Cyprid larvae of the acorn barnacle Semibalanus balanoides (Linnaeus, 1767) (Cirripedia: Sessilia: Archaeobalanidae) can metamorphose to juveniles without being permanently attached to a substrate

It is commonly assumed that the pelagic cyprid larvae of acorn barnacles must permanently attach to a substrate before metamorphosing to the benthic juvenile stage. We show that this is not always the case and demonstrate that some cyprids can metamorphose in the water column, i.e. without first becoming cemented to a surface. We observed early-metamorphosing cyprids to fully developed juveniles in coastal plankton samples during the 2018 recruitment season of Semibalanus balanoides (Linnaeus, 1767) in Atlantic Canada. We demonstrated through a laboratory experiment that cyprids can be induced to fully metamorphose into pelagic juveniles. These novel findings raise the question of whether this phenomenon may also occur in other barnacle species.

Ontogenetic and inter-annual variation in the diet of Japanese jack mackerel (Trachurus japonicus) juveniles in the East China Sea

We examined the diet of pelagic juveniles ofTrachurus japonicusin the surface layer in April, and subsequent demersal juveniles in the near bottom layer during May to June of the East China Sea (ECS) in 2005, 2008 and 2009. Diet composition of the pelagic juveniles showed a significant difference between 2005 and the other two years. That is, they preyed mainly onParacalanus parvuss.l. and occasionally onCalanus sinicusin 2008 and 2009, while they fed mainly onCorycaeus affinisin 2005, partly corresponding with the between-year difference in prey densities. The demersal juveniles depended heavily on the fifth copepodites and females ofC. sinicuswhich store lipids in the body, i.e. high-energy food for the juveniles, without a significant inter-annual difference. The markedly low occurrence ofP. parvuss.l. andC. sinicus, which are considered to be energetically more favourable thanC. affinis, from the stomach of the pelagic juveniles in 2005 corresponded with the lowest growth rates of the pelagic juveniles in the three years. Potentially, this resulted in the lowest observed recruitment level of the demersal juveniles for the ECS in 2005 over these three years.

Changes in spatial and temporal variability of prey affect functional connectivity of larval and juvenile cod

Changes in structural connectivity as it can affect functional connectivity, the biological and behavioural responses of an organism, has been examined here over 2 contrasting years when the spatial distribution of larval and juvenile cod and their prey shifted from the flank to the crest on Georges Bank. New data on the gut contents of pelagic juvenile cod are compared with known prey distributions, potential growth and how climate warming can change connectivity in this region. Centropages spp. was the most important prey for pelagic juveniles, especially in June 1987 when they had high abundance on the crest and were dominant in the cod stomachs. In June 1986, copepod abundance was low where the juveniles were distributed along the flank. The potential growth of juvenile cod was greater in June 1987, consistent with the higher abundance of Centropages spp., and with higher recruitment survival, compared with June 1986. Annual changes in the spatial distribution of cod early life stages within the environment of cold or warm years can have different impacts on their growth and survival. Whereas the small copepods, Pseudocalanus spp., are primary prey for cod larvae and very abundant in cold years, larger copepods, Calanus finmarchicus and Centropages spp., are important prey for the pelagic juveniles and the latter species can have a high impact in warm years on the crest. The different spatial structure during cold or warm years provides an explanation why different year classes respond differently to environmental change. Depending on the presence or absence of specific prey, the functional connectivity response changes pathways that determine the growth and survival of early life stages and ultimately a role in recruitment.

The combined effect of transport and temperature on distribution and growth of larvae and pelagic juveniles of Arcto-Norwegian cod

Temperature has been identified in field studies as the physical parameter most influential on growth and recruitment of Arcto-Norwegian cod. However, it has been pointed out by many authors that temperature in this context has not only direct effects on the cod, but also indirect effects through lower trophic levels. Moreover, it has been said that temperature might also be a proxy for other climatic parameters. The present paper analyses the direct quantitative effects of temperature on larval and pelagic juvenile growth from spawning in Lofoten until the 0-group fish settle in the Barents Sea. The approach taken is that of a modelling study, supported by analysis of existing data on fish stocks and climate. It is shown that transport and temperature alone can reproduce key features of the 0-group weight distribution and concentration in the Barents Sea for two consecutive years. The extent of the dispersion of the larvae and pelagic juveniles, as well as the ambient temperature they experience on their route, are shown to depend upon their depth in the water column and, to a lesser degree, the time of spawning.