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Algae & isopods - a unique symbiosis

When I set essays for my first-year students to write during the semester, I try to give them a scientific paper on each topic to start them off. This means that I need to do some extra bedtime reading as I need to select those papers carefully. Today’s post is based on one of those: a paper about a fascinating mutualistic relationship between marine algae and a species of isopod (the same crustacean group as the more familiar slater).

Symbiotic relationships are fairly common in the oceans – perhaps the best-known example is the relationship between reef-building corals and a group of algae called dinoflagellates (zooxanthellae). The coral animals gain sugars from their algal partners, & may die if the algae leave (which happens if water temperatures get too high, resulting in the coral ‘bleaching’ as at least some of their colour is due to the algae). Another example of such a food-based symbiosis is the one between chemosynthetic bacteria and invertebrate animals at deep-see hot-water vents (Lindquist, Barber & Weisz, 2005). Because many bacteria & algae produce toxic chemicals as a byproduct of their metabolism, it’s been suggested that some microbe-invertebrate relationships also have an element of defence about them. Niels Lindquist & his colleagues investigated this possibility in a species of marine isopod belonging to the genus Santia.

Most marine isopods are either cryptically coloured or only come out at night, when many predators are less active. But this particular Santia species is a bright fluorescent red colour – and spends the day sitting on sponges or corals & basking in the sun, in groups of up to a few thousand individuals. They’re quite big – about 5mm long - & so they’re readily visible to potential predators, which makes their colouration & behaviour particularly interesting.
When the research team examined the animals more carefully, they found that the red colour was due to a carpet of single-celled algae covering the animals’ exoskeletons. At the same time, they never saw fish eating the isopods (despite the fact that fish normally feed quite happily on isopods), which suggested that the Santia sp. were inedible, an hypothesis supported by the isopods’ bright colour and risky sun-bathing behaviour. Linquist & his colleagues decided to see whether the Santia really were unpalatable; whether the epibiont algae caused this unpalatability; just which microbes were living on the animals’ exoskeletons; and how the algae were passed on to new generations of isopods.
For their feeding tests the scientists caught red Santia and also ‘normal’ brown isopods and then released them upstream of schools of reef fishes. This was after releasing isopod-sized food pellets into the same water current – the fish snaffled these up & weren’t put off their meals by the presence of nosey human divers. They were quite happy to eat brown isopods as well. But it was a different story for the red isopods, which were mostly completely ignored. If they were ingested, the fish spat them out again immediately.
To test whether this aversion was due to the carpet of microbes, the researchers kept red isopods in the dark for 2 days. They also extracted pigments from ‘unbleached’ isopods and then carried out feeding trials in which fish were offered: normal food pellets, food pellets with the algal extract, isopods with normal symbionts, and the ‘bleached’ isopods whose algae had lost most of their colour after being kept in the dark. The results: 100% of the food pellets and 80% of bleached Santia were eaten by the fish, while no normal isopods were eaten and only 30% of the treated food pellets, which strongly suggested that the microbes (which turned out to be cyanobacteria, or ‘blue-green algae’) were producing some off-putting chemical that deterred predation by fish. These protective microbes appear to be transmitted vertically, as the scientists observed juvenile Santia – which had just emerged from their mother’s brood pouch – climbing over the mother’s exoskeleton & picking up their own episymbiont populations.
Lindquist et al. concluded that they were looking at a unique symbiosis, one based on both nutrition and defence. Protected by their cyanobacteria, red Santia can be active while it’s daylight, which opens up new niches to them and may mean that they have more chance to feed without competition from other invertebrates. They also eat the microbes, which would certainly be a disadvantage to the individuals consumed. However, overall the cyanobacterial population benefits, because the isopods’ sunbathing habits would maximise opportunities for photosynthesis and hence microbial population growth. The paper concludes:
Although these isopods represent only the second marine example of microbial symbionts producing a chemical defence against host predators, the risk of predation is high in many marine habitats, suggesting that defensive symbioses may be more common in the marine environment than presently believed.
N.Lindquist, P.H.Barber & J.B.Weisz (2005) Episymbiotic microbes as food and defence for marine isopods: unique symbioses in a hostile environment. Proceedings of the Royal Society B (Biological Sciences) 272: 1209-1216. doi: 10.1098/rspb.2005.3082
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