Seeds, pits, and eggs

When I think of biological dispersal, the first few examples that come to mind all involve plants and seeds – the white puff of a dandelion head scattered by a gust of wind (based on the main photograph illustrating the Wikipedia page on dispersal, I’m not the only one who thinks of dandelions first); cherry pits spread far and wide by the lucky birds who found a tree laden with ripe fruit; even the squash plants sprouting from seeds tossed in a backyard compost pile.

But many organisms, animals as well as plants, disperse themselves, or are dispersed by the environment in which they find themselves: wind, or water, or humans move them around.

A team of Japanese researchers recently published a study in the journal Ecological Research detailing the dispersal of a particular type of coastal stick insect found in Japan and Taiwan, Megacrania tsudai, also known as Tsuda’s giant stick insect. Members of the genus Megacrania are large insects, usually between four and five-and-a-half inches long as adults. Only female Megacrania tsudai are found in the wild – they reproduce via parthenogenesis, an asexual process during which the young develop from unfertilized eggs. Megacrania tsudai eggs look like tiny, brown potatoes (pdf), the right size and shape for a dollhouse dinner plate, and adult females lay them often – the insects the scientists studied typically produced one egg per day.

The researchers were interested in whether or not Megacrania tsudai eggs, which are buoyant in seawater, can survive extended periods of time in the ocean – as a coastal species, they suspected that the insect might use ocean currents as a means of dispersal. They found that even after over a year spent floating in seawater, eggs hatched at the same rate as unexposed eggs (about 60% of the eggs eventually hatched, regardless of how long they had been in the water).

They also found that prolonged exposure to seawater delayed development – eggs not exposed to seawater began hatching after 120 days, while those floating in seawater for over a year didn’t begin hatching in substantial numbers until after 150 days, and some eggs were still in a pre-hatch development phase after 200 days.

In their paper, the scientists discuss the dangers Megacrania tsudai larvae and adults face as a terrestrial species living in coastal areas that regularly experience typhoons, strong winds, and floods. “However,” they write, “one or a few M. tsudai eggs are laid every day, and these eggs possess seawater tolerance and have a high variation in egg period duration. All of these characteristics are advantageous for an insect species living in a coastal forest habitat. Therefore, even if adults and larvae are completely lost due to natural disturbances, the eggs are an effective means of recovering and maintaining their population since they hatch normally after seawater exposure[.]”

Dispersal is an important way for populations to endure in the face of challenging local conditions. Megacrania tsudai eggs, just like dandelion seeds and cherry pits, can survive voyages beyond the realm of possibility for their progenitors.

Adult Megacrania tsudai on screw pine leaves in Taiwan; screw pine leaves are a primary food source for the insect. 

(Image by Bettaman via Flickr)