Ciliate-zooplankton epibiosis in Lake St Lucia.

Abstract

Epibiosis is a symbiotic association of two organisms in which one species (epibiont) uses the surface of another species (basibiont or host) as an attachment substrate. An increasing number of studies are revealing that epibionts have mainly deleterious effects on crustacean meso-zooplankton (hereafter referred to simply as zooplankton) hosts. In spite of its widespread occurrence, there are very few studies in Africa that address epibiosis in the aquatic environment, particularly involving zooplankton as hosts. Epibiotic ciliates are often found attached to zooplankton in the St Lucia Estuary, in northan KwaZulu-Natal, South Africa. St Lucia is the largest estuarine lake in Africa and is globally recognized for its ecological importance. A study was conducted in St Lucia between 2015 and 2017, with the aim of determining: the identity of the epibiotic ciliates; their species-specific association with the zooplankton of St Lucia; the effects they have on their hosts and the environmental conditions that promote their proliferation. Based on live observations and images obtained from protargol staining and scanning electron microscopy, the epibiotic ciliates in the St Lucia Estuary were identified as the peritrich sessilid Epistylis sp. (Chapter 1). The results of the experimental study in Chapter 2 were that Epistylis sp. is species-specific, attaching only to the dominant calanoid copepod Pseudodiaptomus stuhlmanni (mainly adults) and that this relationship is host density dependent. Another finding of Chapter 2 was that Epistylis sp. exerts a negative effect on the survivorship of heavily covered P. stuhlmanni. The results of Chapter 3 revealed a low RNA content and RNA:DNA ratio in epibiont-hosting P. stuhlmanni compared with their non-hosting counterparts, which implies a compromised nutritional status of epibiont-hosting copepods. Laboratory-based experiments detailed in Chapters 4 and 5 revealed that Epistylis sp. is: a) unaffected by temperature; and b) favoured by salinities below 20 and organically rich turbidity within the range 250–500 NTU. Results obtained from monthly field observations throughout 2016 (Chapter 6) showed no correlation of Epistylis sp. with these physico-chemical parameters and with the abundance of P. stuhlmanni. The latter result may be due to the uncharacteristically low abundance of the host P. stuhlmanni during the sampling period (January–December 2016). Overall, findings of this study suggest that peritrich epibionts can substantially and negatively affect host species and that they have a complex, context-dependent relationship with environmental conditions. The ecological implications of ciliate-zooplankton epibiosis in the St Lucia Estuary and in similar systems are discussed.