Interview with Sarah Kraak

Conducted October 2007 to April 2008 by Paul Cooijmans


Dr Sarah B. M. Kraak is an evolutionary biologist who has studied the mating behaviour and the evolution of fish species like blennies and sticklebacks, and works in fisheries science, advising on quota for European fisheries.

Questions and answers


For your Ph.D. you have researched the courtship behaviour of a fish with female mate choice and paternal care for eggs in a nest. Why does this particular topic interest you?


It is the case that two of my passions coincided.

The first is my love for the sea. Already from a young age onwards I loved the sea. I love the shells found on the sandy beaches of the Dutch coasts, and I love the never-ending chaotic regularity of the waves, and I love the colours of the seascape: grey, blue, grey, grey, blue, grey, grey, grey...

The Sea 2 - painting by Sarah
The Sea 2 - painting by Sarah

More photos: [The Sea] [The Sea - painting by Sarah]

Remark: To return from a photo, push <Backspace> on your keyboard or use the "Previous page" button of your web browser. In some cases below, a larger version of the photo can be obtained by mouse-clicking on it.

As a second year student of marine biology I went on a field course for two weeks to Roscoff, a town at the coast of Brittany, France (1982). To me this coast is the most beautiful place on earth, especially because it seems so unearthly. A whole new world opened up to me in those weeks. It is the world of a landscape that changes radically about two times per day from 'just the sea' to a landscape that is absolutely fantastic.

[The port of Roscoff at high tide and at low tide]
[View from Roscoff at high tide and at low tide]

The phenomenon itself of the extreme tidal changes adds to the beauty of the place and my fascination for it. The tidal changes follow an intricate rhythm of superposed sinusoid waves. There is the cycle of 12 hours and a bit, which makes that every day the clock times of lowest and highest tide fall later and later in the day. It is thus independent of the day/night cycle, which is intriguing to experience. Furthermore, the tidal differences themselves (the differences between the lowest and the highest water level within one cycle) vary from let's say 5 or 6 meters to 10 or 11 meters according to a moon cycle, with spring tide (the biggest difference) a few days after full and new moon, and neap tide (the smallest difference) around half moon. The amplitude of the spring and neap tides again varies cyclically, such that the most extreme spring tides occur around the 21st of March and September. I tell you: if your work depends on these tides this phenomenon impacts you deeply.

Tidal wave
The tidal wave. These figures are constructed from the tides predicted for 2008 at Brest, Brittany, France ( On the y-axis is the relative water level and on the x-axis is time. The time span in the upper graph is the part between the two red lines in the lower graph. Low tide and high tide alternate about twice per day. Spring tide (large tidal difference) and neap tide (small tidal difference) alternate about twice per month. The most extreme spring tide occurs in the beginning of April, with the least extreme neap tide occurring in the week before. The least extreme spring tide occurs around mid-year, with the most extreme neap tide occurring in the week after. The tidal differences are determined by the extent to which moon and sun pull in the same direction and their distances to the earth.

In our field course in Roscoff we studied the ecological zonation of the flora and fauna living on the rocky shore in the intertidal range. Ecological zonation is the phenomenon that some plants and animals are adapted to be out of the water when it recedes and be exposed to air for longer times than others. Also the small rock pools that are high on the rocky slopes contain an entirely different community of flora and fauna than the ones lower down. This is because higher up temperature and salinity conditions vary more extremely due to sunshine, evaporation, and rainfall. In general, the green seaweeds are more resistant and live higher up the rocky shores. Then you get the brown weeds. Closest to the low water line live the red weeds and the brown Laminarias (very long sea weeds). At low tide the landscape at Roscoff consists of a sandy bottom with rocky hills jutting out that are covered with the prettiest combinations of shiny, slimy, slivery, feathery fine or leathery coarse plants of shapes that are definitely unearthly.

Representation of a relatively sheltered shore. From: Lewis, J. R. 1964. 'The ecology of rocky shores'. The English Universities Press, London.
Representation of a relatively sheltered shore. From: Lewis, J. R. 1964. The ecology of rocky shores. The English Universities Press, London.

the sandy and rocky seafloor at Roscoff - the long weeds are Himanthalia elongata
the sandy and rocky seafloor at Roscoff - the long weeds are Himanthalia elongata

[a mixture of brown, red, and green weeds]
[mainly brown weeds]
[a rock pool surrounded by green, red, and brown weeds]
[the rocky shore near Roscoff]
[Standing on the sea floor at Roscoff at low tide]

Song by Sir Edward Elgar (1857-1934) Sea Pictures, Opus 37, IV. Where Corals Lie (Dr. Richard Garnett)

The deeps have music soft and low
           When winds awake the airy spry,
It lures me, lures me on to go
           And see the land where corals lie.

