Adaptive specialization and constraint in morphological defenses of planktonic larvae

Morphological defenses of plankton can include armor, spines, and
coloration. Spines defend from gape-limited fish predators while
pigmentation increases visibility to fishes but defends from ultraviolet
radiation (UVR). Planktonic crab larvae (zoeae) exhibit inter- and
intra-specific variability in the lengths of defensive spines, extent of
pigmentation, and body size. The determinants of this variability and the
relationships among these traits are largely unknown. Larvae may employ
generalized defenses against the dual threats of UVR and predation or
specialized defenses against their primary threat, with an unknown role of
allometric or phylogenetic constraints. Generalization would result in
longer spines compensating for the increased predation risk imposed by
darker pigments, while specialization would lead to more investment in
either defense from predation (long spines) or UVR (dark pigments), at the
expense of the other trait. We examined 1) the relationship between spine
lengths and pigmentation, 2) the scaling of spine lengths with body size,
and 3) phylogenetic constraint in spine lengths, pigmentation, and body
size, among and within 21 species of laboratory-hatched and 23 species of
field-collected crab larvae from Panama and California. We found a
negative relationship between spine length and pigmentation among species
from laboratory and field. Within species, we found a marginally
significant negative relationship among field-collected larvae. Spine
lengths showed positive allometric scaling with carapace length while
spine and carapace lengths, but not pigmentation, had significant
phylogenetic signals. The negative relationship we observed between
pigmentation and spine length supports our defense specialization
hypothesis. Positive allometric scaling of spine lengths means larger
larvae are better defended from predators, which may indicate that larvae
face greater predation risk as they grow larger. Phylogenetic constraint
may have arisen because related species encounter similar predation
threats. Conversely, phylogenetic constraint in the evolution of spine
lengths may induce convergent behaviors resulting in related species
facing similar predation threats. Our results improve understanding of the
evolution of the larval morphology of crabs, morphological defenses in the
plankton, and evolutionary responses of morphology to multiple
spatially-segregated selective forces.