Inducible defenses are responses activated through a previous encounter with a consumer or competitor that confer some degree of resistance to subsequent attacks. While the importance of inducible resistance has long been known in host-parasite interactions, it is only recently that its importance has emerged in other natural systems. Althought the structural defenses produced by invertebrates to their competitors and predators are by no means the same as an immune response triggered by parasites, these responses all share the properties of (1) specificity, (2) amplification and (3) memory. This review discusses the following ecological consequences and evolutionary causes of inducible defenses: (1) Inducible defenses render historical factors important in biological interactions and can affect the probability of individual survival and growth, as well as affect population dynamics of consumers in some circumstances. (2) Although the benefits of inducible defenses are often balanced by fitness costs, incl...

The ecology and evolution of inducible defenses

It is well documented that animals take risk of predation into account when making decisions about how to behave in particular situations, often trading-off risk against opportunities for mating or acquiring energy. Such an ability implies that animals have reliable information about the risk of predation at a given place and time. Chemosensory cues are an important source of such information. They reliably reveal the presence of predators (or their presence in the immediate past) and may also provide information on predator activity level and diet. In certain circumstances (e.g., in the dark, for animals in hiding) they may be the only cues available. Although a vast literature exists on the responses of prey to predator chemosensory cues (or odours), these studies are widely scattered, from marine biology to biological control, and not well known or appreciated by behavioural ecologists. In this paper, we provide an exhaustive review of this literature, primarily in tabular form. We highlight so...

The scent of death: Chemosensory assessment of predation risk by prey animals

Within a population there are frequently several discrete morphs. While in some cases, particularly color polymorphisms, this variation can be explained by simple Mendelian modes of inheritance, in many cases the evidence suggests a polygenic pattern of inheritance. The threshold model of quantitative genetics, in which discrete morphs are determined by some underlying continuously distributed trait and a threshold(s) of expression, is applied appropriately in these cases. The discrete morphs exhibited in cyclomorphosis, pedomorphosis, pedogenesis, "protective" dimorphisms, trophic dimorphisms, wing dimorphism, and mating strategies can all be analyzed by using this model. Analyses of a wide range of different types of threshold traits show that there is typically a large additive genetic component, but that there is also strong environmental induction. A review of studies shows that no morph has a universally higher fitness, but that there is a tradeoff, with the relative fitnesses of two morphs being co...

The Evolution of Threshold Traits in Animals

This study was designed to measure and separate the physiological costs of inducible defenses from life history trade—offs and maternal effects in the waterflea Daphnia pulex. Juveniles of D. pulex produce morphological changes ("neckteeth") and undergo life history shifts as defenses against predatory Chaoborus (phantom midge) larvae. These traits are induced by a chemical cue (kairomone) released by the predator. I performed life history experiments with and without Chaoborus kairomones at different food levels to quantify the induced changes and their potential physiological costs. The Daphnia clone used in this study also increased its body depth in response to the predator substance. Life history shifted toward a larger body size (both length and depth) and higher fecundity, which was balanced by an increased time to reach maturity and by increased adult instar durations. Reproductive effort was higher in the typical morph in the first adult instar, indicating resource allocation shifts towards growth in the protected morph. However, even in the absence of predation the chemically induced protected morph tended to show an increased intrinsic rate of population growth (r). The longer time to reach maturity was not a direct physiological cost of neckteeth production, but a trade—off for larger body size. The life history shifts are independent of neckteeth formation. Developmental mechanisms leading to life history changes occurred after neckteeth were induced and could thus be uncoupled from neckteeth formation and its direct costs. In this study no direct costs were found. Carbon incorporation rates for the two morphs, at high and low food, were not different. As a maternal effect, the large females of the induced morph produced larger neonates which, in turn, matured at a larger size. Morphological changes, life history shifts, and maternal effects acted in concert to form defenses against Chaoborus. This study shows that the often assumed high physiological costs resulting from the formation or maintenance of the defenses are not necessary to explain the evolution of inducible defenses. As morphological changes increase the visibility of Daphnia pulex, a fitness disadvantage can be caused by a changing predator regime (e.g., fish). The results of this study suggest that environments with changing predator selectivities favor the evolution of inducible defenses.

https://www.jstor.org/stable/pdfplus/1940703.pdf

Predator‐Induced Morphological Defenses: Costs, Life History Shifts, and Maternal Effects in Daphnia Pulex

