Abstract
Barnacle naupliar larvae are differentiated from other zooplankton by their unique pair of frontal lateral horns, frontal filaments, and a pear-shaped cephalic shield. Their morphology impose constraints on their ecological functions and reflect their evolutionary history. To explore the potential functional basis underlying the similarities and differences in barnacle larval form, we conducted a meta-analysis on the shape of the barnacle nauplii’s cephalic shield and examined its relation to larval size, trophic mode, pelagic larval duration and habitat. Nauplii cephalic shield morphology of 102 species were quantified with normalized elliptic Fourier analysis. Most of the species were distributed around the center of the morphospace but a few extreme groups occupied the periphery: nauplii that were large and lecithotrophic. Subsequent principal component regression analyses showed that larval size was a good predictor of the first shape variations axis (aspect ratio). After allometry adjustment, nauplii from different trophic modes differentiated along the second axis of the major shape variations (relative frontal horn length). Habitat was a poor predictor of variations in naupliar body form, but it could be used to differentiate extreme morphology groups from other nauplii. Our result suggests that size-related biomechanical or developmental constraints and feeding requirements are important in shaping the evolution of the naupliar body form. Within the limitations of these functional constraints, habitat drives the divergence of extreme morphology groups from the majority of species. Our comparative morphometrics analysis demonstrated how variations in larval body form can be quantitatively linked to the functional needs that constrain or drive their diversity, and inform further empirical experiments on larval functional morphology.
Highlights
The study of larval morphology has important phylogenetic and ecological implications
We identified the major directions of shape change for the cephalic shield of nauplii and tested for differences in habitats or biomechanical requirements to perform the ecological functions of swimming and feeding, i.e. if size and trophic modes account for the observed variations
Nauplii shape variations summarized by aspect ratio and relative frontal horn length
Summary
Outlines of 102 species of barnacle nauplius larvae were analyzed. The majority of data were sourced from published descriptions (89 species from 63 publications) and the remaining 13 species were obtained from larvae reared in our laboratory (Table A in S1 Text). For data from published descriptions, digitization was performed on the outline drawings presented or, in a few cases, on the photos of larvae (n = 4). Publications without a scale bar for the drawings or Comparative morphometrics of barnacle nauplii photos, or without upright dorsal/ ventral view, were later excluded from analysis. Stage II larvae were chosen as the focus of this study for two reasons: 1) they sufficiently captured the among-species variations (none of the stages differed in their shape disparity, see Fig A in S1 Text); and 2) data on stage II larvae are more readily available than those on other stages in both the literature and our collection due to the difficulty of rearing to later stages. For species with differentiation of nauplius sex, female nauplii were used
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