Detrimental effects of Ulva lactuca (L.) exudates and low oxygen on estuarine crab larvae

Conventional theory postulates that associations between marine macrophytes and animals are generally positive. This paper presents evidence, however, that a common species of green macroalga, Ulva lactuca (L.), is detrimental to estuarine invertebrates due to the production of toxic exudates and low oxygen tensions which occur in the seaweed beds at night. Bioassays of the responses of zoeae of five species of estuarine crabs (Callinectes sapidus Rathbun, Carcinus maenas L., Eurypanopeus depressus Smith, Neopanope texana savi Smith and Rhithropanopeus harissii Gould), using water in which Ulva lactuca was cultured for 24 h, produced 100% mortality after 22 days. No crabs survived the molt into megalopa. Hypoxic water, 0.5 ± 0.3 ppm oxygen, caused a decline in larval activity (movement), but there was no mortality over an 8-h period, Ulva-water purged to 0.4 ± 0.1 ppm oxygen caused 100% mortality in 13–40 min. These synergistic effects could be critical in estuaries where dense U. lactuca beds cause periods of low dissolved oxygen. We hypothesize larval recruitment may be limited in such systems, particularly in areas where flushing is poor.