Amphipods and watercress: potential chemical defense in a stream macrophyte

TitleAmphipods and watercress: potential chemical defense in a stream macrophyte
Publication TypeJournal Article
Year of Publication1988
AuthorsNewman R.M
JournalBulletin of the North American Benthological Society
Volume5
Issue1
Pagination103(Abstr
KeywordsNASTURTIUM
Abstract

While investigating the role of amphipod shredders on litter (watercress, Nasturtium officinale) decomposition, I learned that Nasturtium, after it had been frozen, was toxic to Gammarus pseudolimnaeus. Observations were made and experiments were then performed on Gammarus and watercress from two springbrooks: Carp Creek, MI (in Aug) and Squabble Brook, CT (Oct-Nov). Toxicity tests with fresh frozen watercress leaves gave 48 h LC50 values (trimmed Spearman-Karber methods) or 475 mg wet tissue/L (95% CI=402-528 mg/L) in MI and 1122 mg/L (996-1262 mg/L) in CT. These differences may be due to either temporal or geographic variability. Watercress, a crucifer, is known to contain glucosinolates, which upon tissue damage are hydrolyzed to isothiocyanates or nitriles. A 2 degree compound, phenylethylisothiocyanate, which occurs in watercress was found to be quite toxic in solution. Forty-eight h LC50 values for phenylethylisothiocyanate were 3.62 ug/L (95% CI=2.77-4.74 ug/L) in MI and 0.96 ug/L (0.67-1.36 ug/L) in CT. Choice (preference) trials in CT indicated little consumption of fresh green watercress leaf disks. Overall, less than 10% of the fresh cress offered was consumed (% area remaining = 94.7% for 12 h starvation; 92.5% for 48 h starvation). Yellow (light deprived) cress was consumed much more readily, with only 51.4% remaining for 12 h starvation and 30.8% remaining for 48 h starvation. The most fresh cress was consumed when it was the only choice (about 90% remaining) and the least was consumed when there was an equal or greater number of yellow disks (2:2 or 2:1, ca 94%). These results suggest that watercress may possess defensive chemicals which reduce herbivory by invertebrates such as Gammarus.