The grass squid Pickfordiateuthis pulchella is a paedomorphic loliginid

[Display omitted] •Pickfordiateuthis pulchella is a loliginid, sister to Doryteuthis and Lolliguncula.•The phylogeny supports a shift in body size optimum in Pickfordiateuthis.•There is some support for shifts in body size in Sepioteuthis and Loligo.•Three loliginid lineages show signs of convergence with other decapodiform taxa. The wide disparity in adult body size observed both within and among animal taxa has long attracted widespread interest, with several general rules having been proposed to explain trends in body size evolution. Adult body size disparity among the cephalopod mollusks is remarkable, with adult body sizes ranging from a few centimeters to several meters. Some of the smallest cephalopods are found within Pickfordiateuthis, a group comprising three described species of squid found in the western Atlantic and tropical eastern Pacific. Pickfordiateuthis pulchella, the type species of the genus, was initially proposed to be closely related to the loliginid squids (Loliginidae), with subsequent descriptions of additional species supporting a placement within Loliginidae. Pickfordiateuthis is remarkable in that all species reach sexual maturity at about one-fifth to one-tenth the size seen in most loliginid species. To date, no phylogenetic analyses have included representatives of Pickfordiateuthis. To infer the phylogenetic position of Pickfordiateuthis and explore its implications for body size evolution, we collected specimens of Pickfordiateuthis pulchella from Brazilian waters and sequenced regions of two loci—the mitochondrial large ribosomal subunit (rrnL a.k.a. 16S) gene and the nuclear gene rhodopsin. Maximum likelihood and Bayesian analyses of these sequences support a placement of Pickfordiateuthis pulchella as sister to a clade comprising the Western Hemisphere loliginid genera Doryteuthis and Lolliguncula. Analyses of body size evolution within Loliginidae suggest that a shift to a smaller body size optimum occurred along the lineage leading to P. pulchella, with some evidence of shifts toward larger sizes in the ancestors of Loligo and Sepioteuthis; these inferences seem to be robust to phylogenetic uncertainty and incomplete taxon sampling. The small size and juvenile-like morphological traits seen in adult Pickfordiateuthis (e.g., sepiolid-like fins and biserial sucker arrangement in the tentacles) may be due to paedomorphosis.