**2. Aquatic environments**

In aquatic environments, there is a great diversity of photosynthetic organisms that interact and maintain complex ecological relationships with herbivores. In aquatic habitats, these interactions are considered very important since they affect the nutrient cycle and energy flows of food chains [7, 8].

Generally, when a plant is attacked, its defense mechanisms are activated through the production of diverse compounds generically termed plant secondary metabolites (PSM) [9]. It has been recognized that the secondary compounds may either serve as feeding deterrents or attractants in terrestrial plant-animal interactions or function as allelopathic chemicals or antibiotics; the same evolutionary pressures responsible for the many biologically active compounds found in terrestrial vegetation have been predicted to have parallels in marine [1, 10] and freshwater vegetation [11]. However, it is possible that the constraints in aquatic habitat lead to some differences in the production and action of these natural compounds.

#### **2.1. Marine environments**

The primary producers most widely distributed in marine habitats are the seaweeds (red, brown, and green algae); these photosynthetic organisms have developed several defenses in response to herbivores, for example, by having a resistant or unpalatable physical structure or a morphology that makes the feed difficult for the herbivore or by having spatial and temporally diverse stages of life cycle and by the production of chemical defense against herbivores ranging from unpalatable to toxic. Marine algae are known to produce a wide range of secondary metabolites with various biological actions [1], many of them with medicine and agriculture human uses [12].

On the sea, the most common grazers are generalist such as fishes, sea urchins, gastropods, polychaetes, and a great variety of marine crustaceans [7], which usually feed on the stalks of seaweed. Over 2400 natural products have been isolated from marine red, brown, and green algae, the majority are terpenoids and acetogenins; very few nitrogenous compounds have been isolated. In general, these compounds occur in relatively low concentration (0.2–2.0% dry mass), even so several ecological roles have been documented, and some of them are produced as protection against grazing [13, 14].

Brown algae produce about 1000 secondary metabolites, terpenoids and acetogenins are the most frequent, and they are the only seaweeds that produce polyphenolic compounds [12]. Polyphenolic compounds may function like terrestrial tannins, but they are structurally different so they are often termed phlorotannins to distinguish from them [12, 15, 16]. Phlorotannins are usually associated with a chemical defense: protection against grazing, pathogen attack, epiphytism, microfouling, and ultraviolet (UV) damages [16]. In red algae, the greatest variety of secondary metabolites is found, about 1240 reported; in Rhodophyta, all classes of compounds except phlorotannins can be found; most of them are halogenated (methanes, haloketones, phenolics, and complex terpenes) [15] recognized as antibacterial, antifungal, antiviral, antiinflammatory, antiproliferative, antifouling, antifeedant, cytotoxic, ichthyotoxic, and insecticidal activity [17]. In contrast, the green algae are the ones with the least secondary metabolites isolated; about 290 are known and most of them are sesquiterpenes and diterpenes; only few species produced halogenated compounds [18].

These diverse compounds are consumed directly by the herbivore when it feeds algae, but many of them may be released into the aquatic environment during algal growth or at cell lysis. In the last cases, chemical information is transmitted by diffusion and adventive lamina flow [19]; a major problem in the aquatic environment is dilution of the secreted products, so small molecules are favored because of their faster diffusion.

As we see, the natural compound production differs among seaweeds, as well as between and within species [7, 20], these differences suggest separate historical origin [11]. In the same way, the type and quantity of secondary compounds in algae differ from vascular plants; in algae, the absence of alkaloids and the presence of halogens compounds have been detected, contrary to terrestrial plants [7].

In marine environments, sea grasses are the only true submerged angiosperm, and as vascular plants, they are more complex morphologically and physiologically than algae; they produce some secondary compounds against herbivores, such as phenolic acids, phenolic acid sulfate esters, and sulfated flavonoids [21].
