An unexpected morphotype of Chrysaora hysoscella in the Gulf of Trieste (North Adriatic Sea)

The genus Chrysaora (Semaeostomeae: Pelagiidae) comprises 16 species distributed worldwide, yet its taxonomy remains under continuous revision. Chrysaora hysoscella (the compass jellyfish) is common in the Mediterranean Sea, including the Adriatic Sea. In the Gulf of Trieste, its presence has been documented since 1874, with an increase in abundance and frequency reported since the 1980s. In 2025, several “unusual” Chrysaora-like jellyfish were observed in the Gulf of Trieste, with reports from both the Italian and Slovenian coasts. All individuals displayed striking morphological traits, in particular extremely elongated oral arms measuring 2 to more than 3 m, far exceeding those typically described for C. hysoscella. In contrast, the umbrellas, which showed different pigmentations, ranging from traditional compass patterns to whitish or pale coloration, were relatively small (≤ 20 cm in diameter), resulting in a marked disproportion between umbrella size and oral arm length. To determine whether these specimens represent a novel morphotype of C. hysoscella or an undescribed species, molecular analyses were conducted. DNA was extracted from umbrella margins, oral arms, and gonadal tissues, and a fragment of the mitochondrial Cytochrome Oxidase I (COI) gene was amplified following Folmer et al. (1993). Molecular identification confirmed all individuals as Chrysaora hysoscella, with the highest BLASTn similarity to specimens from Cork (Ireland). These findings suggest that the observed morphological divergence likely reflects a high degree of phenotypic plasticity within C. hysoscella. To further assess intraspecific variability and regional patterns, genetic analyses are currently being expanded with additional samples from across the Mediterranean basin.