Authors

G S van Beest¹; F E Montero¹; F Padrós³; J A Raga¹; A Born-Torrijos²;  ¹ Cavanilles Institute for Biodiversity and Evolutionary Biology, Science Park, University of Valencia, Spain, Spain;  ² Institute of Parasitology, Biology Centre of the Czech Academy of Sciences, České Budějovice, Czech Republic;  ³ Servei de Diagnòstic Patològic en Peixos, Facultat de Veterinària, Universitat Autònoma de Barcelona, Spain

Discussion

Transmission and infection strategies are crucial to complete trematode life cycles, being the transmission step between first and second intermediate hosts especially important in aquatic habitats. Cercariae, free-swimming larval stages, are at the mercy of environmental conditions during their short life span. During this time, they need to display a series of behaviours intended to find, attach and penetrate a suitable target host. To better understand the behaviour and locomotion of cercariae, we studied the strigeid Cardiocephaloides longicollis (Rudolphi, 1819) Dubois, 1982. These cercariae are released in thousands into the seawater, where they swim, locate and penetrate the skin of fish hosts, migrating and encysting in the brain as metacercariae before they are consumed by the definitive host, gulls. Freshly-emerged cercariae were collected from a pool of six infected snails and used in different observational assays to study their locomotion. Histological analyses helped to locate C. longicollis on its way to the fish brain.
First, the locomotion of free-living cercariae was recorded with a video camera, which allowed the identification of different swimming patterns in the water column to facilitate encounters with fish hosts. Observations suggested that C. longicollis displays up to six different behaviours that take them in water levels frequented by the fish hosts, that could facilitate its interaction with the host (as described in other species as Diplostomum spathaceum). Second, in vivo stained cercariae allowed their tracking during their interaction with fish. Over 30% cercariae were found successfully attached to fish skin. Following the contact with the host, cercariae released their tails and display a creeping motion on fish skin that helped them to look for an adequate spot to enter the host. Creeping on the skin as well as burrowing within the tissue after penetration, were recorded for posterior analysis. Results suggested that once the tail is detached, cercarial motion relies on their body locomotion; this means that while cercariae creep on the fish surface, they show caterpillar-like movement, with strong participation of the oral and the ventral suckers, thus moving onward unidirectionally in large steps. However, to burrow the fish tissue, cercariae move onward peristaltically in small steps, using mouth structures to open the path, while the rose-thorn spines, located on cercariae flanks surface, act as anchors.
Histological sections, representing different post-penetration time points into fish, helped to detect cercariae locations at its earliest post-penetration stages during the migration to the brain. No relevant tissue damage or host reaction was detected in the fish tissues during cercariae migration.
These results point out that the complex strategies displayed by C. longicollis, integrated by a variety of behaviours, allow the successful transmission of larval stages. These behaviours are further discussed in relation to closely related trematodes.