Geometric morphometry has been widely used in decapods’ studies for taxonomic needs, and for eco-morphological adaptation and intraspecific variations recordings. Among the 40 species of the genus Hippolyte, the Mediterranean endemic Hippolyte sapphica is the only one with two distinct conspecific morphotypes, without intermediate forms: morph-A with a long, dentate and morph-B with a very short, toothless rostrum. Previous studies have shown that the “rostral loss” in morph-B seems to be controlled by a single pair of alleles, with a complete dominance of allele b, expressed in morph-B. We aim to elucidate morphotypes’ rostral pattern in relation to size, sex, and season. Shrimps were collected during two different (dry/wet) seasons from two sites: s.1 with a mixed (morph-A and B) and s.2 with a pure, unmixed (morph-A) species populations. After morph and sex identification, individuals were photographed and geometric morphometric analysis of rostrum was carried out on a set of landmarks. The data suggest that only morph-A rostral shape seems to be influenced by shrimp’s size, sex, and time of the year. Interestingly, two distinct morph-B clusters appear, which probably correspond to the homozygous and heterozygous state (BB and BA) of the gene site that controls the species morphotypes’ phenology.

One of the most exceptional, loud paradigm of shape polymorphisms constitutes the “rostral loss” condition in hippolytid shrimps. The intertidal shrimp Hippolyte sapphica includes two conspecific morphotypes, one of which demonstrates a neotenic rostrum (morph-B). Morphs’ rostral elongation is controlled by a single genetic locus, with long rostra (morph-A) representing the recessive state and short, larval-like ones the completely dominant state. Geometrics morphometry on the species morphotypes revealed also the homozygous/heterozygous state of the gene site along with some induced body’ adaptations, which compete the micro-evolutionary disadvantage of the “rostral loss”. We found recently that females’ viability and maternal energy investment selectively favors morph-A. The present contribution detects and discuss comparatively demographic and reproductive traits in species mixed (both morphs) and unmixed populations. Our results show that this sharp dimorphic rostral condition is a sex-related marker and that the species is gonochoric. Presence of morph-B results to (a) lower egg production (b) higher seasonally males’ percentage (c) morph-A females’ earlier maturation and (d) higher fecundity in morph-A mixed populations. It seems that the “rostral loss” state induces complex adaptations between the two morphotypes through sex ratios equilibria, morphotypes’ growth rates, and morphs’ fecundity differentiate inputs throughout the seasons.