First Occurence of the Invasive Hydrozoan Eucheilota maculata (Cnidaria: Hydrozoa) in New Jersey, USA

Eucheilota maculata (Hartlaub, 1894) is a small hydrozoan native to the Mediterranean and northeast Atlantic Ocean. Like many diminutive hydrozoans, their presence in new environments may go unnoticed due to their small size, limited medusae production, and morphological similarities with native species. We report on the first observation of this species in the western North Atlantic Ocean using 16S molecular sequence analysis and morphological evaluation. Additionally, medusae were collected using standardized zooplankton tows providing density estimates of their blooms. 232 individuals were present in our sampling efforts in 2019 and 2021. 65 individuals were collected in September 2019 from 4 different sites with density ranging from 0.19-11.7 individuals m-3. In 2021, the primary bloom occurred in July with densities exceeding 13 m-3 at one site, but their presence was also recorded in August and September, but at lower densities. Their increase in abundance may be related to the closure of the Oyster Creek Nuclear Generating Station in 2018, as none were observed during sampling in that year or the years prior in Barnegat Bay, New Jersey.