This paper examines the contrasting survival strategies employed by zooplankton in the benthic and pelagic zones of the ocean. Beginning with the role of phytoplankton as the primary marine food source, the paper explains how the inability to actively swim forces many zooplankton to rely on passive defense mechanisms. Benthic organisms such as algae, sponges, and soft coral employ physical structures, noxious chemical secretions, and behavioral adaptations like nocturnal growth to minimize predation. In contrast, pelagic zooplankton — particularly nekton in the mesopelagic zone — rely on light-based strategies including diel vertical migration, camouflage coloration, guanine crystal light reflection, and transparency to evade visual predators.
Phytoplankton is the primary food source for most marine organisms, either directly or indirectly. Since phytoplankton converts sunlight into energy in the form of carbohydrates, their habitat is necessarily confined to the upper pelagic layers. The organisms that feed on phytoplankton — the zooplankton — are thus forced to either remain in the upper pelagic layers or migrate vertically between the upper and lower layers. An important functional division within zooplankton is the ability to actively travel from location to location (nekton) or simply drift with the ocean currents (plankton) (Miller, 2004, p. 111). Whether an organism can swim or not determines, to a significant extent, what survival strategies are utilized.
Since plankton cannot evade predators, they rely on more passive defense mechanisms. Gastropods grow hard shells with narrow openings or elaborate sharp spines, and microalgae restrict the production of digestible structural material to deter foragers (Duffy and Hay, 2004, p. 137). Some species of benthic algae alter their form of growth to minimize predation. For example, Lithophyllum congestum grows as a smooth crust on reef slopes where parrotfish prefer to forage, and in upright branches on reef flats where parrotfish rarely go. The advantage of upright branches is a more rapid rate of growth and reproduction. Benthic plankton that lack physical defenses — such as sponges, ascidians, soft coral, and microbes — secrete noxious chemicals to discourage predation, or hide in habitats where predators are not active or are too large to fit.
Some forms of algae (seaweed) produce new growth at night when herbivore activity ceases, thus minimizing losses (Duffy and Hay, 2004, p. 134). During this period of vulnerability, the new growth secretes noxious chemicals until calcification can occur the following day. Algae may also escape vigorous foraging by growing in a refuge or garden protected by herbivores. Reef damselfishes engage in such behavior and will aggressively defend their "garden."
"Light-based and migratory strategies of pelagic zooplankton"
"Comparative synthesis of benthic and pelagic strategies"
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