Behind anemone lines: factors affecting division of labour in the social cnidarian Anthopleura elegantissima

Uncertainty and anxiety: Evolution and neurobiology

Anxiety is a complex phenomenon: Its eliciting stimuli and circumstances, component behaviors, and functional consequences are only slowly coming to be understood. Here, we examine defense systems from field studies; laboratory studies focusing on experimental analyses of behavior; and, the fear conditioning literature, with a focus on the role of uncertainty in promoting an anxiety pattern that involves high rates of stimulus generalization and resistance to extinction. Respectively, these different areas provide information on evolved elicitors of defense (field studies); outline a defense system focused on obtaining information about uncertain threat (ethoexperimental analyses); and, provide a simple, well-researched, easily measured paradigm for analysis of nonassociative stress-enhanced fear conditioning (the SEFL). Results suggest that all of these-each of which is responsive to uncertainty-play multiple and interactive roles in anxiety. Brain system findings for some relevant models are reviewed, with suggestions that further analyses of current models may be capable of providing a great deal of additional information about these complex interactions and their underlying biology.

Strategic growth in social vertebrates

Individual differences in growth and size of vertebrates often represent adaptive, plastic responses to contrasts in ecological conditions. Recent studies show that vertebrates can also modify their growth and size in an adaptive fashion in response to fine-grain changes in social conditions (which we refer to as strategic growth). Here, we review experimental evidence for strategic growth in social vertebrates. We describe a set of conditions under which strategic growth commonly occurs, and highlight potential examples of convergent evolution of strategic growth across the tree of life. This synthesis has implications for the way we think about organismal growth and size, because it underscores that the size of individuals can often be fine-tuned to their social environment.

The weird eusociality of polyembryonic parasites

Some parasitoid wasps possess soldier castes during their parasitic larval stage, but are often neglected from our evolutionary theories explaining caste systems in animal societies. This is primarily due to the polyembryonic origin of their societies. However, recent discoveries of polyembryonic trematodes (i.e. flatworms) possessing soldier castes require us to reconsider this reasoning. I argue we can benefit from including these polyembryonic parasites in eusocial discussions, for polyembryony and parasitism are taxonomically vast and influence the evolution of social behaviours and caste systems in various circumstances. Despite their polyembryony, their social evolution can be explained by theories of eusociality designed for parent–offspring groups, which are the subjects of most social evolution research. Including polyembryonic parasites in these theories follows the trend of major evolutionary transitions theory expanding social evolution research into all levels of biological organization. In addition, these continued discoveries of caste systems in parasites suggest social evolution may be more relevant to parasitology than currently acknowledged.

The Contribution of Clonality to Population Genetic Structure in the Sea Anemone, Diadumene lineata

Ecological and evolutionary processes differ depending on how genetic diversity is organized in space. For clonal organisms, the organization of both genetic and genotypic diversity can influence the fitness effects of competition, the mating system, and reproductive mode, which are key drivers of life cycle evolution. Understanding how individual reproductive behavior contributes to population genetic structure is essential for disentangling these forces, particularly in species with complex and plastic life cycles. The widespread sea anemone, Diadumene lineata, exhibits temperature-dependent fission, which contributes to predictable variation in clonal rate along the Atlantic coast of the United States, part of its non-native range. Because warmer conditions lead to higher rates of clonality, we expected to find lower genotypic and genetic diversity in lower versus higher latitude populations. We developed primers for 11 microsatellite loci and genotyped 207 anemones collected from 8 sites ranging from Florida to Massachusetts. We found clonal influence at all sites, and as predicted, the largest clones were found at lower latitude sites. We also found genetic signatures of sex in the parts of the range where gametogenesis is most common. Evidence of sex outside the native range is novel for this species and provides insights into the dynamics of this successful invader. Our findings also illustrate challenges that partially clonal taxa pose for eco-evolutionary studies, such as difficulty sampling statistically robust numbers of genets and interpretating common population genetic metrics. For example, we found high among-locus variation in FIS, which makes the meaning of mean multilocus FIS unclear.