By mount and steed, by lawn and rill,
           When night is deep, and moon is high,
That music seeks and finds me still,
           And tells me where the corals lie.

Yes, press my eyelids close, 'tis well;
           But far the rapid fancies fly
To rolling worlds of wave and shell,
           And all the lands where corals lie.

Thy lips are like a sunset glow,
           Thy smile is like a morning sky,
Yet leave me, leave me, let me go
           And see the land where corals lie.

[A l'Ile - painting by Sarah] [De l'Ile - painting by Sarah]
[Sea weeds - painting by Sarah] [Sea weeds 2 - painting by Sarah]

I completely fell in love with the sea back then. A desire grew in me to be part of the sea. At the time, whenever I sailed the sea in a boat I felt a longing to be in and not on the sea. I was not satisfied anymore to belong to the world above the sea surface. I wanted to belong to the world below the sea surface. Apparently some of my fellow students felt the same, because the whole group of the Roscoff field course joined the student SCUBA diving club. And our dreams came true of floating and flying in three-dimensional space looking at the more colourful and more fancifully shaped part of our planet's flora and fauna [Something colourful and fancifully shaped - Corynactis viridis] [Something colourful and fancifully shaped - Halcurias pilatus]. A year later most of the students of the Roscoff group went on a second marine field course of three weeks which took place in Corsica (1983). That is when and where I met my study subject you ask about.

A. sphynx male displaying from his nest
A. sphynx male displaying from his nest

The other passion is my scientific interest which concentrated on, among many other things, scepticism towards the theory of evolution by natural selection. Not that I favoured any alternatives such as creationism or variations thereof. I just did not take for granted what my mother - she is a biologist and scientist as well - told me, namely that life on earth and its species came about by the process of natural selection. I wanted to understand HOW. Around 1979 my mother introduced me to the book The selfish gene by Richard Dawkins. Later I got a copy of the book from my father. I guess this is one of the books that influenced me most. It made the case for natural selection (but not for its role in the origin of species... which I came to understand much later when I did research on speciation) totally convincingly, yes, it appeared to be natural necessity. The book focuses on adaptation by natural selection, not so much of anatomical features of animals such as long necks of giraffes, but more of behavioural strategies found in nature. Strategies such as why an individual should or should not cooperate with another individual. Upon reading this book a world opened up to me - again! The world of science in its most interesting guise.

Then, in 1983 in Corsica, the field course teacher suggested that I study the mating decisions of the fish that later became the subject of my Ph.D. For a few moments I thought that mating behaviour of fish would be uninterestingly simplistic. I thought it would be a case of male and female meet and then mix their gametes (= eggs and sperms) and that's it. But the teacher suggested to study decisions, strategies, and choices! And then I realized I could become a real scientist studying the behavioural strategies of animals in the light of evolution and natural selection.

And then I became a real scientist.

[a real scientist - at field station STARESO, Corsica, holding a test tube with blenny eggs]
[a real scientist - observing blenny behaviour in nest cavities in an artificial hard substrate]

a real scientist several years later_determining embryo stages in stickleback eggs - while smoking
a real scientist several years later - determining embryo stages in stickleback eggs - while smoking

Post hoc, after I had become a scientist of sexual strategies, I can add another reason for my interest in this particular topic. Sex is not only one of the most pleasant activities to indulge in privately, but for me also one of the most interesting subjects to contemplate and discuss. Not only human sexuality interests me, with questions about the psychology of sex roles and whether sex-specific behaviour can be explained biologically. But it also became apparent to me that sexuality has been perhaps by far the most creative force in nature: almost all diversity of life has come into existence owing to sex. So, studying sex is studying (almost) everything. It also turned out to be great fun to study the biology of sex, because its terminology is always reminding us of our own sexuality. It always triggers jokes on human mating decisions such as extramarital affairs and the like.

Why then this particular species with paternal care for eggs in a nest? No reason at all I think. Any other fish, or bird, or spider, or lizard would have meant the same to me, except that the study of all but the first would not coincide with my love for the sea. I do not think I have a special interest for the species with male parental care. Or do I? Well yes, being a female myself I find it amusing to see that females exert their power over males through mate choice (which is ubiquitous in the animal and plant kingdoms) and let the machos work and care for the kids. But the world of biology is full of nice arrangements! And of awful ones.

[To the rest of this interview...]