The small intertidal New Zealand crab, Halicarcinus cookii undergoes a terminal pubertal moult, but this moult takes place over a wide range of pre-moult sizes. Relative growth of the abdomen width in immature females is positively allometric, but negatively allometric in mature females. Male abdomen growth is negatively allometric. Growth of the cheliped propodus in males is positively allometric, but in females it is negatively allometric or isometric. Overlap in the size-range of mature and pre-pubertal immature female H. cookii is 72% and in other hymenosomatids it can be as high as 87%. This overlap is probably the result of crabs having a variable number of pre-pubertal instars, but seasonal change in water temperature, with crabs moulting to a smaller final size during colder months and to a larger size during the warmer months is also possible. The net reproductive rate (R o ) of early and delayed moulters is compared and for H. cookii R o e / R o d ¼ 1.0 so the size overlap is stable. Most hymenosomatids have determinate growth, but Hymenosoma orbiculare and Elamenopsis lineata continue to moult after maturity and have retained the ancestral link between moulting and mating.
Non-indigenous species (NIS) in the Arctic have an increased likelihood of arrival from ship traffic in the region, while the survival potential of the species becomes more likely in a warming environment. Monitoring is essential to detect the rate and magnitude of the establishment of NIS. In this study, a list of 123 potential marine NIS for Svalbard was drafted and the presence of marine NIS in soft sediment of Kongsfjorden in Svalbard was assessed using molecular metabarcoding techniques. For 37 species, including eight potential Arctic NIS, we generated new 18S and/or COI barcode sequences to improve the available online reference databases. In total, 299 species were identified in the sediment samples, including seven potential NIS. Three of these potential NIS have not been reported before in Svalbard: the harpacticoid copepod Euterpina acutifrons, and the ascidians Botrylloides violaceus and Molgula manhattensis. Another novel observation for Svalbard was the polychaete Chone mollis. Additional studies are needed to assess whether the NIS have been established on Svalbard and what their potential impact on the local system may be. Metabarcoding proved to be an effective monitoring tool to detect the presence of new species in Svalbard marine waters. We advise its use to set up a baseline record for the presence of NIS at points of entry, especially harbours. This approach is also valuable for biodiversity monitoring, in particular the detection of small organisms and life stages that are hard to identify using current visual techniques.
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