Plant defence against herbivores is a crucial theme in the evolution of both plants and animals, yet it is expressed very differently in water and on land. Spines, hairs, and distance signalling are common in land plants but very rare in submerged aquatic plants.
To account for this hitherto unrecognized dichotomy, Vermeij suggests that animal sensory capacities are in part to blame. Herbivores, pollinators and plant dispersers on land often rely on long-range visual or olfactory signals, whereas aquatic herbivores do so on a much smaller scale, all because of the contrasting properties of air and water. Both contact and long-range defences of plants are much more developed in land plants, whose ecological connections with animals are far more complex than in water. Host specialization by small herbivores is very high on land, but generally less in marine ecosystems, especially in the tropics.
Geerat J. Vermeij, 2016, 'Plant defences on land and in water: why are they so different?', Annals of Botany, vol. 117, no. 7, pp. 1099-1109 http://dx.doi.org/10.1093/aob/mcw061