Growth rate and survival of terrestrial isopods is related to possibility to acquire symbionts

•Symbiotic microbes often play a key role if animals rely on low quality food.•Isopod mancae were dissected from brood pouch before contact with environment.•After in-vitro hatching, the modes of transmission were simulated.•Symbionts were seemingly acquired horizontally and environmentally, not vertically.•Symbionts may act as a source of vitamins and fatty acids.

The acquisition and maintenance of symbiont-host associations is considered an important prerequisite for the successful colonisation of land by animals because symbionts allowed the hosts to dwell on low-quality food sources. Digestive tract symbionts are suggested to either enhance digestive efficiency of cellulose or supply the host with nutrients otherwise limited in the food. Terrestrial isopods are the most successful land colonisers among the Crustacea, and although symbiotic bacteria have been identified in digestive tract, the mechanism of transmission and the nutritional role of the symbionts are at present poorly understood.We used a novel in vitro approach to isolate and culture juveniles of Porcellio scaber at a time when only the vertical mode of symbiont acquisition could have occurred, which allowed us to follow juvenile growth and to simulate alternative modes of symbiont acquisition through feeding manipulations. Thus, we experimentally obtained groups of juveniles that could have acquired symbionts only through vertical transfer (mother-offspring), or additionally, through horizontal (through faeces or contact with conspecifics) and environmental (through leaves) transfer. We quantified survival and growth rates over two months for the different experimental acquisition modes, and significantly different growth rates were observed (p < 0.001). Growth of juveniles suggests that first, symbiont inoculation is mediated through horizontal and environmental transfer and second, the symbionts may in fact serve as a source for fatty acids and vitamins. The growth rates further question that vertical transfer occurs in woodlice. Although survival did not differ significantly between different acquisition modes (p = 0.051), juveniles supplemented with potential sources of symbionts showed a tendency towards increased survival. The successful invasion of land may thus have been facilitated through the uptake of symbionts from the surroundings.