Carbonic anhydrases are influenced by the size and symbiont identity of the aggregating sea anemone Anthopleura elegantissima

Carbonic anhydrases (CA; EC 4.2.1.1) play a vital role in dissolved inorganic carbon (DIC) transport to photosynthetic microalgae residing in symbiotic cnidarians. The temperate sea anemone Anthopleura elegantissima can occur in three symbiotic states: hosting Breviolum muscatinei (brown), hosting Elliptochloris marina (green) or without algal symbionts (aposymbiotic). This provides a basis for A. elegantissima to be a model for detailed studies of the role of CA in DIC transport. This study investigated the effects of symbiosis, body size and light on CA activity and expression, and suggests that A. elegantissima has a heterotrophy-dominated trophic strategy. We identified putative A. elegantissima CA genes and performed phylogenetic analyses to infer subcellular localization in anemones. We performed experiments on field-collected anemones to compare: (1) CA activity and expression from anemones in different symbiotic states, (2) CA activity in brown anemones as a function of size, and (3) CA activity in anemones of different symbiotic states that were exposed to different light intensities. CA activity in brown anemones was highest, whereas activity in green and aposymbiotic anemones was low. Several CAs had expression patterns that mirrored activity, while another had expression that was inversely correlated with activity, suggesting that symbionts may induce different DIC transport pathways. Finally, CA activity was inversely correlated with anemone size. Our results suggest that the observed CA activity and expression patterns are affected not only by symbiosis, but also by other factors in the host physiology, including trophic strategy as it relates to body size and cellular pH homeostasis.

How counterfactuals of Red-Queen theory shed light on science and its historiography

A historical episode of evolutionary theory, which has lead to the Red Queen theory of the evolutionary maintenance of sex, includes two striking contingencies. These are used to explore alternative what-if scenarios, in order to test some common opinions about such counterfactuals. This sheds new light on the nature of science and its historiography. One counterfactual leads to an unexpected convergence of its result to that of the actual science but, nevertheless, differs in its causal structure. The other diverges towards an incompatible alternative, but this requires further contingent choices that also diverge from actual science. The convergence in the first counterfactual is due to a horizontal transfer of knowledge. Similar transfers of knowledge are typical for innovations of actual science. This suggests that contingent choices can merge as well as fork research traditions both in actual research and counterfactual history. Neither the paths of the actual history of science nor those of its counterfactual alternatives will form a tree of exclusively diverging bifurcations, but a network instead. Convergencies in counterfactuals may, therefore, be due to the web-structure of science as much as to the aims of the historians in question. Furthermore, the difference in causal structure between the actual science and its convergent counterfactual might become diagnostic for external factors rather than internal aims forcing a historian towards convergence.

Social niche construction and evolutionary transitions in individuality

Social evolution theory conventionally takes an externalist explanatory stance, treating observed cooperation as explanandum and the positive assortment of cooperative behaviour as explanans. We ask how the circumstances bringing about this positive assortment arose in the first place. Rather than merely push the explanatory problem back a step, we move from an externalist to an interactionist explanatory stance, in the spirit of Lewontin and the Niche Construction theorists. We develop a theory of 'social niche construction' in which we consider biological entities to be both the subject and object of their own social evolution. Some important cases of the evolution of cooperation have the side-effect of causing changes in the hierarchical level at which the evolutionary process acts. This is because the traits (e.g. life-history bottlenecks) that act to align the fitness interests of particles (e.g. cells) in a collective can also act to diminish the extent to which those particles are bearers of heritable fitness variance, while augmenting the extent to which collectives of such particles (e.g. multicellular organisms) are bearers of heritable fitness variance. In this way, we can explain upward transitions in the hierarchical level at which the Darwinian machine operates in terms of particle-level selection, even though the outcome of the process is a collective-level selection regime. Our theory avoids the logical and metaphysical paradoxes faced by other attempts to explain evolutionary transitions.

Growing coral larger and faster: micro-colony-fusion as a strategy for accelerating coral cover

Fusion is an important life history strategy for clonal organisms to increase access to shared resources, to compete for space, and to recover from disturbance. For reef building corals, fragmentation and colony fusion are key components of resilience to disturbance. Observations of small fragments spreading tissue and fusing over artificial substrates prompted experiments aimed at further characterizing Atlantic and Pacific corals under various conditions. Small (∼1–3 cm²) fragments from the same colony spaced regularly over ceramic tiles resulted in spreading at rapid rates (e.g., tens of square centimeters per month) followed by isogenic fusion. Using this strategy, we demonstrate growth, in terms of area encrusted and covered by living tissue, of Orbicella faveolata, Pseudodiploria clivosa, and Porites lobata as high as 63, 48, and 23 cm² per month respectively. We found a relationship between starting and ending size of fragments, with larger fragments growing at a faster rate. Porites lobata showed significant tank effects on rates of tissue spreading indicating sensitivity to biotic and abiotic factors. The tendency of small coral fragments to encrust and fuse over a variety of surfaces can be exploited for a variety of applications such as coral cultivation, assays for coral growth, and reef restoration.

