2
Food Choice and Evolutionary Constraints
The choice, by any animal grazer, of plant foods available in the same geographical
and ecological range [7], is of paramount importance to determine its fitness and
represents a selective advantage. Mechanical, physiological and chemical factors
influence these choices [8]. For example, the selection of macroalgal food by various
marine invertebrates is hardly influenced by the toughness and the mechanical properties of thalli [9], but also by the chemical defenses (both constitutive and activated)
produced by various algae. Mechanical properties represent a first level of defense, but
plants are mainly efficient for the production of a large variety of chemical weapons
[10], including oxylipins, terpenoids, alkaloids, quinones, terpenoids, flavonoids,
glycosides, organic acids and other compounds (comprising cyanogenic and carcinogenic compounds) able to make them unpalatable or even toxic for various potential
consumers [11]. Diatoms, in this view, are among the most productive organisms,
since they are mechanically protected by silica frustules and chemically protected by
interesting and quite innovative compounds [12]. For example, the hatching success of
herbivore copepods is impaired by the feeding on diatom-dominated blooms, because
these microalgae produce wound-activated oxylipins and aldehydes [13] triggering
mortality or teratogenic modifications in copepod larvae. The production of woundactivated chemical defenses is widespread within marine microorganisms [14]. The
evolution of chemical weapons, according to the evolution of invertebrates able to
detoxify [15], incorporate [16] or use them for various purposes [17] represent
spectacular examples of invertebrate-microorganism co-evolution [18]. Thus, various
substances produced by algae and diatoms as repellents may acquire important
physiological roles and become decisive in patterns of food plant selection, as also
observed since the last century in terrestrial organisms [19].
3
The Diversity of Marine Diatoms
Diatoms are a major group of microalgae colonizing any marine and freshwater
environment, including humid zones, and they are quite abundant both as part of the
phytoplankton and in the benthos [20]. The large variability of environmental factors
influencing their ecology and morphology produced an extraordinary diversity of
forms and adaptations [21]. The impact of diatoms on the primary production of
oceans is quite high and their global scale contribution is estimated to be 30–40% of
the world’s primary production. Apparently, the diversity of diatom genomes (and,
consequently, their physiological plasticity) is so high to justify their success and
dominance in any aquatic environment, thanks to their ability to react to environmental changes, also producing spores and defense compounds [22]. In addition,
environmental factors drive their morphological variations and their frustules,
appearing as static structures, may adapt to both physicochemical conditions and
ecological constraints, leading to morphological variations of valves towards the
best solutions to face adverse or favorable conditions [23], according to their genetic
diversity. Most planktonic [24] and benthic diatoms have been demonstrated to
7 Co-evolution of the Shrimp Hippolyte inermis and the Diatoms Cocconeis. . .
137
Food Choice and Evolutionary Constraints
The choice, by any animal grazer, of plant foods available in the same geographical
and ecological range [7], is of paramount importance to determine its fitness and
represents a selective advantage. Mechanical, physiological and chemical factors
influence these choices [8]. For example, the selection of macroalgal food by various
marine invertebrates is hardly influenced by the toughness and the mechanical properties of thalli [9], but also by the chemical defenses (both constitutive and activated)
produced by various algae. Mechanical properties represent a first level of defense, but
plants are mainly efficient for the production of a large variety of chemical weapons
[10], including oxylipins, terpenoids, alkaloids, quinones, terpenoids, flavonoids,
glycosides, organic acids and other compounds (comprising cyanogenic and carcinogenic compounds) able to make them unpalatable or even toxic for various potential
consumers [11]. Diatoms, in this view, are among the most productive organisms,
since they are mechanically protected by silica frustules and chemically protected by
interesting and quite innovative compounds [12]. For example, the hatching success of
herbivore copepods is impaired by the feeding on diatom-dominated blooms, because
these microalgae produce wound-activated oxylipins and aldehydes [13] triggering
mortality or teratogenic modifications in copepod larvae. The production of woundactivated chemical defenses is widespread within marine microorganisms [14]. The
evolution of chemical weapons, according to the evolution of invertebrates able to
detoxify [15], incorporate [16] or use them for various purposes [17] represent
spectacular examples of invertebrate-microorganism co-evolution [18]. Thus, various
substances produced by algae and diatoms as repellents may acquire important
physiological roles and become decisive in patterns of food plant selection, as also
observed since the last century in terrestrial organisms [19].
3
The Diversity of Marine Diatoms
Diatoms are a major group of microalgae colonizing any marine and freshwater
environment, including humid zones, and they are quite abundant both as part of the
phytoplankton and in the benthos [20]. The large variability of environmental factors
influencing their ecology and morphology produced an extraordinary diversity of
forms and adaptations [21]. The impact of diatoms on the primary production of
oceans is quite high and their global scale contribution is estimated to be 30–40% of
the world’s primary production. Apparently, the diversity of diatom genomes (and,
consequently, their physiological plasticity) is so high to justify their success and
dominance in any aquatic environment, thanks to their ability to react to environmental changes, also producing spores and defense compounds [22]. In addition,
environmental factors drive their morphological variations and their frustules,
appearing as static structures, may adapt to both physicochemical conditions and
ecological constraints, leading to morphological variations of valves towards the
best solutions to face adverse or favorable conditions [23], according to their genetic
diversity. Most planktonic [24] and benthic diatoms have been demonstrated to
7 Co-evolution of the Shrimp Hippolyte inermis and the Diatoms Cocconeis. . .
137