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Simple Summary: A striped dolphin was found dead in Maltese waters in July 2020, displaying a severe infestation of the mesoparasite Pennella balaenoptera. This parasite is the largest known mesoparasite to grow on cetaceans and the level of infestation is typically associated with the health status of its host. This incident emphasizes the importance of monitoring the presence of these parasites, to better understand the health of dolphins in the area and to contribute to conservation efforts. Here, we document the stranding of a striped dolphin Stenella coeruleoalba (Meyen, 1833) (Mammalia: Delphinidae), which was found dead in Maltese waters in July 2020. The stranded dolphin exhibited a severe infestation of the mesoparasitic copepod, Pennella balaenoptera Koren and Danielssen, 1877 (Copepoda: Pennelidae). Parasites of this genus represent the largest known mesoparasites to infest cetaceans. Under normal circumstances, cetaceans may have a few P. balaenoptera individuals attached to them, but cetaceans with compromised health are more prone to heavy infestations. The identification of the parasite was accomplished through morphological and genetic analyses. This incident highlights the significance of monitoring mesoparasitic infestations, offering valuable insights into the health of cetacean populations and emphasizing the potential implications for conservation efforts in the region. [ABSTRACT FROM AUTHOR]

This case report describes the first identification in Israel of the mesoparasitic copepod Pennella balaenoptera, in the carcass of a marine mammal. A Fin whale (Balaenoptera physalus) washed ashore on the Israeli coast was autopsied in contiguity to a large oil spill that occurred along Israel's coastline. At the external examination, a large number of Pennella balaenopterae were embedded within the whale's blubber on its ventral and lateral sides. Further toxicological analyses of lung and bronchus samples, revealed the presence of higher-thanacceptable levels of toluene without any evidence of other above-normal petroleum components, suggesting the inhalation of the substance. This paper proposes to consider toluene toxicity as a possible etiological factor affecting the host's immunity and consequently increasing the parasitic pressure. [ABSTRACT FROM AUTHOR]

Pennella balaenoptera is a mesoparasitic copepod that has been reported in at least 17 cetacean species. Subtle morphological differences in the first antennae of adult females have been used to discriminate this species from P. filosa, a species infecting fishes. Other morphological traits are unreliable because of their high plasticity, and no molecular data are available to confirm the taxonomic status of P. balaenoptera as an independent species. We found no consistent morphological differences of the first antennae between P. balaenoptera and P. filosa collected from cetaceans and fish in the western Mediterranean. Molecular data on the mitochondrial cytochrome oxidase subunit I failed to show reciprocal monophyly for the 2 species, and nucleotide divergence between them was low (mean ± SD [range]: 4.1 ± 0.006% [0.5-8.9]). Thus, P. balaenoptera and P. filosa are considered conspecific. We also obtained data on infection parameters of P. balaenoptera based on 450 individuals of 6 cetacean species stranded on the Spanish Mediterranean coast between 1980 and 2017. Prevalence was significantly lowest in the most coastal species, the bottlenose dolphin Tursiops truncatus (3.6%) and highest in the most oceanic species, Cuvier's beaked whale Ziphius cavirostris (100%). This suggests that the life cycle of P. balaenoptera is primarily oceanic. Interestingly, P. filosa also occurs in the oceanic realm infecting large fishes. This ecological similarity further supports the hypothesis that P. balaenoptera and P. filosa are conspecific.