A RNA-seq approach to identify putative toxins from acrorhagi in aggressive and non-aggressive Anthopleura elegantissima polyps

BACKGROUND

The use of venom in intraspecific aggression is uncommon and venom-transmitting structures specifically used for intraspecific competition are found in few lineages of venomous taxa. Next-generation transcriptome sequencing allows robust characterization of venom diversity and exploration of functionally unique tissues. Using a tissue-specific RNA-seq approach, we investigate the venom composition and gene ontology diversity of acrorhagi, specialized structures used in intraspecific competition, in aggressive and non-aggressive polyps of the aggregating sea anemone Anthopleura elegantissima (Cnidaria: Anthozoa: Hexacorallia: Actiniaria: Actiniidae).

RESULTS

Collectively, we generated approximately 450,000 transcripts from acrorhagi of aggressive and non-aggressive polyps. For both transcriptomes we identified 65 candidate sea anemone toxin genes, representing phospholipase A2s, cytolysins, neurotoxins, and acrorhagins. When compared to previously characterized sea anemone toxin assemblages, each transcriptome revealed greater within-species sequence divergence across all toxin types. The transcriptome of the aggressive polyp had a higher abundance of type II voltage gated potassium channel toxins/Kunitz-type protease inhibitors and type II acrorhagins. Using toxin-like proteins from other venomous taxa, we also identified 612 candidate toxin-like transcripts with signaling regions, potentially unidentified secretory toxin-like proteins. Among these, metallopeptidases and cysteine rich (CRISP) candidate transcripts were in high abundance. Furthermore, our gene ontology analyses identified a high prevalence of genes associated with "blood coagulation" and "positive regulation of apoptosis", as well as "nucleoside: sodium symporter activity" and "ion channel binding". The resulting assemblage of expressed genes may represent synergistic proteins associated with toxins or proteins related to the morphology and behavior exhibited by the aggressive polyp.

CONCLUSION

We implement a multifaceted approach to investigate the assemblage of expressed genes specifically within acrorhagi, specialized structures used only for intraspecific competition. By combining differential expression, phylogenetic, and gene ontology analyses, we identify several candidate toxins and other potentially important proteins in acrorhagi of A. elegantissima. Although not all of the toxins identified are used in intraspecific competition, our analysis highlights some candidates that may play a vital role in intraspecific competition. Our findings provide a framework for further investigation into components of venom used exclusively for intraspecific competition in acrorhagi-bearing sea anemones and potentially other venomous animals.

What determines the duration of war? Insights from assessment strategies in animal contests

Interstate wars and animal contests both involve disputed resources, restraint and giving up decisions. In both cases it seems illogical for the weaker side to persist in the conflict if it will eventually lose. In the case of animal contests analyses of the links between opponent power and contest duration have provided insights into what sources of information are available to fighting animals. I outline the theory of information use during animal contests and describe a statistical framework that has been used to distinguish between two strategies that individuals use to decide whether to persist or quit. I then apply this framework to the analysis of interstate wars. War duration increases with the power of winners and losers. These patterns provide no support for the idea that wars are settled on the basis of mutual assessment of capabilities but indicate that settlement is based on attrition. In contrast to most animal contests, war duration is as closely linked to the power of the winning side as to that of the losing side. Overall, this analysis highlights a number of similarities between animal contests and interstate war, indicating that both could be investigated using similar conceptual frameworks.

