Marine Parasites and Disease in the Era of Global Climate Change

Annual Review of Marine Science

White shrimp (Litopenaeus setiferus) in the southeastern United States are increasingly infected with Hyalophysa lynii, an apostome ciliate that parasitizes penaeid shrimp. The top shrimp shows heavy melanized gill tissue due to infection; the bottom shrimp is healthy (or asymptomatic). Photo by Marc Frischer.

Corresponding Author: James Byers,

Summary by Ethan Hackmeyer

Though oceans cover approximately 70% of Earth’s surfaces, parasitism in marine ecosystems is relatively poorly understood compared to terrestrial host-parasite relationships. As climate change shifts conditions within the world’s oceans, it is imperative to better understand the consequences of environmental change on marine host-parasite dynamics. In a new review paper, CEID member Jeb Byers gives an overview on the current knowledge of marine parasitism, diagnoses gaps in the current data, and identifies several shifting oceanic conditions that will likely affect marine host-parasite dynamics as climate change continues to accelerate. While temperature change is the most well-known and widely studied factor that Dr. Byers identifies, he also discusses the impacts that changes in dissolved oxygen, salinity and acidity could have on parasite dynamics, as well as the impact that changing host-parasite relations will have on the wider ecological community.

When discussing the influence of temperature change on marine host-parasite relations, Byers advocates Thermal Performance Curves (TPCs) to help predict how specific host-parasite relationships could shift due to variations in temperature. TPCs track the performance of a host and its parasite over changing temperatures, and can be used to assess whether a host or parasite will outperform the other at a certain temperature. However, Byers concedes that TPCs are only a starting point for temperature sensitivity comparison, and the information needed to create them is missing for a large amount of host-parasite relationships.

The increase in the average global ocean temperature is generally hypothesized to increase marine parasite transmission and severity. Byers agrees that this is true in some cases, as rising temperatures increase the metabolism of parasites, causing them to eat and replicate quicker, causing more damage to hosts. Additionally, higher water temperatures result in less dissolved oxygen in the environment, causing more physiological stress on hosts that could further cause an imbalance in many host-parasite dynamics. Rising temperatures may also lead to longer periods of time in which conditions are suitable for parasite transmission, allowing for parasites to spread to more hosts than they can under current conditions. Conversely, Byers also notes that some parasites may already be in their optimal temperatures, and a warmer marine climate could result in a severe diminishing, or even extinction, of some parasite species.

Byers notes several other factors that will affect marine host-parasite dynamics as they shift, including oxygen, salinity, acidity and changes in organism range. In addition to higher temperatures driving down dissolved oxygen levels, high nutrient flows resulting from human development can cause eutrophication, which  decreases oxygen levels within near-shore ecosystems, especially estuaries and bays. This can cause hypoxia, as species that are not used to prolonged periods with lower oxygen levels may develop depressed immune systems. Increasing salinity, related to higher evaporation rates due to the rising climate, has a variable effect on marine host-parasite relations similar to temperature. While some parasites, especially ectoparasites, may enjoy increasing salinity, others may not be affected by it or may be harmed. Increasing ocean acidity could also shift host-parasite relations as it causes stress to both the host and parasite organisms, though not enough research has been done on acidity and parasitism to construct a concrete trend. Changes in all of the aforementioned factors can also shift the ranges and distributions of parasite and host species.

In addition to the effects of abiotic factors on the host-parasite pair, the community in which a host and parasite are embedded will impact, and be impacted by, shifts in the host-parasite dynamic. For example, though temperature increasing metabolism could boost parasites in a lab setting, in a field setting, higher metabolism will also increase in consumers of parasites, possibly increasing predation on free-living larvae. Hosts weakened by parasites and changing ocean conditions are also more vulnerable to predation, and keystone species that are weakened or reduced in number due to predation can result in altered ecosystem structures and compositions.

As climate change continues to accelerate, it becomes increasingly important to understand how marine host-parasite pairs respond to changes in their environment. Byers strongly suggests that more research be done that investigates how all changing oceanic conditions affect host-parasite dynamics, not just increasing temperature. Furthermore, Byers calls for more studies that take into account how the community responds to, and influences, shifting host-parasite relationships, rather than solely focusing on the pair.

Byers 2020. Marine Parasites and Disease in the Era of Global Climate Change. Annual Review of Marine Science. 13:3.1-3.24.