Horseshoe Crabs Teach Us About Heterochrony

A new method for quantifying heterochrony in evolutionary lineages

James C. Lamsdell

Summarized by Anna Geldert

What data were used? A total of 20 traits that display heterochronic conditions for 54 species of horseshoe crabs (both living and extinct) were studied. 256 traits were examined and documented in these horseshoe crabs and 99 related species to make a character matrix. Of the 54 horseshoe crabs, environmental data from previous studies was also collected to determine the species’ habitat..

Methods: This paper presents a new method for quantifying heterochrony through a process called “heterochronic weighting.” Heterochrony is a process that alters the timing and length of developmental stages of organisms, and is characterized as either paedomorphism (retaining juvenile characteristics as an adult) or peramorphism (developing beyond what is seen in related species; more “adult-like”.) For each characteristic, paedomorphic traits were assigned a score of -1, peramorphic traits were assigned a score of +1, and neutral characteristics were assigned a score of 0. The heterochronic weighting of a species was then defined as the sum of all scores divided by the number of characteristics. The author also looked at heterochrony in an evolutionary context. He generated a probable evolutionary tree using a computer model that related species based on shared traits. He then used the tree to determine the heterochronic weighting of the clade (i.e., evolutionary group) by averaging those of the individual species. The differences in heterochronic wightings between habitat preferences (marine or nonmarine) and clades were tested for statistical significance. Lastly, the author tested to see if there were concerted trends towards paedomorphy or peramorphy in each clade.  The evolutionary tree was also tested to determine the most likely habitat for ancestry species of horseshoe crabs, which gave insight to when shifts from marine to nonmarine environments occurred.

The figure shows a diagram of the heterochronic conditions as seen in limb length. Three drawings of the underside of the head shield of a horseshoe crabs are shown side by side. There is one small pair of claw-like appendages towards the front of the head shield and ten longer walking limbs visible. The first diagram has the longest limbs, extending outside the shell. It is labeled “-1,” representing a paedomorphic condition. The second diagram, labeled “0” for a neutral condition, has shorter limbs that are all contained under the shell, though some extend nearly to the edge. Lastly, the final diagram is labeled “+1” for a peramorphic condition. The limbs of the crab in this diagram are the shortest, spanning only a third to a half the width of the shell.
Fig 1. Variations in limb length serve as an example of a heterochronic characteristic in horseshoe crabs. Paedomorphic (-1), neutral (0), and peramorphic (+1) conditions are shown.

Results: Overall, heterochronic weighting proved successful in quantifying the paedomorphic and peramorphic changes in horseshoe crab characteristics. Of the four clades studied, two (Bellinurina and Austrolimulidae) were found to have statistically significant occurrences of heterochrony, with Bellinurina trending towards paedomorphic characteristics and Austrolimulidae trending towards peramorphic characteristics. The Paleolimulidae clade was characterized as having non-significant  heterochronic weightings, while the Limulidae showed a slight peramorphic trend that could be explained by random evolution, not necessarily a concerted trend. More extreme heterochronic weightings (both positive and negative) were associated with the evolutionary transition to non-marine habitats, as was the case for both Bellinurina and Austrolimulidae clades.

Why is this study important? First and foremost, this study is important because it developed a method for quantifying instances of heterochrony, which has not been studied in a combined phylogenetic and ecological context. This gives insight into the interaction between ecology and heterochrony, especially as an evolutionary mechanism. For example, it is noteworthy that both clades that transitioned to non-ancestral nonmarine environments (Bellinurina and Austrolimulidae) experienced higher rates of heterochrony, suggesting that greater ecological change may correlate with increased likelihood for developmental changes in horseshoe crabs. However, it is also important to recognize that environmental affinity is not the only factor influencing heterochrony, or else Bellinurina and Austrolimulidae would have developed in the same way, trending towards either paedomorphic or peramorphic characteristics. The opposite trajectories of the two clades suggests that environmental pressures may increase heterochrony, but underlying genetic factors determine the direction of development.

The big picture: The process of heterochronic weighting developed in this study has the potential to advance the field of paleobiology, as the author was now able to quantify paedomorphy and peramorphy throughout evolutionary history. This allows for a deeper understanding of the relationship between an evolutionary mechanism and other factors, such as ecological affinity or evolutionary relatedness. However, as this study is so far the only study to have employed heterochronic weighting so far, the success rate of this process is limited to horseshoe crabs. Therefore, further research is needed to determine the effectiveness of this method for heterochrony in other species groups.

Citation: Lamsdell, J. C. (2020). A new method for quantifying heterochrony in evolutionary lineages. Paleobiology, 47(2), 363–384.

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