Abstract
Mate-guarding amphipods have, for a long time, attracted the attention of researchers interested in pairing patterns and selection processes. Furthermore, in recent years, attention has been focused on how these patterns and processes may be modulated by parasites. This study investigates sexual selection and patterns of precopulatory mate-guarding in freshwater amphipods of the genus Gammarus, these being the model organisms of choice for such studies. It also examines how parasites impact upon a suite of fitness components in Gammarus.The first test of the "timing hypothesis" regarding the generation of size- assortative pairing in amphipods is presented. The timing hypothesis proposes that, since large males are better able to afford the costs of mate-guarding than small males, the former can take larger females into precopula earlier in the female moult cycle than is feasible for the latter. This leaves small males to form pairs with smaller females closer to moult, thus generating size-assortment. Male G. pulex, collected both in precopula and as singletons, were presented with females that were (1) previously guarded and, therefore, near to copulatory moult and (2) previously unguarded and, therefore, far from copulatory moult. This tested the prediction of the timing hypothesis that size- assortment should break down when the opportunity for time-based male decisions is removed, but that size-assortment should occur where timing is not disrupted. Counter to the hypothesis, size-assortment did not break down upon removal of the time factor. Large males tended to initiate mate-guarding earlier than small males in both female moult groups. However, only in the previously unguarded group did large males guard for longer than did small males. This suggests that, although size-assortment occurred in all groups, the causative mechanisms that generated this pattern may differ between these groups. Therefore, the possible importance of other mechanisms in producing size- assortment where males encounter numerous females that are close to moult is considered, for example, direct competition, simultaneous manipulation of females and female resistance. In addition, it was observed that prior recent guarding experience by males had no effect on latency to or duration of guarding or on size assortative pairing.
The possible functions of the enlarged posterior gnathopods in male G. pulex were investigated in relation to inter- and intra-sexual interactions. Pairing success in the field was assessed relative to posterior gnathopod size, however, no sexually selected pairing advantage of large posterior gnathopods was observed. Furthermore, using a series of laboratory experiments, in which the posterior gnathopods of some males were ablated, it was shown that these appendages are neither necessary for, nor advantageous in, initiation or maintenance of precopula, irrespective of competition from intact males. However, males lacking their posterior gnathopods cannot copulate, despite repeated attempts. It was concluded, therefore, that the posterior gnathopods should be classified as naturally selected primary sexual traits. However, the possibility that the size of these appendages is sexually or ecologically selected via some process not investigated in this study is discussed. In addition, it was shown that females require some signal associated with copulation to induce them to extrude eggs into their brood pouches. In the absence of this signal, however, females may simply reabsorb the unused material.
G. pulex is the intermediate host of an acanthocephalan parasite, Echinorhynchus truttae, the final hosts of which are salmonids. A field survey revealed that G. pulex display altered use of microhabitat when parasitised by E. truttae, such, that infected animals are less likely than uninfected animals to occur in patches offering shelter and protection from currents and predators. A laboratory simulation corroborated this, showing that parasitised animals were less likely than unparasitised animals to exploit protected microhabitats, such as macrophyte and substrate, and more likely to occur in the water column or at the water surface. Furthermore, parasitised individuals displayed hyperactivity and increased positive phototaxis, behaviours that are associated with a greater tendency to enter the drift. These altered behaviours are discussed in terms of adaptive manipulation of the intermediate host by the parasite to increase its transmission via predation to its final host. Furthermore, in male G. pulex, E. truttae was associated with reduced pairing success, possibly as a result of infected animals being less able than uninfected animals to invest in mate-guarding.
G. duebeni celticus may be infected by the microsporidian parasite, Pleistophora. An electron microscopy study revealed that Pleistophora caused complete lysis of the abdominal muscle tissue in the foci of infection. A field survey showed that in males, but not in females, infection was associated with reduced body condition. Overall, male pairing success was not affected by infection, however, the difference in size between paired and unpaired males tended to be more marked among infected than uninfected males. This suggests that size was a more important determinant of pairing success in infected than in uninfected males, perhaps indicating the competitive superiority of the latter. Female pairing success was markedly reduced by parasitism. This may have been due to males preferentially pairing with uninfected females as these were more fecund. That Pleistophora may be transmitted vertically from mothers to
offspring via the ova was also discussed.
| Date of Award | Dec 2002 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Northern Ireland Department for the Economy |
| Supervisor | Robert Elwood (Supervisor) & Jaimie Thomas Allan Dick (Supervisor) |