DEVELOPMENT, SURVIVAL AND TIMING OF METAMORPHOSIS OF PLANKTONIC LARVAE IN A VARIABLE ENVIRONMENT - THE DUNGENESS CRAB AS AN EXAMPLE

We use models to show how a variable environment can affect developmen t, survival and timing of metamorphosis of meroplanktonic larvae. For a general case, we use an analytical model to explore the effect of te mperature-dependent development and mortality rates on temporal patter ns of metamorphosis. The distribution of metamorphosis over time is a lagged version of the pattern of larval release, distorted by changes in development and mortality rates. If temperature causes development rate to increase (decrease) linearly with time through the larval peri od, the timing of metamorphosis has a short (long) temporal range, a l arge (small) amplitude, and is shifted to the left (right). This impli es that at locations with a decreasing trend in development rate (e.g. due to decreasing temperature), the timing of metamorphosis is more s ensitive to larval release time than at locations with an increasing t rend. Mortality of larvae can be influenced directly by the environmen t through sub-optimal conditions, or indirectly by environmentally ind uced changes in development rate, which change the duration of the lar val period. If mortality rate is constant, the longer the larval perio d the greater the mortality. However, this result is not true if morta lity rates change with the environment in the same way that developmen t rates change. The common assumption of constant mortality rates need s to be critically reassessed. To apply these results to a specific ex ample, we develop a model of the temperature- and salinity-dependence of Dungeness crab Cancer magister Dana larval development, using avail able laboratory data. We then use historical records of daily sea surf ace temperatures and salinities to examine the impact of these 2 varia bles on larval development and survival at coastal sites along the U.S . west coast. Based on calculations from 7 locations over a total of 2 33 yr, Dungeness crab larvae can take between 74 and 182 d to metamorp hose, depending on location, year and time of release. Larvae in the n orthernmost part of the study region (Washington) have the longest mea n annual development times (133 to 163 d), those in central and northe rn California have intermediate development times (81 to 134 d), and t hose in southern California have the shortest development times (74 to 84 d). Greatest relative intra- and interannual variability in mean d evelopment times and survival occurred in upwelling areas off central California. Observed latitudinal differences in timing of metamorphosi s from zoeal to megalopal stages were consistent with differences pred icted by the model. We conclude that variability in temperature and sa linity can cause Dungeness crab larval periods to vary by a factor of 2. For meroplanktonic populations in general, complicated patterns of metamorphosis can result from variable temperatures during the larval period, even if hatching of larvae occurs at a constant rate.