When English biologist and mathematician JBS Haldane was asked what he’d learnt of the Creator after a lifetime of studying nature, he is said to have replied that He must have “an inordinate fondness for beetles”. Haldane’s famous answer tells us that the world is crawling with beetles – some 390,000 species and almost a quarter of animal species – but also that only SOME groups have had this kind of success. There are winners in the history of life on Earth, but also plenty that are now extinct or failed to diversify.

The question of why some groups flourish while others do not has puzzled biologists for centuries. One way to tackle the question is to look for special features or abilities shared by the successful groups – adaptations. Perhaps they evolved a new defence against predators or a new way of getting around. The ability to eat new foods helps explain the incredible number of species of insects. Herbivorous insects like beetles, butterflies and true bugs are clearly among the most diverse animal groups on Earth.

Like insects on land, many of the 70,000 species of crustaceans (crabs, lobsters and their relatives) eat plants and seaweeds in the kelp forests and coral reefs in the sea, and in rivers and lakes around the world. Some crabs even climb mangrove trees to feed on fresh leaves, and others eat seedlings from the rainforest floor (like the famous Red Crabs from Christmas Island).

Red Land Crab, Gecarcoide natalis
Red Land Crab, Gecarcoide natalis Red Land Crab, Gecarcoide natalis There are an estimated 45 million Red land crabs on Christmas Island. Image: John Tann
© John Tann

That many crustaceans would eat plants may be no surprise. As humans, we take it for granted that we can eat fruit and vegetables. Plants are clearly abundant across most of the planet and should be a major potential food source for animals. From a physiological perspective, however, plants, are tough to eat. They have little of the nitrogen needed to make proteins for growing bodies and are often loaded with physical and chemical defences (e.g., spines, toxins), making them hard to digest. Special adaptations are required inside and out to be able to eat plants. Consequently, few major groups of animals can feed on plants alone – some insects, birds and mammals on land, and some crustaceans, molluscs, sea urchins and fish in the sea. So, how have crustaceans fared with eating plants?

We recently showed that the ability to eat seaweeds and plants promotes diversity among crustaceans, just as it does among herbivorous insects.

To do this, we closely examined the evolutionary tree of the crustaceans and found animals eating plants in at least 31 different lineages. Then, to test whether plant-feeding promotes diversity, we compared the number of species in each plant-feeding group with their nearest relatives.

These comparisons showed that the herbivores had, on average, 21 times more species than their nearest predatory or scavenging relatives. As with insects on land, groups that took on fresh plants as a food source have become much more diverse than those that did not.

Our work with crustaceans shows that if an animal group can overcome the challenges of eating plants, the evolutionary benefits are great. The very difficult evolutionary path to vegetarianism, paradoxically, leads to the most diverse lineages. Plants represent a more extensive and more reliable food source than animals. Once plant eating gets started, the large population sizes of herbivores promote speciation while the non-herbivores are more vulnerable to extinction.

Maybe there is an inordinate fondness for crustaceans too.

Shane Ahyong (Australian Museum)
Alistair Poore (University of New South Wales)
Erik Sotka (College of Charleston)

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