and cite the presence of something supernatural, as Sibelius suggested; others may feel I am being overly sentimental and Romantic. I'll live with that.

¹ http://www.sibelius.fi/english/elamankaari/sib_symbosion.htm

² http://www.sibelius.fi/english/musiikki/ork_sinf_05.htm

Commentaries on Lake Keitele

http://www.vangoghmuseum.nl/vgm/index.jsp?page=276110&lang=en

http://www.nationalgallery.org.uk/paintings/akseli-gallen-kallela-lake-keitele

Recording 

https://www.youtube.com/watch?v=PF3TS7yZb6s&ab&ab_channel=%E5%A4%A9%E7%9A%87%E9%99%9B%E4%B8%8B%E4%B8%87%E6%AD%B3

Giant Albatrosses: mythology and mastery of flight

The most well-known albatross is the fictional one that played a central role in Samuel Taylor Coleridge’s The Rime of the Ancient Mariner. ¹ During a voyage in the Southern Ocean, the Mariner’s ship became surrounded by ice and four verses of the poem describe what happened next:

The Ice was here, the Ice was there,

The Ice was all around:

It crack’d and growl’d, and roar’d and howl’d -

Like noises of a swound.

At length did cross an Albatross,

Thorough the Fog it came;

And an it were a Christian Soul,

We hail’d it in God’s name.

The Marineres gave it biscuit-worms,

And round and round it flew:

The Ice did split with a Thunder-fit;

The Helmsman steer’d us thro’.

And a good south wind sprung up behind;

The Albatross did follow;

And every day, for food or play

Came to the Marinere’s hollo!

The albatross is seen to have supernatural powers and is interpreted as a religious symbol by Coleridge. Unfortunately, we then learn that the Mariner kills the albatross with a shot from a crossbow:

And I had done an hellish thing

And it would work ‘em woe:

For all averr’d, I had kill’d the Bird

That made the Breeze to blow.

 

The Mariner then had to wear the slain bird around his neck, while the ship was becalmed because the albatross, such a good indicator of winds, was no longer their guide. The crew faced increasing heat and the Mariner’s shipmates were killed by dehydration, but he survived and, when rains came, he was able to drink the fresh water. After the Mariner blessed the sight of many-coloured “water-snakes” that surrounded the ship, the albatross fell from his neck and he managed to get back to tell the tale.

There’s more in the poem, of course, but the albatross joins the Mariner as the main player in the drama. It is fitting that albatrosses are regarded as having supernatural powers as they fly efficiently and without apparent effort. They are also very large birds, with the six species of Giant Albatrosses (Diomedea spp.) being found throughout the Southern Oceans, where they complete long-distance flights, little seen by humans.

Albatrosses have long, narrow wings just like those found on glider aircraft. This high aspect ratio (we can refer to this as the length of the wing divided by its depth from front to back) means that lift is generated over most of the wing’s surface when it is fully extended. Lift is achieved by the wing profile that is convex above and less concave below; so air travels further over the upper surface than it does under the wing. This results in low pressure over the upper surface that “sucks” the wing, and thus the albatross, upwards, counterbalancing sinking resulting from the bird’s body mass. The same principle of generating lift applies to all birds using gliding flight but lift is also affected by the angle of the wing and by active flapping.  The advantages of the long, narrow wing of the albatross are clear when the dominant mode of locomotion is by gliding, but quite different attributes are needed for rapid take-off, when each cycle of flapping displaces air downwards and rearwards. Albatrosses taking off from the ground generate airspeed by running with their long wings flapping in a necessarily narrow arc. Takeoff, whether from land or sea, is often clumsy and sometimes requires a more than one attempt. Once the albatross is airborne, flapping flight is maintained for a short period before winds are used to allow gliding.

In the Southern Ocean haunts of the Giant Albatrosses there are near continuous strong winds and they are at their fastest at some distance above the ocean. Albatrosses can thus fly into the wind and generate lift before turning and gaining momentum as they fly down towards the sea surface, before once again turning into the wind. This is achieved without flapping and it allows energy-efficient flying over long distances. There is also a fascinating use of the winds that blow over the surface of the sea to create waves. Albatrosses fly parallel to the wave front using the upward deflection of the winds here to provide lift and, flying close to the water surface, they also use ground effect. This results from the pressure difference between the upper and lower surfaces of the wing and is pronounced at the wing tip where turbulent vortices result. In the picture below, an albatross is gliding close to the sea surface and, with the wing tips pointing downwards (anhedral), uses vortices projected to the water surface, resulting in enhanced lift and reduced drag. Albatrosses are not the only birds to use this method of flight enhancement; pelicans use it too and that is why the Boeing Corporation developed the Pelican aircraft concept, ² using lift from the wing and also from the pronounced ground effect at the wing tip. The Boeing project is a theoretical extension of the Ekranoplan flying prototypes. ³

We thus come to designs made by humans using features that birds have had for millions of years. Creationists believe that birds are also designed, but Evolutionists suggest the selection of adaptations over time. For example, reptiles developed feathery coverings and these became modified into feathers of various forms, some of which clothed the fore limbs to produce the wing structure and covering. There are many other changes that allow birds to fly, but few can match Giant Albatrosses as masters of flight, even though they are challenged by takeoff. It’s no wonder that sailors like the Ancient Mariner, encountering albatrosses in the Southern Ocean, afforded them mythological status.

¹ Samuel Taylor Coleridge (1798) The Rime of the Ancient Mariner in Lyrical Ballads, with a few Other Poems. London, J. & A. Arch.

² http://www.boeing.com/news/frontiers/archive/2002/september/i_pw.html

³ http://news.bbc.co.uk/1/hi/magazine/7638659.stm