How reversible is development? Contrast between developmentally plastic gain and loss of segments in barnacle feeding legs

Influence of a changing wave climate on the quality and morphometry of the stalked barnacle Pollicipes pollicipes (Gmelin, 1789), along the coasts of NW Iberia

Wave climate is shifting over the last decades along the Atlantic coasts of Europe ultimately driven by large-scale patterns of atmospheric variability forced by anthropogenic global warming. Changes in wave height and surf zone orbital currents are hypothesized to drive marked shifts in the shape of intertidal organisms such as the stalked barnacle Pollicipes pollicipes, whose quality and market price are known to decrease non linearly with the peduncle length: width ratio S. This study evaluates wave trends in NW Iberian Peninsula, using the Spanish Port System 2006-2020 SIMAR wave hindcast. On the other hand, trends in stalked barnacle morphology and quality are estimated from 26 sites at the management regions of Baiona and A Guarda between 2011 and 2020. Results show evidence of temporal changes in barnacle quality and, especially, morphometry caused by simultaneous shifts in winter wave induced orbital currents. Because of the non linear relationship between S and the high quality threshold, large increases in S are usually translated to small reductions in quality. However, we identified a tipping point around S = 2.4 that if surpassed can lead to great drops in barnacle quality. In addition, changes in wave forcing will have different effects at each extraction site, as trends in wave climate are decoupled from barnacle morphometry at steeper sites sheltered from the predominant wave direction. In conclusion, this knowledge could be applied to develop site specific barnacle harvesting strategies based on annual wave climate forecasts.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s11160-024-09838-2.

Phenotypic Switching Resulting From Developmental Plasticity: Fixed or Reversible?

The prevalent view of developmental phenotypic switching holds that phenotype modifications occurring during critical windows of development are "irreversible" - that is, once produced by environmental perturbation, the consequent juvenile and/or adult phenotypes are indelibly modified. Certainly, many such changes appear to be non-reversible later in life. Yet, whether animals with switched phenotypes during early development are unable to return to a normal range of adult phenotypes, or whether they do not experience the specific environmental conditions necessary for them to switch back to the normal range of adult phenotypes, remains an open question. Moreover, developmental critical windows are typically brief, early periods punctuating a much longer period of overall development. This leaves open additional developmental time for reversal (correction) of a switched phenotype resulting from an adverse environment early in development. Such reversal could occur from right after the critical window "closes," all the way into adulthood. In fact, examples abound of the capacity to return to normal adult phenotypes following phenotypic changes enabled by earlier developmental plasticity. Such examples include cold tolerance in the fruit fly, developmental switching of mouth formation in a nematode, organization of the spinal cord of larval zebrafish, camouflage pigmentation formation in larval newts, respiratory chemosensitivity in frogs, temperature-metabolism relations in turtles, development of vascular smooth muscle and kidney tissue in mammals, hatching/birth weight in numerous vertebrates,. More extreme cases of actual reversal (not just correction) occur in invertebrates (e.g., hydrozoans, barnacles) that actually 'backtrack' along normal developmental trajectories from adults back to earlier developmental stages. While developmental phenotypic switching is often viewed as a permanent deviation from the normal range of developmental plans, the concept of developmental phenotypic switching should be expanded to include sufficient plasticity allowing subsequent correction resulting in the normal adult phenotype.

How adaptive plasticity evolves when selected against

Adaptive plasticity allows organisms to cope with environmental change, thereby increasing the population's long-term fitness. However, individual selection can only compare the fitness of individuals within each generation: if the environment changes more slowly than the generation time (i.e., a coarse-grained environment) a population will not experience selection for plasticity even if it is adaptive in the long-term. How does adaptive plasticity then evolve? One explanation is that, if competing alleles conferring different degrees of plasticity persist across multiple environments, natural selection between genetic lineages could select for adaptive plasticity (lineage selection). We show that adaptive plasticity can evolve even in the absence of such lineage selection. Instead, we propose that adaptive plasticity in coarse-grained environments evolves as a by-product of inefficient short-term natural selection: populations that rapidly evolve their phenotypes in response to selective pressures follow short-term optima, with the result that they have reduced long-term fitness across environments. Conversely, populations that accumulate limited genetic change within each environment evolve long-term adaptive plasticity even when plasticity incurs short-term costs. These results remain qualitatively similar regardless of whether we decrease the efficiency of natural selection by increasing the rate of environmental change or decreasing mutation rate, demonstrating that both factors act via the same mechanism. We demonstrate how this mechanism can be understood through the concept of learning rate. Our work shows how plastic responses that are costly in the short term, yet adaptive in the long term, can evolve as a by-product of inefficient short-term selection, without selection for plasticity at either the individual or lineage level.