the balance of tik is must much better than whale. whale image looks like a 3rd graders story, and it just doesn’t balance well with the text.

tikta1 says it all, with all elements working together.

Wasp1 is clean, balanced, and works well with the title, as well as capturing the concept of “tangled bank” perhaps a bit better.

More contrast. More use of the available range of values.

Brighter glints at the surface. Maybe some light streaking through those glints.

Darker darks in the water, especially down below.

Maybe some focused light ‘caustics’ wrapping down from the tops of the creatures.

Some sense of churn or trail or at least slight air exhalation to impart direction and speed cues.

An even bigger mouth on the left about to eat the whale thing. (Okay – I was kidding about that last part)

For the target audience, I’d pick one of the first two, but with revised text placement and fonts choices.

Firstly, they are the only ones to have action. Although in principle the last does too, as you wrote, you wouldn’t know unless you knew the story.

Secondly, Tiktaalik doesn’t convey a sense of scale and may look a bit “goofy” to some. On the scale issue, I can’t but help think some will confuse it with some kind of salamander or the like. (The third does a little better on that front.)

Thirdly, I think it’s helpful for the picture of have depth if it is presenting living things in action. The first two and Tiktaalik3 qualify for that.

I do think the use of the placement of the text and choice of fonts (and colour) in the Whale ones needs attending to, though. Because of this my vote would be split over these two, without favouring either one of them.

So the question is: which readers do you want to aim at: students already enrolled in the class, who are expecting a textbook, and who have to buy it in any case; or people outside the classroom who might be intrigued by it enough to buy it.

I’d aim at the latter group, myself, both because it doesn’t hurt to cast the nets wide, and because it might startle students expecting something more textbooky (read, expensive and boring).