In a freshwater amphipod, we found that individuals formed pairs quicker under high predation pressure. This result suggested that search time is costly in terms of vulnerability to predation. Adaptations that would decrease search time in a high predation environment like marine systems should be strongly selected. One possible variable to look at would be whether predation risk is higher for highly dimorphic species. .

At first, I think on sexual selection.

Take a look: http://en.wikipedia.org/wiki/Sexual_selection

Thanks Daniel!

I was aware of sexual dimorphism as the best general explanation. See a quote from a recent paper of myself and coworkers (2012):

"Sexually dimorphic sensory systems can be found across various Arthropoda (e.g. Schafer & Sanchez 1976; Martens 1987; Jourdan et al. 1995; Koh et al. 1995; Fernandes et al. 2004). In most of these cases, males show an increased size of sensory organs (e.g., antennae) and number of olfactory sensillae (i.e. chemoreceptors), which has been attributed to the search for and location of (receptive) females. As an example, for several species of oniscid isopods, Lefebvre et al. (2000) found evidence for scramble-competition polygyny (Alcock 1980) as the prevalent mating system. Males compete indirectly by fertilizing as many mates as they can find in their fertile period. They bear longer antennae compared to the females that they apply to compete intensively in searching and locating receptive females (Lefebvre et al. 2000). Mating strategies for Macrostylidae cannot be inferred from morphological data only. Because of the unavailability of genetic data (as discussed below) and the difficulties associated with keeping live specimens, morphology and collection data make our primary sources for ecological and evolutionary implications. However, given low densities in the deep-sea benthic environment (Sanders & Hessler 1969), the search for a mating partner itself is likely to be among the dominating forces for the evolution of sexually dimorphic traits in olfactory organs. The evolution of the dimorphism found in the males’ enlarged antennulae and increased number of aesthetascs implies importance of this chemosensory organ for mating in general and would hence be driven by sexually selective pressure (Lande 1980)."

The questions that remain for me are whether the mere search for the females is the reason in some species or would scramble competition be an important factor as well? If finding the females was the issue, why do some have extreme dimorphisms and others not? Would some probably have an "outbreeding strategy" while others tolerate inbreeding? And how would you test that?

I see. In this case, someone with expertise in the group ecology could be of more help than me.

I guess that as you pointed that polygyny was the mating system found, sexual selection involving intra and intersexual selection would be the major force shaping those differences between sexes. The differences between species are more difficult to access without genetic and ecological data. Many factors from lack of variability from resources distribution could be related to those differences.

In a freshwater amphipod, we found that individuals formed pairs quicker under high predation pressure. This result suggested that search time is costly in terms of vulnerability to predation. Adaptations that would decrease search time in a high predation environment like marine systems should be strongly selected. One possible variable to look at would be whether predation risk is higher for highly dimorphic species. .

Do species with extreme dimorphism differ from those without it in limbs affected by dimorphism, or just in extent of differences between sexes (and dimorphic limbs are the same)? If just in extent, may this be due to morpho-phisiological correlations?

We do not find any obvious adaptations for mate guarding as present e.g. in Asellus aquaticus. Behavioral observations are currently impossible due to the remoteness of the habitat, the small size of the animals and their digging behavior. The body part affected by the sexual dimorphism are the body (more slender and smaller in males), antennula (and sometimes antenna) hypertrophied, posterior walking legs extremely elongated. The shape of the pleotelson can also be quite different from the female. I do not see any more "meaning" in that than an increase in the range size of the males. I cannot imagine a reason why they should be more vulnerable to predation than other species without such dimorphism except maybe during exactly this dimorphic adult stage. The reason why I think so is that those males are probably less burrowing, more walking or maybe even drifting. The elongated walking legs might reduce their sinking speed when spread out. Could benefits from an outbreeding strategy explain the evolution of such a dimorphism?

Sexual dimorphism is strongly driven by ecological factors. an important factor that can result in extreme dimorphism between sexes is the ecological preferences of each sex.

For instance the dietary requirement for the female may differ from that of the male for reproductive reasons. thus an environment that is healthy enough to sustain the variety of resources will allow for morphological and physiological specializations suitable to efficiently harness this resource.

In essence sexual dimorphism will never be expressed in extreme forms unless it confers an ecological advantage on the species and its population as a whole.