Familial strife on the seashore: aggression increases with relatedness in the sea anemone Actinia equina

Pairwise contests occur when two individuals compete directly over ownership of an indivisible resource. Contests vary in the degree of escalation, some encounters being settled through non-injurious behaviour while others are only resolved after dangerous fighting. Here, we investigate the role of relatedness, assessed using AFLP analysis, on the occurrence of stinging during staged contests in the beadlet sea anemone Actinia equina. Contrary to our expectations, we found that the chance of stinging, and hence the chance of inflicting damage, increased with the degree of relatedness between the two opponents. This result may be explained by the negative relationship between asymmetry in fighting ability and escalation level predicted by theory. We suggest that in order to fully understand how relatedness influences aggression, predictions from kin selection theory should be incorporated with those from contest theory.

Cellular differentiation and individuality in the 'minor' multicellular taxa

Biology needs a concept of individuality in order to distinguish organisms from parts of organisms and from groups of organisms, to count individuals and compare traits across taxa, and to distinguish growth from reproduction. Most of the proposed criteria for individuality were designed for 'unitary' or 'paradigm' organisms: contiguous, functionally and physiologically integrated, obligately sexually reproducing multicellular organisms with a germ line sequestered early in development. However, the vast majority of the diversity of life on Earth does not conform to all of these criteria. We consider the issue of individuality in the 'minor' multicellular taxa, which collectively span a large portion of the eukaryotic tree of life, reviewing their general features and focusing on a model species for each group. When the criteria designed for unitary organisms are applied to other groups, they often give conflicting answers or no answer at all to the question of whether or not a given unit is an individual. Complex life cycles, intimate bacterial symbioses, aggregative development, and strange genetic features complicate the picture. The great age of some of the groups considered shows that 'intermediate' forms, those with some but not all of the traits traditionally associated with individuality, cannot reasonably be considered ephemeral or assumed transitional. We discuss a handful of recent attempts to reconcile the many proposed criteria for individuality and to provide criteria that can be applied across all the domains of life. Finally, we argue that individuality should be defined without reference to any particular taxon and that understanding the emergence of new kinds of individuals requires recognizing individuality as a matter of degree.

Prolonged exposure to elevated CO(2) promotes growth of the algal symbiont Symbiodinium muscatinei in the intertidal sea anemone Anthopleura elegantissima

Some photosynthetic organisms benefit from elevated levels of carbon dioxide, but studies on the effects of elevated PCO(2) on the algal symbionts of animals are very few. This study investigated the impact of hypercapnia on a photosynthetic symbiosis between the anemone Anthopleura elegantissima and its zooxanthella Symbiodinium muscatinei. Anemones were maintained in the laboratory for 1 week at 37 Pa PCO(2) and pH 8.1. Clonal pairs were then divided into two groups and maintained for 6 weeks under conditions naturally experienced in their intertidal environment, 45 Pa PCO(2), pH 8.1 and 231 Pa PCO(2), pH 7.3. Respiration and photosynthesis were measured after the 1-week acclimation period and after 6 weeks in experimental conditions. Density of zooxanthellal cells, zooxanthellal cell size, mitotic index and chlorophyll content were compared between non-clonemate anemones after the 1-week acclimation period and clonal anemones at the end of the experiment. Anemones thrived in hypercapnia. After 6 weeks, A. elegantissima exhibited higher rates of photosynthesis at 45 Pa (4.2 µmol O(2) g(-1) h(-1)) and 231 Pa (3.30 µmol O(2) g(-1) h(-1)) than at the initial 37 Pa (1.53 µmol O(2) g(-1) h(-1)). Likewise, anemones at 231 Pa received more of their respiratory carbon from zooxanthellae (CZAR  = 78.2%) than those at 37 Pa (CZAR  = 66.6%) but less than anemones at 45 Pa (CZAR  = 137.3%). The mitotic index of zooxanthellae was significantly greater in the hypercapnic anemones than in anemones at lower PCO(2). Excess zooxanthellae were expelled by their hosts, and cell densities, cell diameters and chlorophyll contents were not significantly different between the groups. The response of A. elegantissima to hypercapnic acidification reveals the potential adaptation of an intertidal, photosynthetic symbiosis for high PCO(2).

