Data for: The grass squid Pickfordiateuthis pulchella is a paedomorphic loliginid

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 four 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.

动物类群内部及类群间成年个体体型的显著差异长期以来广受学界关注，学界已提出多条通用规则以阐释体型演化的趋势。头足类软体动物（cephalopod mollusks）的成年体型差异尤为突出，其成年个体体型跨度从数厘米至数米不等。目前已知体型最小的头足类类群中包含皮氏枪乌贼属（Pickfordiateuthis），该属涵盖分布于西大西洋及热带东太平洋的4种枪乌贼。该属的模式种为美丽皮氏枪乌贼（Pickfordiateuthis pulchella），最初被认为与枪乌贼科（Loliginidae）的枪乌贼类群亲缘关系密切，后续新增物种的分类学研究均支持其隶属于枪乌贼科。皮氏枪乌贼属的独特之处在于，其所有物种的性成熟体型仅为多数枪乌贼科物种的1/5至1/10。截至目前，尚无任何系统发育分析纳入皮氏枪乌贼属的代表物种。为推断皮氏枪乌贼属的系统发育位置并探讨其对体型演化研究的启示，本研究从巴西海域采集了美丽皮氏枪乌贼的标本，并对两个基因座的区域进行了测序：线粒体大亚基核糖体RNA（mitochondrial large ribosomal subunit，即rrnL，又称16S）基因，以及视紫红质（rhodopsin）核基因。对上述序列的最大似然法（maximum likelihood）与贝叶斯（Bayesian）系统发育分析结果显示，美丽皮氏枪乌贼与西半球枪乌贼科的多氏枪乌贼属（Doryteuthis）以及枪鱿属（Lolliguncula）构成的支系互为姊妹群。对枪乌贼科内部体型演化的分析表明，在通往美丽皮氏枪乌贼的演化支系中，存在向更小体型最优值演化的转变；同时有证据显示，枪乌贼属（Loligo）与拟乌贼属（Sepioteuthis）的祖先类群则存在向更大体型演化的趋势。上述推断对于系统发育不确定性及类群采样不完整的情况均表现出较强的稳健性。成年皮氏枪乌贼所展现出的小型体型及幼态形态特征（例如类似耳乌贼类的鳍，以及触腕上的双排吸盘排布），可能源于幼态延续（paedomorphosis）。