What happens when sculpins cross the aquatic aisle?

There are approximately 100 freshwater sculpin species found in temperate and boreal river systems throughout the northern hemisphere, nearly all of which evolved from a single marine ancestor. Considering that sculpins successfully invaded these freshwater systems, one might speculate that adaptive radiation would follow the shift into novel habitats that contain a new prey base or resources to expand into. So, the question must be asked: is there evidence for adaptive radiation in freshwater sculpin fishes, or do they occupy similar ecological guilds/niches and retain morphological traits possessed by their marine cousins? Buser and colleagues have an answer!

In similar cases around the world, expansion into new habitats proceeded morphological adaptation. For example, the 30 Silversword alliance species currently occupy a niche on the Hawaiian Islands but radiated from a single ancestral plant species, or Cichlid fishes that evolved morphologic traits (e.g., modifications to the pharyngeal jaws) to take advantage of nearly every trophic niche in the African Great Lakes. However, investigating adaptive radiation in sculpins turns out to be rather complex given that they occupy vast geographic ranges with different species-specific prey taxa, and no such method for quantifying diet data at this scale existed – until recently.  

Using traditional descriptive categories (e.g., insectivore, molluscivore, piscivore) can be too broad for marine-to-freshwater comparisons and, conversely, categorizing prey too narrowly restricts comparisons between habitats. Nevertheless, that did not stop Buser and colleagues from inventing a new quantitative method to tackle this perplexing problem. To determine if and how adaptive radiation has occurred in sculpins, they needed to quantify prey data in such a way that it can be applied across both marine and freshwater sculpins and be compared with the functional feeding traits of each species.

Buser et al. 2019, Figure 1: Functional feeding traits measured for each species in study
Link to rotatable 3D image
 

To do this, they first performed a literature review of all sculpins to obtain life-history and diet data. Then, to quantify diet across all sculpins, they used functional traits common among prey taxa (e.g., capable of swimming, possess defensive spines) and performed a cluster analysis which groups taxa according these traits. Lastly, they assigned each sculpin species to a synthetic prey category, obtained from clustering analysis, and based on the highest percent importance in the diet.

It turns out that most freshwater sculpins maintain the same ecological niche as marine sculpins (i.e., niche conservatism). When diet, life-history, and feeding functional morphology are compared, nearly all marine and freshwaters sculpins retain similar morphological characters and consume benthic arthropods. Why is this? Buser and colleagues make the case that biological and ecological constraints likely maintain niche conservatism in freshwater sculpins. Unlike Cichlids or other fishes that have radiated to fill various niches, sculpins do not possess a swim bladder and are therefore more likely to maintain their presence on the benthos in both environments. It is possible that ecological factors are constraining the adaptive radiation of sculpins. Freshwater sculpins are found throughout the northern hemisphere, but they typically occupy habitats that are low in biodiversity and high in prey abundance. So, they have plenty to eat, but less diversity to choose from.

Buser et al. 2019, Figure 5: Functional feeding traits compared with diet and life-history traits
Link to interactive figure

However, there are a handful of notable exceptions. For example, Comephorus dybowskii is found in Lake Baikal, Siberia and has managed to depart form the typical sculpin morphology. Lake Baikal maintains an ecologically diverse and rich community of organisms throughout the water column and C. dybowskii has evolved an elongate body and jaws to take advantage of this; these pelagic predatory traits are clearly visible in the interactive above. Likewise, sculpins in the genus Clinocottus are intertidal inhabitants that almost exclusively consume algae. The jaws in Clinocottus are reduced and are ideal for gripping and tearing algae from rocks. For a better visualization, I recommend using the interactive link in the figure above to see how these groups fall into morphospace and how they compare to each other based on synthetic prey diet and aquatic habitat.

This study demonstrates a novel approach for quantifying species-specific diet that can be used in comparative studies on morphology and dietary ecology. The application of this method to future studies is promising and may provide new insight into previous work that used more traditional qualitative methods.

By Alvaro Cortes

Alvaro Cortes is a MS student at Oregon State University studying the systematics of freshwater sculpin fishes (genus Cottus). You can contact him at alvaro.cortes@oregonstate.edu and/or follow him on Twitter @Sea_of_Cortes.

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