Species-specific patterns in cercarial emergence of Diplostomum spp. from snails Radix lagotis

Highlights

• Species-specific emergence patterns of three Diplostomum spp. were studied.

• Daily periodicity-dependent patterns in cercarial emergence were observed.

• There was seasonal intraspecific variation in the daily cercarial output.

• Patterns synchronised with reproduction and activity of fish second intermediate hosts.

• Diplostomum spp. cercariae evolutionarily adapted to fish behaviour.

Abstract

The cercarial emergence patterns of three species of Diplostomum (Diplostomum ‘mergi’, Diplostomum spathaceum and Diplostomum parviventosum) parasitizing freshwater first intermediate host Radix lagotis sampled in Most Lake, Czech Republic, were studied under various experimental conditions, i.e. field, laboratory and incubator, and seasons, i.e. spring, summer and autumn. We discovered unexpected daily periodicity-dependent species-specific emergence patterns among the three Diplostomum spp. depending on experimental conditions. At the same time, the intraspecific variation of D. spathaceum cercarial release in response to seasonal conditions was observed. We found that a complex array of mechanisms can affect Diplostomum species-specific patterns in cercarial emergence, of which behavioural characteristics of fish related to reproduction and feeding processes are considered the most important factors. This might represent a specific adaptive evolutionary mechanism to maximise transmission success while avoiding competition for host resources. Our results contribute to a better understanding of ecological and epidemiological aspects with respect to specific adaptive strategies compartmentalised among species of Diplostomum and consequences for infection risk in fish hosts.

Introduction

Trematodes (Digenea) are common and abundant in marine and freshwater habitats worldwide, and their typically three-host life cycle requires molluscs as first intermediate hosts and a wide range of invertebrates and vertebrates as second and definitive hosts (Combes et al., 2002, Galaktionov and Dobrovolskij, 2003). Free-swimming larvae, cercariae, which emerge from molluscs, represent significant ecological elements with a strong potential to transfer substantial biomass within the whole ecosystem’s food chains as they are usually produced in tremendous numbers (Kuris et al., 2008, Preston et al., 2013, Rosenkranz et al., 2018, Soldánová et al., 2016, Thieltges et al., 2008a) and are frequently subject to predation (e.g. Kaplan et al., 2009, Welsh et al., 2017, Vielma et al., 2019). Cercariae, being non-feeding and short-lived (typically 24–72 h), are functionally crucial stages which aim to infect as many hosts as possible (Esch et al., 2001, Combes et al., 2002, Morley, 2012). The short transmission opportunity is balanced by a great diversity of cercarial behaviours related to the processes of host finding and recognition, as well as emergence patterns from their molluscan hosts as a result of long coevolutionary processes/interactions with molluscs and challenging environments (e.g. Combes et al., 1994, Haas, 2003). Besides the strong dilution effect described for many different predator–prey systems (e.g. Johnson et al., 2010, Johnson and Thieltges, 2010, Thieltges et al., 2008b, Goedknegt et al., 2012), other environmental factors contribute to the reduction of the cercarial population, thereby preventing successful completion of the life cycle (e.g. Pietrock and Marcogliese, 2003).

There are many species-specific variations in patterns of cercarial emergence, of which the most common is their release from molluscs in high numbers with continuous production (Combes et al., 1994). In many mollusc-trematode systems the timing of cercarial output is synchronised with the activity and/or behaviour of the next hosts to ensure their mutual overlap in space and time (Anderson et al., 1976, Combes et al., 1994, Combes et al., 2002, Théron, 2015).

