Drunk on algal toxins

Harmful algal blooms (HABs) pose a significant threat to coastal economies and human health. Moreover, evidence suggest that HABs are increasing in intensity and frequency. Copepods have a key role in the transfer of toxins in food webs, as they generally are the first consumers of the algae and act as vectors for the toxins. To date, research on copepod responses to HABs has mostly been limited to studying algae rejection, algae avoidance and incapacitation of the copepod. In their paper, Lasley-Rasher et al. (2016) examined the effect of HABs on the swimming behaviour of copepods and if the changes affected encounter rates with predators. 

Specimens of the neurotoxic alga Alexandrium fundyense and the copepod Temora longicornis were collected from the Damariscotta River estuary in the Gulf of Maine. The dinoflagellate produces toxins (e.g. saxitoxins) that cause paralytic shellfish poisoning and red tides. HPL-chromatography showed the total amount of saxitoxins in the A. fundyense culture to be 2.9 pg/cell. Copepods were placed in a tank with filtered seawater and fed with either i) A. fundyense, ii) a 50/50 mixture containing A. fundyense and Rhodomonas lens (a non-toxic control alga) or iii) R. lens. After a 2h exposure period the copepods were transferred to other tanks. Incubation lasted for another 15h during which the copepods were fed with Tetraselmis sp. (another non-toxic alga) to minimize variation due to hunger levels. Subsequently the swimming behaviour of T. longicornis was tracked using the LABTRACK software to generate 3D tracks, of which only the first 10s of movement were analysed. Survivorship, ingestion and egg production were tested separately in beakers.

After being exposed to A. fundyense, the copepods swam faster (25%) and displayed straighter swimming paths than those conspecifics fed only on the control alga. Survival was not affected by exposure the harmful alga, nor were there significant differences in the amount of food ingested or in the egg production. Moreover, the copepods did not seem to avoid A. fundyense or become incapacitated.

Unexpectedly, exposure to A. fundyense had no significant effects on either survival, ingestion or egg production and even stimulated swimming behaviour. The authors theorize that this T. longicornis strain may be resistant to saxitoxins, as the characteristic blocking of sodium channels was not observed in this study. However, a growing number of evidence suggests that saxitoxins can sometimes stimulate grazers. Along with path straightness, faster swimming increases the probability of dispersal and of an encounter with a predator. Analysis using a model predicted the 25% increase in swimming speed to theoretically lead to an 56% increase in encounter rates with predators. This has considerable implications for HABs as it could potentially decrease grazing pressure while simultaneously increasing toxin uptake by copepod predators. So far, the bioaccumulation of saxitoxins in T. longicornis has not been studied and they are generally thought to be less likely to bioaccumulate than shellfish

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In conclusion, the paper demonstrates how HABs can alter the behaviour of their grazers. In addition to more direct harmful impacts, the effects on grazer behaviour also have to be addressed when examining HABs. While the paper has not been cited yet, it has garnered quite a bit of attention online. On the whole, it was an interesting paper that had a good structure, was easy to follow and didn’t try to do too much. 

Reviewed Paper:

Lasley-Rasher, R. S., Nagel, K., Angra, A., & Yen, J. (2016, April). Intoxicated copepods: ingesting toxic phytoplankton leads to risky behaviour. In Proc. R. Soc. B (Vol. 283, No. 1829, p. 20160176). The Royal Society. http://rspb.royalsocietypublishing.org/content/283/1829/20160176