The paper “An update and ecological perspective on certain sentinel helminth endoparasites within the Mediterranean Sea” published in Parasitology is freely available to read online.

Marine parasitic helminths (i.e., worms), generally have a “bad reputation” especially when they occur in commercially valuable fish. Their less-than glamorous appearance and the zoonotic risks posed by some members of the “gang” relegate these organisms as unwanted creatures. However, behind these unconventional exteriors, helminth parasites represent a fascinating world.

This paper focuses on the biodiversity and ecology of specific tropically transmitted helminth endoparasites (TTHs) in Mediterranean Sea ecosystems, aiming to elucidate their potential effectiveness as “sentinels” of anthropogenic disturbances in the marine environment. The Mediterranean Sea is recognized as a marine biodiversity hotspot, hosting a significant percentage of the world’s marine species. This enclosed basin is facing several anthropogenic driven threats, including seawater warming, pollution, overfishing, bycatch, intense maritime transport, and invasion by alien species.

Among the TTHs selected as the focus of this study, there is a group of ascaridoid nematodes comprising species of the genera Anisakis, Sulcascaris and Hysterothylacium, and a group of cestodes including species of the orders Trypanorhyncha, Tetraphyllidea and Oncoproteocephalidea. The selected TTHs exhibit complex life cycles, involving several host organisms at different trophic levels, and are directly exposed to abiotic environmental conditions during their free-living phases. Crustaceans, mollusks, fish, and squid can act as intermediate and paratenic hosts, while large organisms of mid-high trophic levels, such as cetaceans, sea turtles, large teleost and elasmobranchs, are the definitive hosts.

Assessing the status of associations and dynamics of TTHs and their hosts in the Mediterranean Sea can offer a baseline for monitoring of the overall “health status” of this marine environment and predict variations under various human-induced scenarios. Anthropogenic disturbances directly impacting the free-living stages of the chosen TTHs and their host population demography can significantly alter their distribution, infection levels and intraspecific genetic variability. Changes in the distribution of particular TTHs species can also serve as indicators of various ecological perturbations, such as sea temperature variations, currents switches, and hosts migrations.

Gathering data over a temporal and spatial scale and gaining new ecological insights into these marine parasite species, we attempted to update their biodiversity with the perspective of using these parasites’ abundance, distribution, and their genetic variability values as predictors of anthropogenic-driven perturbation effects on marine food webs. Our findings so far suggest that

marine ecosystems with stable food webs and abundant host populations are associated with large parasitic populations. Consequently, the overall abundance of parasites and their genetic variability, including some of the TTHs considered here, can reflect the state of trophic webs in the Mediterranean marine ecosystem. The findings also suggest that the “metrics” proposed here, such as estimates of genetic variability and infection rates of parasites, could serve as indicators to monitor the status of diverse marine food chains over temporal and spatial scales. In the light of current global anthropogenic-driven changes, it is advised that we study their effects on the alteration of intraspecific genetic variability of TTHs, representing the basis for any evolutionary change and a basal component of natural biodiversity.

Parasites as “sentinels” of global changes in the marine Mediterranean ecosystem.

The figure illustrates possible anthropogenic impacts on abiotic (i.e. sea temperature, salinity) and biotic factors (i.e. demography of intermediate and definitive hosts) (on the right) of the life-cycle (simplified in the center) of selected trophically transmitted helminth parasites (TTHs).

On the left, photos of TTHs, from the top to the bottom: adult worms of Anisakis; adult worms of Anisakis around discarded plastic, found in the stomach chamber of a stranded sperm whale; larvae of Trypanorhyncha infecting the muscle of a teleostean fish; third stage larvae of Anisakis infecting a fish host (UV image).