A wide range of environmental factors affect cercarial emergence: exogenous abiotic conditions such as salinity, water pressure, tidal cycles or UV radiation (Mouritsen, 2002, Koprivnikar and Poulin, 2009, Studer et al., 2012), biotic factors such as the size of molluscan hosts (e.g. Poulin, 2006, Morley et al., 2010), age of infection with increasing cercarial emergence with duration of the infection (e.g. Faltýnková et al., 2009, Karvonen et al., 2004a, Massoud, 1974), history of trematode infection related to the number of miracidia infecting snails (e.g. Massoud, 1974, Sluiters et al., 1980) or co-infections with multiple genotypes of conspecific trematodes (e.g. Berkhout et al., 2014). Cercarial emergence generally increases with an increase in temperature (e.g. Lyholt and Buchmann, 1996, Fingerut et al., 2003, Poulin, 2006) until stabilising in the optimum temperature range (emergence plateau, i.e. thermostability), then declinies as the temperature increases beyond this upper limit (Morley and Lewis, 2015). The photocycle and changes in light intensity were also shown to control cercarial emergence and output rates (e.g. Umadevi and Madhavi, 1997, Kaewkes et al., 2012, Théron, 2015, Soldánová et al., 2016). In addition, seasonal variation in cercarial output in temperate habitats is strongly temperature-dependent, peaking in warm summer months and decreasing towards the winter period when temperatures drop to a minimum threshold, ceasing emergence (Galaktionov and Dobrovolskij, 2003, Morley, 2012). Cercarial emergence rhythms are usually circadian (one peak during a 24 h period) and diurnal (a maximum number of cercariae emerge during the daylight period) (Combes et al., 1994).

Cercarial output measurements directly in the field under natural light and temperature conditions are generally scarce (Brassard et al., 1982, Fingerut et al., 2003, Preston et al., 2013, Prokofiev et al., 2015). Diplostomum spp. were studied intensively, mainly with respect to the infection capability of fish intermediate hosts and induced changes in their behaviour, including reproduction and somatic growth (e.g. Majoros, 1999, Seppälä et al., 2004, Seppälä et al., 2007, Karvonen and Seppälä, 2008, Gopko et al., 2015, Johansen et al., 2019), but little is known about patterns of cercarial emergence (periodicity) and daily output rates (production). There are a few studies for Diplostomum spp. in lymnaeid snails Lymnaea stagnalis – Karvonen et al., 2004a, Karvonen et al., 2006b, Lyholt and Buchmann, 1996, Morley et al., 2003; Lymnaea arctica – Brassard et al. (1982) and Myxas glutinosa – Karvonen et al. (2006a). Only Brassard et al. (1982) followed cercarial emergence in the field under the natural photocycle and thermal regime. The cercarial release of Diplostomum spathaceum from L. arctica was investigated by placing snails in the shallow pond zone. However, this was in a subarctic region in North America and no similar study has been conducted in freshwater temperate systems in Europe. The remaining studies were conducted in the laboratory or an incubator under standardised conditions where the cercarial output was monitored every 24–48 h, focusing on seasonal differences, diurnal versus nocturnal preferences in daily emergence (Karvonen et al., 2004a, Karvonen et al., 2006b), the effects of light and temperature (Lyholt and Buchmann, 1996) or cadmium toxicity on cercarial emergence (Morley et al., 2003).

Metacercariae of Diplostomum spp. are important pathogens of fish, causing eye cataracts leading to impaired vision and even blindness, making fish more vulnerable and prone to predation (Chappell et al., 1994, Seppälä et al., 2004, Karvonen, 2012). Cyprinids, the main second intermediate hosts for Diplostomum spp., are commonly farmed and fished as highly important food fishes and are popular targets for recreational fishing (e.g. Faltýnková et al., 2016, Hartman and Regenda, 2016). Diplostomum spp. are common in freshwater lentic habitats including commercial fish farms in temperate European regions where parasites frequently cause problems involving high fish mortality and ultimately leading to severe economic losses (Hakalahti et al., 2006, Karvonen et al., 2006a, Karvonen and Seppälä, 2008). Detailed research into cercarial emergence strategies of Diplostomum spp. is therefore essential for understanding their transmission biology and dynamics, which could be further used in the practical application of preventive methods.

The present study aimed to investigate the emergence of cercariae of Diplostomum spp. from the first intermediate snail host Radix lagotis (Lymnaeidae) in a temperate region under natural habitat and laboratory conditions. Specifically examined were: (i) the overall emergence output to determine the daily productivity of Diplostomum spp., and (ii) rates of cercarial release to characterise the circadian periodicity in emergence. We discovered unexpected daily periodicity-dependent species-specific emergence patterns among the three Diplostomum spp. in R.lagotis depending on experimental conditions. At the same time, the intraspecific variation in D. spathaceum cercarial release in response to seasonal conditions was observed.