Endoparasites affect eel swimming behaviour

A new paper entitled ‘Relationship between European eel Anguilla anguilla infection with non-native parasites and swimming behaviour on encountering accelerating flow‘ has just been published in the Journal of Fish Biology by Lynda Newbold et al. It examines the effect of the Eel swimbladder nematode Anguillicoloides crassus and other endoparasite (the gill monogeneans Pseudodactylogyrus bini and P. anguillae) infections on the behaviour of downstream migrating silver eels. The authors used an experimental flume to look at the how the eels behaved when they encountered accelerating water velocity, common at engineering  structures  in rivers  where flow is constricted, for example at dams and bypass systems.

this need to be considered when designing eel bypasses to ensure that the hydraulic conditions at the entrance encourage efficient downstream silver eel passage

The study found that the probability of reacting to, and rejecting, the velocity gradient was positively related to swimbladder nematode abundance.  They found that high abundance of Pseudodactylogyrus spp. reduced this effect, but A. crassus was the strongest parasitic factor associated with eel behaviour, and abundance was positively related to delay in downstream passage. The authors noted that delayed downstream migration at hydraulic gradients associated with riverine engineering structures could result in additional energetic expenditure for migrating silver eels already challenged by swimbladder nematode infection.

It is now clear that there is a possible energetic cost of A. crassus infection induced through host behavioural change. When combined with a decreased swimming performance and impaired swimbladder function this could reduce the probability of European eels successfully completing their spawning migration. Furthermore, greater avoidance of velocity gradients at dam bypass entrances will reduce passage efficiency for eels parasitized with the swimbladder nematode. The authors recommend that this need to be considered when designing eel bypasses to ensure that the hydraulic conditions at the entrance encourage efficient downstream silver eel passage. Newbold et al (2015) concluded that “the results of this study suggest that the combined effect of barriers and parasite infection on energetic costs during migration should be recognised in European eel conservation efforts to promote passage, escapement and spawner quality“.

Anguillicoloides crassus in eel body cavity
Anguillicoloides crassus in eel body cavity
Eel migration barrier
The cumulative impact of migration barriers and endoparasites on migrating silver eels is likely to be highly significant.

Eel swimbladder nematode Anguillicoloides crassus

Anguillicoloides crassus (formerly Anguillicola crassus) is a parasitic nematode worm that lives in the swimbladders of eels (Anguilla spp).  It is native to Southeast Asia and its native host is the Japanese eel (A. japonica). The endoparasite has spread throughout Europe since its arrival in Europe in the 1980’s.  Once introduced to a population the nematode can spread rapidly. Intermediate hosts include copepods and ostracods. Once introduced into a lake or river it may spread rapidly among the eel population. A. crassus is a successful coloniser due to its large production of eggs and low specificity regarding intermediate hosts. It has been estimated that A. crassus is present in at least 70% of Ireland’s wetted area and is spreading. It is widely considered that A. crassus played a role in the collapse of the European eel population by decreasing growth, increasing mortality and impeding spawning success.

This endoparasite causes damage through inflammation of the swimbladder and secondary bacterial infection. This results in reduced function or failure of the gas glands thus reducing the eel’s ability to maintain and adjust buoyancy. In severe infections a total loss of function of the swimbladder may occur. It is understood that when European eel migrates across  the Atlantic to spawn it travels at great depths. It is clear that swimbladder damage due to heavy A. crassus infestation is very likely to impair their ability to migrate normally and is a serious threat for the reproductive success of infected and previously infected European eels.

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