Evolution of functional specialization and division of labor

Division of labor among functionally specialized modules occurs at all levels of biological organization in both animals and plants. Well-known examples include the evolution of specialized enzymes after gene duplication, the evolution of specialized cell types, limb diversification in arthropods, and the evolution of specialized colony members in many taxa of marine invertebrates and social insects. Here, we identify conditions favoring the evolution of division of labor by means of a general mathematical model. Our starting point is the assumption that modules contribute to two different biological tasks and that the potential of modules to contribute to these tasks is traded off. Our results are phrased in terms of properties of performance functions that map the phenotype of modules to measures of performance. We show that division of labor is favored by three factors: positional effects that predispose modules for one of the tasks, accelerating performance functions, and synergistic interactions between modules. If modules can be lost or damaged, selection for robustness can counteract selection for functional specialization. To illustrate our theory we apply it to the evolution of specialized enzymes coded by duplicated genes.

Is boldness a resource-holding potential trait? Fighting prowess and changes in startle response in the sea anemone, Actinia equina

Contest theory predicts the evolution of a stable mixture of different strategies for fighting. Here, we investigate the possibility that stable between-individual differences in startle-response durations influence fighting ability or 'resource-holding potential' (RHP) in the beadlet sea anemone, Actinia equina. Both winners and losers showed significant repeatability of pre-fight startle-response durations but mean pre-fight startle-response durations were greater for eventual losers than for eventual winners, indicating that RHP varies with boldness. In particular, individuals with short startle responses inflicted more attacks on their opponent. Both repeatability and mean-level responses were changed by the experience of fighting, and these changes varied with outcome. In losers, repeatability was disrupted to a greater extent and the mean startle-response durations were subject to a greater increase than in winners. Thus, following a fight, this behavioural correlate of RHP behaves in a way similar to post-fight changes in physiological status, which can also vary between winners and losers. Understanding the links between aggression and boldness therefore has the potential to enhance our understanding of both the evolution of animal personality and the 'winner and loser effects' of post-fight changes in RHP.

Evolutionary causes and consequences of consistent individual variation in cooperative behaviour

Behaviour is typically regarded as among the most flexible of animal phenotypic traits. In particular, expression of cooperative behaviour is often assumed to be conditional upon the behaviours of others. This flexibility is a key component of many hypothesized mechanisms favouring the evolution of cooperative behaviour. However, evidence shows that cooperative behaviours are often less flexible than expected and that, in many species, individuals show consistent differences in the amount and type of cooperative and non-cooperative behaviours displayed. This phenomenon is known as 'animal personality' or a 'behavioural syndrome'. Animal personality is evolutionarily relevant, as it typically shows heritable variation and can entail fitness consequences, and hence, is subject to evolutionary change. Here, we review the empirical evidence for individual variation in cooperative behaviour across taxa, we examine the evolutionary processes that have been invoked to explain the existence of individual variation in cooperative behaviour and we discuss the consequences of consistent individual differences on the evolutionary stability of cooperation. We highlight that consistent individual variation in cooperativeness can both stabilize or disrupt cooperation in populations. We conclude that recognizing the existence of consistent individual differences in cooperativeness is essential for an understanding of the evolution and prevalence of cooperation.

Incomplete division of labor: error-prone multitaskers coexist with specialists

Division of labor (DoL) occurs when individual members of a group specialize by performing particular tasks toward some common goal. Under complete DoL, every individual acts as a specialist and so performs only one kind of task. But under incomplete DoL, some individuals may act as generalists and so have the capacity to perform more than one kind of task. This persistence of generalists in the presence of specialists presents a theoretical challenge, particularly if generalists must pay an extra cost, an inefficiency penalty, for their capacity to perform more than one type of task. Prior work focused on how such costs tend to drive evolution toward complete DoL, with only specialists typically remaining at equilibrium [Wahl, L.M., 2002a. Evolving the division of labor: generalists, specialists and task allocation. J. Theoret. Biol. 219, 371-388; Wahl, L.M., 2002b. The division of labor: genotypic versus phenotypic specialization. Am. Nat. 160, 135-145]. Relaxing this key assumption, we show that generalists, despite paying some extra costs, can coexist with specialists. Relaxing another assumption, we also show that this coexistence can hold even when generalists often perform the wrong task. How can stable multitasking emerge despite this flawed decision-making? From the perspective that cognitive errors matter only when they translate into fitness decrements, we observe that error-prone generalists may persist most commonly in situations in which their mistakes do little to jeopardize group success. Our findings show that incomplete DoL can emerge even when generalists often err and must pay extra costs for their multitasking capacity.