Population dynamics of three brachyuran crab species (Decapoda) in Icelandic waters: impact of recent colonization of the Atlantic rock crab (Cancer irroratus)

The Atlantic rock crab (Cancer irroratus) was first found in Icelandic waters in 2006. Since then, the species has dispersed rapidly and is currently found clockwise from the southwest coast of Iceland to the east, corresponding to >70% of the coastline. Here, we present a monitoring study on this non-indigenous crab species in Iceland from 2007 to 2019. The study shows that the rock crab is now the most abundant brachyuran crab species on soft substrate bottoms in Southwest Iceland, both as adults and planktonic larvae, indicating that it is outcompeting its rival native species, the European green crab (Carcinus maenas) and the spider crab (Hyas araneus). The average size of the rock crab was similar over time (2007–2019), although it fluctuated between years in a pattern similar to that for the green crab, while significant reduction in size was observed for male spider crabs. The rock crab population is still in a growth phase in Icelandic waters, as seen in increasing distributional range, and can be found in densities comparable to the highest reported for the species in its native range in North America.

SIZE COMPOSITION AND FECUNDITY OF THE ROCK CRAB GRAPSUS GRAPSUS (LINNAEUS, 1758) IN THE ISLANDS OF NAVACHISTE BAY, SINALOA, MEXICO

The present study analysed the size composition and fecundity of Grapsus grapsus (Linnaeus, 1758) in the intertidal regions of Guasayeye, La Ventana and San Lucas islands in Navachiste Bay, Sinaloa, Mexico. Samples were collected from November 2017 to October 2018 with a bimonthly periodicity and sampling stations were established in each island. Crab samples were collected randomly at nocturnal low tide. Sex of the crabs was determined and carapace width (CW), length (L) and height (H) in mm, and their weight (W) in g were measured. Biometric correlations were performed. Fecundity was estimated in gravid females. The highest percentage of capture for females was 53.3% whereas for males was 46.7%; and the sex ratio was 1.15:1. The highest percentage was in San Lucas Island (41.4%) and the lowest in La Ventana (27.7%). Maximum CW (69.3 mm) and weight (137.5 g) were recorded in San Lucas. Island, while the lowest CW (9.3 mm) and weight (0.5 g) were recorded in Guasayeye Island. No significant difference in size (CW and W) were observed between sexes or between islands. The average fecundity for female was 9,245 eggs. A high biometric correlation was obtained (r2=0.98) between the morphometric variables and G. grapsus was found to have an isometric growth.

A field-based investigation of behavioural interactions between invasive green crab (Carcinus maenas), rock crab (Cancer irroratus), and American lobster (Homarus americanus) in southern Newfoundland

Marine species invasions pose a global threat to native biodiversity and commercial fisheries. The European green crab (Carcinus maenas) is one of the most successful marine invaders worldwide and has, in the last decade, invaded the southern and western coastal waters of the island of Newfoundland, Newfoundland and Labrador (NL), Canada. Impacts of green crab on the American lobster (Homarus americanus), which are native to Newfoundland, are not well understood, particularly for interactions around deployed fishing gear. Declines in lobster catch rates in invaded systems (i.e., Placentia Bay, NL), have prompted concerns among lobster fishers that green crab are interfering with lobster catch. Here, we conducted a field experiment in a recently-invaded bay (2013) in which we deployed lobster traps pre-stocked with green crab, native rock crab (Cancer irroratus) (a procedural control), or empty (control). We compared catch per unit effort across each category, and used underwater cameras to directly observe trap performance in situ. In addition, we used SCUBA surveys to determine the correlation between ambient density of lobster and green crab in the ecosystem and the catch processes of lobster in traps. We found: (1) Regardless of the species of crab stocked, crab presence reduced the total number of lobster that attempted to enter the trap, and also reduced entry success rate, (2) lobster consumed green crab, rock crab and other lobster inside traps and (3) there was a positive association between lobster catch and ambient lobster density. Our results suggest that while there was a relationship between in-trap crab density and trap catch rates, it was not linked to the non-native/native status of the crab species.

Missing link: California rock crabs serve as intermediate hosts for the parasite Helicometrina nimia

Parasites can have strong effects on invertebrate host behaviour, fecundity and survival in marine ecosystems. However, parasites are often poorly documented and still rarely integrated into marine ecological modelling; comprehensive surveys of infection in marine invertebrates are sporadic at best. For example, rock crabs are an important part of Californian coastal ecosystems, both as regulators of mussel populations and non-native species, and as prey items for predators like sea otters, but their parasite communities and potential effects on crab population dynamics are seldom studied or understood. Here, we present the first report of infection by the trematode Helicometrina nimia in the economically and ecologically important red rock crab (Cancer productus) and Pacific rock crab (Romaleon antennarium). As intermediate hosts, they are a missing link for infection by H. nimia in Californian fish that was unreported until now. Based on these findings, we advocate for further research into parasite diversity and their potential effects on ecologically and commercially important species.