Crowding in the first intermediate host does not affect infection probability in the second host in two helminths

Area of Science:

• Ecology
• Parasitology
• Evolutionary Biology

Background:

• Parasitic worms often exhibit reduced growth (crowding effect) when co-infecting a host.
• The fitness consequences of this crowding effect on parasite establishment are rarely studied.

Purpose of the Study:

• To investigate if larval crowding affects the establishment success of two parasites, *Camallanus lacustris* (nematode) and *Schistocephalus solidus* (cestode), in their next host.
• To examine the fitness consequences of density-dependent growth in parasites with complex life cycles.

Main Methods:

• Infected copepods (intermediate host) were fed to sticklebacks (second host).
• Parasite infection intensity in copepods was experimentally varied.
• Establishment success of worms in sticklebacks was measured.

Main Results:

• Larval crowding did not significantly reduce the establishment probability of *C. lacustris* or *S. solidus* in sticklebacks.
• Crowded treatments showed a non-significant trend towards more extreme infection outcomes (all or none).

Conclusions:

• Reduced larval size due to crowding may not necessarily decrease establishment success in the subsequent host.
• Parasite transmission dynamics can be complex, with crowding potentially influencing the probability of successful infection establishment.