ENHS Nature Trails Dec 98 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Why are there no insects in the open ocean? A fifteen year conversation

Insects are, without question, the most successful animal colonizers on earth. Found on land, in the air and in the water, insects are incredibly well adapted, having acclimated to some of the harshest climes on this planet. The number of insect species on land is estimated to be as high as 20 million. Yet for all their success on land, insects are nearly completely absent from the ocean. The question, of course, is why?

This issue has been the subject of numerous discussions with my friend and colleague, Dr. Simon Maddrell, an infectiously enthusiastic, comparative physiologist and insect biologist par excellent from the University of Cambridge, England with none of the stuffy pretensions that characterize many of his Catabrigian (those hailing from Cambridge) colleagues. After many years of cogitating on this subject, Simon has finally published his views on this intriguing subject in the Journal of Experimental Biology (201:2461-4, 1998) and I am pleased to be able to encapsulate them in this column.

Our discussions on this and other subjects often follow the general format of a Cambridge tutorial. Until fairly recently, Cambridge (and Oxford) students traditionally did not attend classes. Their major contact with University Dons, the learned teaching staff of Cambridge colleges, consisted of weekly tutorial sessions where the Tutor would discuss specific subjects with one or two students. These sessions are intense, highly intellectual, and very beneficial since a student cannot hide and thus has to come prepared or leave humiliated.

So, one evening about 15 years ago, after several bottles of very expensive vintage wine had disappeared --the Cambridge colleges are the richest institutions in England after the Queen and the Church of England--, Simon closed his eyes, sighed, and popped the question. "Hmm, hmm", he intoned spiritedly, "Nathan, dear boy, why aren't there insects in the ocean?" Being 15 years his junior and a teetotaler to boot, I figured I had a clear mental advantage and responded immediately with the youthful bravado of a know-it-all recent PhD graduate (which I was), "Because insects cannot survive in a salt water environment." A smile slowly formed on Simon's face, his eyes opened, and, looking in the way that Ken Starr must have looked at Clinton during the latter's deposition, immediately said, "Nosiree! Insects live just fine in saline water. Some insects rear their larvae in sea water and a number of insects thrive just dandy in the high salt environs of your west coast inland lakes, like Mono Lake in California and the Great Salt lake in Utah. Try again!" Advantage, the Brits.

The English love victory but this Yank was loath to give them one, so I dug deeper. "How about the fact that insects evolved from crustacean relatives who appeared in the ocean first and thus appropriated all the niches that insects might have filled if they had gotten there first?" Simon opened his eyes a bit wider and retorted, ever so gently, "Not bad for an American. Think about it more and let's talk again soon".

Several years and many wine bottles passed into oblivion before the subject was broached again. Over a wonderful feast of Indian food at a local Cambridge landmark restaurant, Simon leaned over the table and whispered, "Did you have a chance to think about it?". "About WHAT?" I replied somewhat chastened over the possibility that I had forgotten something important. "You know. The lack of insects in the ocean", he said with glee as if our conversation had continued unabated from years before. "As I remembered", I said slowly while straining my cortex to the breaking point to recount anything of relevance, "our conversation. ...ended ....with ...the ... the idea that crustaceans got there first". I gleamed with triumph over remembering the gist of one sentence said years previously. Simon's reply was brief, "Why?". "Why? Why what?" I blurted. Simon looked at me the way he must look at his students, "Just because crustaceans got there first doesn't mean that they must stay there? But they did, so they question remains. Why?"

I started to fumble and fell back on the only thing I ever remembered from my one and only marine ecology course. "Well, because...hmm...well...probably because insects are poor competitors compared to crustaceans." I took a deep breath and waited. Because our table for 8 was filled with lively chatter, a conversation at the other end of the table requested Simon's attention and thankfully our discussion was remanded to another time and place.

This discourse, albeit delayed, was far from over. Five years ago Simon and I were meandering through the National Gallery of Art in London, marveling at a couple of Mondrians and Monets when it bubbled to the surface again, this time initiated by me. "So," I said assuming a professional tone, "if crustaceans out compete insects in the ocean, then they must either capture prey better which is unlikely or have developed better strategies to avoid predation. And since the main predators in the oceans are fish, then crustaceans must be better adapted to escape predation by fish". Simon nodded in approval. Point and set to the American.

But the match was not quite over. By the time we got to the Picassos, Simon had responded in kind. "If you are correct, then you have to explain why crustaceans are better at avoiding fish predation." "Hard shells" I replied immediately. "Good but not good enough." Simon said generously. "Many fish have developed effective means of disembodying crustaceans for their ever-so-tasty flesh". Oops. Back to the drawing board for a couple more years.

Fast forward to this past spring when Simon and I revived this 15 year old conversation on the train back to Cambridge from London. I was prepared, or so I thought. "Remember our conversation about the lack of insects in the oceans? I think I have it figured out. "Insects have tracheal systems and air sacs for breathing air which when filled would not allow them to move very quickly in water." Simon grinned while replying, "But insects do quite well in many fresh water environments. But there's a catch - they only do well when fish are not very abundant. Think about how crustaceans avoid predation by fishes." I replied tout de suite, remembering my undergraduate days on the Oregon coast where my undergraduate advisor and I used to catch dungeness with crab pots, "They descend to the depths of the ocean during the day,". "Can insects do the same?" Simon queried with a knowing voice of authority. Thinking back to their air-filled tracheal systems, I had to reply in the negative. "So let me get this straight" I said. "The reason why insects haven't colonized the open sea is simply because they cannot descend sufficiently deeply to avoid being eaten by fish?" Simon grinned from ear to ear, thoroughly pleased that this 15 year old conversation had ended so satisfactorily.

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