The caprellid amphipod, Caprella mutica, is a well-known invasive species, originating in the Sea of Japan, which has been rapidly expanding along the coasts of North America, Europe and Oceania for the last forty years. Caprella mutica is frequently associated with man-made structures, especially those dedicated to aquaculture activities, where it can reach high densities of up to 300,000 ind./m2. A well-established population of C. mutica was recently found by SCUBA-divers in Galician waters (north-west Spain) at 6 different man-made floating structures along Ría de Arousa. The record of this species in this location implies a new southernmost limit of distribution, extending the known distribution range in Atlantic European waters and confirming the continuity of the colonization southwards.

To quantitatively assess the fecundity of female Caprella mutica (Crustacea: Amphipoda), an invasive caprellid species in numerous regions of the world, I reared pairs of adult C. mutica under four different temperature regimes (5, 10, 15 and 20°C) and measured the moulting interval, oviposition interval, oviposition–hatch period, and oviposition–juvenile emergence period. Concurrently, I examined changes in the number of eggs per oviposition and hatching success at the different temperatures, and clarified the timing of events from oviposition to juvenile emergence at all temperatures. The findings showed that this species reproduces at temperatures ranging from 5 to 20°C. Moulting interval was closely correlated with oviposition interval, becoming longer at lower temperatures. Similarly, the mean oviposition–juvenile emergence period was also longer at lower temperatures. Eggs developed normally at 15 and 20°C but showed decreased viability at 5 or 10°C. The number of eggs produced per oviposition did not vary with rearing temperature. These findings indicated that reproduction in female C. mutica in the field was frequent with short intervals between oviposition events when temperatures exceeded 15°C. Conversely, at temperatures below 10°C, reproduction was less frequent, resulting in markedly reduced reproductive rate and an increased probability of unusual egg development.

Characteristics of the second gnathopod are traditionally used to distinguish between species of caprellid amphipods. However, these distinctions are often subjective and can be variable within a species. Geometric morphometrics were used to quantitatively assess shape variation of the second gnathopod propodus of three species of caprellids in North America, including the non-native Caprella mutica. Gnathopod shapes of C. mutica specimens from different latitudes revealed distinct morphologies; the factors responsible for the shape variations are unknown. Allometric change of propodus shape was observed in C. mutica. Larger individuals showed a wide array of possible propodus morphologies. Despite this variability, there were clear differences between large specimens of C. mutica and two species native to North America: C. alaskana and C. kennerlyi. The use of geometric morphometrics and the thin-plate spline method can serve to both complement descriptions using traditional keys and aid in identification of non-native species in novel geographical regions.