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The gastropod shell influences important aspects of the hermit crab's life; however, the shells are commonly a limited resource. Therefore, different hermit crab species that coexist in intertidal areas are commonly involved in intraspecific and interspecific competition for shells. We assess if differences in shell preference, exploitation ability, or competition by interference can explain the partitioning of shells between the coexisting species Calcinus californiensis and Clibanarius albidigitus. Clibanarius preferred shells of Nerita funiculata among the six gastropod shells tested, while Calcinus did not establish a hierarchy in shell preference. Therefore, the preference for gastropod shell species does not seem to diminish the competition for shells in the wild. Clibanarius identified and attended to chemical cues signalling potential sites of available shells (chemical cues of dead gastropods); Calcinus did not respond to these cues (competition by exploitation). However, Calcinus was more successful in obtaining a new shell by interspecific shell fighting than Clibanarius. Consequently, the use of better quality shells (intact shells) by Calcinus in the wild can be explained by its greater fighting ability compared with Clibanarius. The bias in shell distributions through dominance by shell fighting, more than by exploitation ability, has also been suggested for other hermit crab species of these genera. [ABSTRACT FROM AUTHOR]

Juvenile growth rate is an important life-history trait that affects the size at maturity, and may influence the development of sexual ornamentation. The sword of several species of the genus Xiphophorus (Teleostei: Poeciliidae) is an elaborate secondary sexual trait that confers an advantage in terms of sexual selection, counterbalanced by locomotive and predatory costs. This study assesses differences in male juvenile growth, age, and size at maturation and their relationship with maximum body size and sword elongation in Montezuma swordtails (Xiphophorus montezumae). Fish size and sword length were measured over the course of 20 months. The size at which individual males initiated sword elongation varied considerably among individuals. Male maximum body size was correlated with early growth rate and age of maturation, and sword growth rate was positively correlated with male size at the onset of sword growth. Early growth in X. montezumae may provide valuable information about performance in subsequent stages and about probable future reproductive success. [ABSTRACT FROM AUTHOR]