Copepod and monogenean infection of fish under sea surface temperature anomalies

Highlights

• Prevalence and intensity of copepods and monogeneans on Pacific sierra were examined

• Infection was assessed in relation to sea surface temperature anomalies

• Some of the highest prevalence values occurred during warm months

• Prevalence of parasites was also moderate or high under colder than average conditions

Abstract

The prevalence and median intensity of infection of Pacific sierra (Scomberomorus sierra) by parasitic copepods (Caligus fajerae, Caligus omissus, and Cybicola buccatus), and monogeneans (Thoracocotylidae) were examined over a six-year period (2015 to 2021) in the southeastern Gulf of California. This period included sea surface temperatures that were warmer and colder than average (SST anomalies). A regression model indicated that the prevalence of parasitic species was higher when conditions were warmer than average, although some of the relationships were non-significant. Prolonged periods of high temperature and heat pulses >2 °C above the average temperature negatively affected the prevalence of parasites. A moderate or high prevalence of parasites was observed during colder than average conditions. These results are in accordance with those of previous studies showing that fish ectoparasites are able to develop under a wide range of thermal conditions.

Introduction

Parasitic copepods (Crustacea: Copepoda) and monogeneans (Platyhelminthes: Monogenea) are two of the most common ectoparasites on marine fish. Both have a direct lifecycle in which early developmental stages (larvae) are free-living and juveniles and adults are parasitic. The development of parasitic copepods involves a number of stages, which may vary among families. In Caligidae, which is the most common family of parasitic copepods on marine fish, mature females produce eggs that hatch as free-swimming nauplii. There are two naupliar stages followed by the copepodid (the infective stage that locates a host), four chalimus and/or pre-adult stages, and then the adult (Venmathi-Maran et al., 2013). Most monogeneans lay eggs that hatch into larvae known as oncomiracidia, which must attach to a specific host species. A few species are viviparous, giving birth to young parasites, which attach to the same host as the parent and may subsequently transfer to a new host when hosts touch (Jansen and Bakke, 1991; Olstad et al., 2006; Brazenor et al., 2020; Hoai, 2020).

During their development, parasitic copepods and monogeneans are directly exposed to variability in water conditions. Water temperature has a particularly strong influence on their lifecycle. The development time, adult body size, and reproductive outputs of caligid copepods are negatively associated with increasing temperature (Samsing et al., 2016; Hamre et al., 2019). Similarly, evidence from Neobenedenia girellae suggests that at low temperatures, monogeneans exhibit slower production of larger, energy-dense eggs, whereas at high temperatures, monogeneans mature faster and produce less energy-rich eggs at a faster rate (Brazenor et al., 2020). Infection levels of parasites may, therefore, peak during warm periods. However, available evidence indicates that seasonal dynamics of fish parasitic infection are far from universal. Strong seasonal dynamics have been found for some parasites and host taxa in certain habitats, but not for others (Poulin, 2020). Additionally, it has been argued that pulse heat events, which are expected to be more frequent and intense in the future, may increase or reduce the transmission of parasites. As no universal patterns have been detected, researchers must measure the effects of warming on focal hosts, parasites, and corresponding communities to develop predictions for a particular system (Claar and Wood, 2020).

The Gulf of California (GC) is a biodiversity hotspot and the most important fishing region in Mexico (Lluch-Cota et al., 2007). This enclosed area is subject to overfishing, loss of biodiversity and habitats, pollution, climate variability, and climate change (Páez-Osuna et al., 2016). The ecological variability in the GC is affected by El Niño-Southern Oscillation (ENSO) events (Lluch-Cota et al., 2010; Herrera-Cervantes et al., 2020) because of its connection to the Pacific Ocean. However, there is a limited knowledge of the effects of ENSO on the dynamics of marine parasites.

In the present study, we analyzed changes in the prevalence and intensity of infection of Pacific sierra (Scomberomorus sierra) from the southeastern GC by parasitic copepods and monogeneans, during a six-year period (2015 to 2021). This period included anomalous warm and cold episodes influenced, respectively, by the very strong El Niño event in 2015–2016 and the moderate La Niña in 2020–2021 (Fig. 1). Scomberomorus sierra is distributed in the eastern tropical and subtropical Pacific and is particularly abundant in the southeastern GC, representing an economically important resource (see Torres-Rojas et al., 2020, and references therein). In Sinaloa state, during 2015 and 2016, around 2000 tons live weight was landed, while for the period 2017–2019 the values were greater than 3000 tons (SIPESCA, 2019). The commercial size captured in the study area ranges from 10 to 75 cm in fork length (FL) and the fishing effort is supported by organisms of 35 to 44 cm FL (Aguirre-Villaseñor et al., 2006). In addition to its abundance, we selected S. sierra because it is a host species for a range of metazoan parasites, including copepods and monogeneans, as recently reported by Santos-Bustos et al. (2020).