A case of modular phenotypic plasticity in the depth gradient for the gorgonian coral Antillogorgia bipinnata (Cnidaria: Octocorallia)

Abstract

Background: Phenotypic plasticity, as a phenotypic response induced by the environment, has been proposed as a key factor in the evolutionary history of corals. A significant number of octocoral species show high phenotypic variation, exhibiting a strong overlap in intra- and inter-specific morphologic variation. This is the case of the gorgonian octocoral Antillogorgia bipinnata (Verrill 1864), which shows three polyphyletic morphotypes along a bathymetric gradient. This research tested the phenotypic plasticity of modular traits in A. bipinnata with a reciprocal transplant experiment involving 256 explants from two morphotypes in two locations and at two depths. Vertical and horizontal length and number of new branches were compared 13 weeks following transplant. The data were analysed with a linear mixed-effects model and a graphic approach by reaction norms.

Results: At the end of the experiment, 91.8% of explants survived. Lower vertical and horizontal growth rates and lower branch promotion were found for deep environments compared to shallow environments. The overall variation behaved similarly to the performance of native transplants. In particular, promotion of new branches showed variance mainly due to a phenotypic plastic effect.

Conclusions: Globally, environmental and genotypic effects explain the variation of the assessed traits. Survival rates besides plastic responses suggest an intermediate scenario between adaptive plasticity and local adaptation that may drive a potential process of adaptive divergence along depth cline in A. bipinnata.

Keywords: Antillogorgia bipinnata; Depth cline; Octocoral; Phenotypic plasticity; Reaction norm.

Fig. 1

Locations and design of the reciprocal transplant experiment. a Map of Bocas del Toro, Panama, signalling the localities of Hospital point (black star) and Crawl key (red circle) (basemap from https://google.com/maps/). b Experiment design with arrows indicating the direction of transplants between habitats and localities with curved arrow for native controls. Eight segments per colony were used to get a fully crossed and replicated design

Fig. 2

Reaction norms for vertical length variation in A. bipinnata. a-f, on the Y-axis is the vertical length variation in mm and on the X-axis the environment. Colours encode source locality of colonies: black for Hospital Point and red for Crawl Key. Data are laterally offset for improved visualisation. Dots represent the median magnitudes and bars the + − MAD (Median Absolute Deviation). HPs = Hospital Point shallow, HPd = Hospital Point deep, CKs = Crawl Key shallow, CKd = Crawl Key deep

Fig. 3

Reaction norms for horizontal length variation in A. bipinnata. a-f, on the Y-axis is the horizontal length in mm and on the X-axis the environment. Colours encode source locality of colonies: black for Hospital Point and red for Crawl Key. Data are laterally offset for improved visualisation. Dots represent the median magnitudes and bars the + − MAD. HPs = Hospital Point shallow, HPd = Hospital Point deep, CKs = Crawl Key shallow, CKd = Crawl Key deep

Fig. 4

Reaction norms for branch density in A. bipinnata. a-f, on the Y-axis the branch density calculated over the length of the main axis and on the X-axis the environment. Colours represent source locality of colonies: black for Hospital Point and red for Crawl Key. Data are laterally offset for improved visualisation. Black and red dots represent the median magnitudes and bars the + − MAD. HPs = Hospital Point shallow, CKs = Crawl Key shallow, CKd = Crawl Key deep