This study was designed to measure and separate the physiological costs of inducible defenses from life history trade-offs and maternal effects in the waterflea D(aphnitl /)ulex. Juveniles of D. pulex produce morphological changes ("neckteeth") and undergo life history shifts as defenses against predatory Ch(uoborus (phantom midge) larvae. These traits are induced by a chemical cue (kairomone) released by the predator. I performed life history experiments with and without Chuioboruis kairomones at different food levels to quantify the induced changes and their potential physiological costs. The Dalphni(u clone used in this study also increased its body depth in response to the predator substance. Life history shifted toward a larger body size (both length and depth) and higher fecundity, which was balanced by an increased time to reach maturity and by increased adult instar durations. Reproductive effort was higher in the typical morph in the first adult instar, indicating resource allocation shifts towards growth in the protected morph. However, even in the absence of predation the chemically induced protected morph tended to show an increased intrinsic rate of population growth (r-). The longer time to reach maturity was not a direct physiological cost of neckteeth production, but a trade-off for larger body size. The life history shifts are independent of neckteeth formation. Developmental mechanisms leading to life history changes occurred after neckteeth were induced and could thus be uncoupled from neckteeth formation and its direct costs. In this study no direct costs were found. Carbon incorporation rates for the two morphs, at high and low food, were not different. As a maternal effect, the larger females of the induced morph produced larger neonates which, in turn, matured at a larger size. Morphological changes, life history shifts, and maternal effects acted in concert to form defenses against Chaioboruts. This study shows that the often assumed high physiological costs resulting from the formation or maintenance of the defenses are not necessary to explain the evolution of inducible defenses. As morphological changes increase the visibility of Daiphnlai pule-V, a fitness disadvantage can be caused by a changing predator regime (e.g., fish). The results of this study suggest that environments with changing predator selectivities favor the evo- lution of inducible defenses.

Costs, life history shifts, and maternal effects in daphnia pulex'

Etude de l'anatomie interne et des structures ciliaires permettant l'identification des especes du sous ordre Urostylina. Description de Bakuella variabilis n.sp. Holosticha estuarii n.sp. et H. polystylata n.sp. On erige Pseudokeronopsis n.g. renfermant des especes assignees precedemment a Keronopsis

The suborder Urostylina Jankowski (Ciliophora, Hypotrichida): Morphology, systematics, and identification of species

. The hypotrich ciliate Onychodromus quadricornutus is remarkable in its potential for voluminous size (up to 900 μm in length), its possession of a unique set of four dorsal spines or horns, and its capability to express two kinds of developmental polymorphism induced by intraspecific predation. Cell length frequencies in replicates of a well-fed clone show normal distributions; starvation followed by intraspecific predation, however, induces cells within a clone to transform into two size classes: small lanceolate cells and cannibal giants. Induction experiments indicate that a substance released by cannibal giants stimulates defensive spine growth in clonemates within 24 h. Giants can also induce spine growth in non-clonemates. Furthermore, O. quadricornutus cells exposed to the predacious ciliate Lembadion magnum also develop hypertrophied spines. Selection experiments show that conspecific giants prey on cells with undeveloped spines ( 40 μm in length). Transformation of a population of similarly sized O. quadricornutus cells into two different size classes may function to increase the range of potential prey sizes available to the O. quadricornutus population; hypertrophied spines appear to function as an inducible defense against intermittent predators appearing in the system including conspecific giants. This is the first reported case of a defensive developmental polymorphism induced by intraspecific predation.

Developmental Polymorphism Induced by Intraspecific Predation in the Ciliated Protozoon Onychodromus quadricornutus

The cortical development during cell division and the interphase ultrastructure of the marine interstitial hypotrich Certesia quadrinucleata is described using light microscopy and both scanning and transmission electron microscopy. Membranelles are paramembranelles; postciliary microtubules from rightmost membranellar kinetosomes line the buccal cavity and separate parallel arrays of pharyngeal discs that border the cytopharynx. A large paroral membrane is present; an endoral membrane is absent. Alveolar plates lie within alveolar membranes except in regions where organelles and organellar complexes (cirri, the condylopallium, dorsal bristles, membranelles, and the paroral membrane) emerge from the cortex. Muciferous-like bodies attach to the plasma membrane in these regions. Dorsal bristles possess transverse and postciliary microtubules as well as kinetodesmal fiber like those of other hypotrichs. Lasiosomes are present. A unique bulbous structure—the condylopallium—protrudes from the anterior right of the cell. The morphogenetic pattern is euplotine in that cortical development begins in one latitudinal zone, and the oral primordium of the opisthe develops within a subsurface pouch apart from the frontal primordia. Microtubular bundles appear beside (later attached to) developing frontal anlagen; they disappear after cirri are in final interphase locations. Although possessing unique characters, Certesia shares a close phylogenetic relationship with Euplotes.

Ultrastructure and Cortical Morphogenesis in the Euplotine Hypotrich Certesia quadrinucleata Fabre‐Domergue, 1885 (Ciliophora, Protozoa)

Discotricha papillifera Tuffrau, a marine interstitial ciliate, is redescribed with the aid of light, scanning, and transmission electron microscopy from cells collected at a New Hampshire beach. Presence of primitive membranelles as well as an advanced stomatogenesis is demonstrated. The ultrastructure, including a unique membrane-bounded septate structure, is described. The cell is tentatively placed in the nassulid suborder Microthoracina, but affinities with other groups are discussed.

Barry J. Wicklow

Papers

The suborder Urostylina Jankowski (Ciliophora, Hypotrichida): Morphology, systematics, and identification of species

Developmental Polymorphism Induced by Intraspecific Predation in the Ciliated Protozoon Onychodromus quadricornutus

Ultrastructure and Cortical Morphogenesis in the Euplotine Hypotrich Certesia quadrinucleata Fabre‐Domergue, 1885 (Ciliophora, Protozoa)

Discotricha papillifera: structure and morphogenesis of a marine interstitial ciliate.