If my previous post, on the role viruses may play in ecological cycles was just a little too close to home, maybe this post will be an antidote. It continues the theme of organisms interacting to create the ecosystems we see in streams, but this time dealing with the effect that invertebrates, rather than microorganisms and viruses, may have on the algae that live in our lakes and streams.
The painting at the top of this post is the end result of picking a mayfly nymph off a stone submerged in the River Ehen and realising that I only had the sketchiest of ideas about how it went about feeding on algae. I brought it home with me, managed to identify the genus, if not the species to which it belonged (Ecdyonurus) and took a photograph. You can see the characteristic flattened head that identifies it as a member of the Heptageniidae, along with the two large, upward-facing eyes, a row of plate-like gills along the abdomen and three tails protruding from the final segment.
What we cannot see in this image are the mouthparts, hidden below that bulbous head. I had to search around the internet to find detailed images of the mouthparts of Ecdyonurus, eventually locating some useful scanning electron micrographs in a paper by Todd Wellnitz and J.V. Ward. These images are quite difficult for a non-entomologist to decipher, because insects have extremely different mouthparts to vertebrates. In brief, they have, like us, upper and lower jaws (the maxillae and labium respectively) but both of these can have jointed extensions (“palps”) that are, in effect, “arms” with built-in cutlery, each adapted to that insect’s particular diet. Imagine that a human ancestor had wanted to do all the things that we do with our arms but didn’t want to stop walking on four legs. Now imagine that humans evolved a short pair of arms on either side of the face. If that doesn’t sound bizarre enough, some arthropods have these palps on both the upper and lower jaws, so there could be four separate items of cutlery descending upon an unsuspecting prey organism.
In the case of Ecdyonurus, the labial palps are brush-like appendages that it uses to sweep the algae that live on the stones towards its mouth. That’s what the structure on the right of the image is supposed to represent, and part of my intention is to show this in relation to the size of the algae that are typically found in the habitats where Ecdyonurusfeeds. The individual bristles that make up this brush are about five or ten micrometres (= 1/200th to 1/100th of a millimetre) apart, whereas the smallest diatoms are about ten micrometres long. So Ecdyonurus should be pretty effective at scraping up most of the algae that it encounters.
By way of analogy, a 1.5 cm Ecdyonurus nymph grazing on stream diatoms of this size is roughly equivalent to a human eating peas. And all this is done, remember, without the mayfly actually seeing what it is doing – its eyes, if you remember, are on the top of the head. Some mayfly nymphs have been shown to have sensory receptors in their labial palps, so it is reasonable to assume that Ecdyonurus has something similar. This doesn’t necessarily mean that it feeds with any great discrimination, rather it chomps through all the algae, just leaving behind those that are too firmly attached to be removed easily. Whether it can separate out two algae of similar sizes mid-meal (selecting peas but leaving sweetcorn, as it were) seems unlikely. But then, if you know anything about insect lifecycles, you’ll know that mayfly nymphs don’t have parents they can easily annoy, which takes away most of the fun of separating peas from sweetcorn. Or, by analogy, one species of Achnanthidium from another.
Wellnitz, T.A. & Ward, J.V. (1998). Does light intensity modify the effect that mayfly grazers have on periphyton. Freshwater Biology 39: 135-149.
Gaino, E. & Reborna, M. (2004). The sensilla on the labial and maxillary palps of Baetis rhodani (Ephemeroptera, Baetidae). Research update on Ephemeroptera and Plecoptera (E. Gaino, editor), University of Perugia, Perugia.
Some other highlights from this week:
Wrote this whilst listening to: Gryphon, early seventies band who blended medieval and folk music with progressive rock.
Cultural highlights: Rewatched the film Amélie, probably my favourite film about Paris.
Currently reading: continuing with Wanderlust: A History of Walking, by Rebecca Solnit.
Culinary highlight: Continued the French theme with hake steaks on a cassoulet made with roast fennel, followed by crêpes suzette, made with garden apples rather than oranges.