What are larvae? How biology affects larval transport How physics affect larval transport Upwelling and larval transport in the California Current
Holoplankton: Plankton that are free-swimming for their entire life cycle. Goals: Eat, avoid being eaten, reproduce Meroplankton: A planktonic life stage (“larvae”) of organisms that are strong swimmers or live on the bottom as adults. Microscopic, ~0.1 to ~5 mm Goals: Develop, avoid being eaten, find habitat
Example of life cycle for species with larvae
Most larvae bear little resemblance to adults Sea star Phoronid worm Octopus Snail
Most seafood species have larvae Mussel Crab Lobster Tuna Dinner Adults Larvae
Most fouling organisms have larvae Barnacle life cycle Feeding Non-feeding (Oceanographers are always looking for better ways to keep barnacle larvae from settling on their boats and instruments!)
What makes larval ecology so important What makes larval ecology so important? ~70% of benthic invertebrates have planktonic larvae Population dynamics Ecologically important (population limited by supply) Edible species (valuable +$) Fouling organisms and invasive species (costly -$) Biogeography -- geographic distributions range expansions Conservation -- Identify “source” and “sink” populations design of marine reserves
Larval Transport: Horizontal movement of larvae from one point to another Larval Dispersal: Spread of larvae from spawning sites to wherever they die or settle Settlement: When a larva metamorphoses and adopts a benthic lifestyle Recruitment: Defined by when we first observe the “new recruit” in the population
What influences larval transport? Biological Processes Development Mode Pelagic Larval Duration Response to Environment Larval Behavior Physical Processes Currents, turbulence Upwelling
Two potential development modes Planktotrophic larvae Feed on other plankton, usually phytoplankton Female produces many small embryos with a long pelagic larval duration (PLD). Lecithotrophic larvae Do not feed on other plankton. Instead they consume yolk that is added to the embryo. Female produces fewer, larger embryos with shorter PLD.
Sea slug (Alderia willowi) switches seasonally from plantotrophic to lecithotrophic larvae Planktotrophic eggs Adult Lecithotrophic eggs P. J. Krug
Feeding larvae tend to be in the plankton longer and disperse farther than non-feeding larvae This is about half the earth’s diameter! This is about half a mile! Shanks et al. 2003
An extreme example -- this Pacific snail can remain in the larval stage for 4.5 years! If average current speed is 20 cm/s, this thing can travel 28,000 km, or 2/3 the distance around Earth, before settling! Strathmann and Strathmann 2007
Comparable odds ratios 1 in 176,000,000 1 in 3,000,000 1 in 20,000 1 in 10 Comparable odds ratios Average number of eggs produced per female per season Brooders Lecitho- trophic Plankto- trophic Thorson 1950
Development rate depends on temperature 25.2 oC 17.5 oC Scheltema 1967
For feeding larvae, development rate also depends on temperature & food availability 10 days 20 days Barnacle development rate Development is fraction of progress towards becoming a cyprid Pfeiffer-Hoyt & McManus 2005
Behavior affects distance and direction of transport SINK SWIM SWIM SINK
Particle Reynolds Number Inertia: an object in motion tends to stay in motion (tendency for gliding) Viscosity: “stickiness” of a fluid, like friction (inhibits gliding) Reynolds number: ratio of inertial forces to viscous forces
Particle Reynolds Number Rep If Rep>1, Inertia dominates. If Rep<1, viscosity dominates. Plankton with Rep1 feel like they’re swimming in molasses.
Swimming velocity scales with body size Most Invertebrate Larvae u 1 mm/s to 1 cm/s Fish Larvae u 1 to 20 cm/s From Huntley & Zhou 2004
Reynolds number scales with body size Most larvae are <0.1 cm long and have Rep<1. Some exceptions include large crustacean larvae, fish larvae At Rep<1, Net velocity = flow + behavior Inertia Viscosity Mann & Lazier, after Okubo 1987
Horizontal advection x = (Ucurrent + Uswim) t [distance] = [distance/time] x [time] Ucurrent 1 to 100 cm/s Uswim 0.01 to 1 cm/s **Currents dominate horizontal advection Vertical advection z = (Wcurrent + Wswim/sink) t [distance] = [distance/time] x [time] Typical Wcurrent 1 to 10 cm/s, but average = 0 Typical Wswim/sink 0.01 to 1 cm/s **Behavior dominates vertical advection Diffusion (Random motion due to turbulent mixing)
Larval Transport: Focus in on California Current, Oregon upwelling zone
Upwelling in California Current has big effect on dispersal of rocky shore species Mussels and barnacles form patches in intertidal zones and stay attached to rock after settlement.
Note the direction arrows
WA OR Point Conception CA Halpin et al. 2004
Primary production in California Current is strongly dependent on upwelling Temperature Chlorophyll MBARI data from August (peak upwelling season)
Separation of coastal jet can be seen in Chl A map WA OR S. Calif. Point Conception CA San Diego
2. Central California Coast 1 2 1. Oregon Coast 2. Central California Coast -Weak, intermittent upwelling -Strong, steady upwelling -High invertebrate recruitment -Low invertebrate recruitment
Barnacle Mussel California Oregon California Oregon 2 years of recruitment data California Oregon California Oregon Connolly et al. 2001
Central California example - barnacle data Counted barnacle larvae #67 - 1969 to 1984 #63,70 - 1982 to 1984 Roughgarden et al. 1988
Upwelling transports larvae offshore Roughgarden et al. 1988
Central California example - barnacle recruitment peaks during relaxation events Farrell et al. 1991
Oregon region: -Seasonal upwelling, weak/intermittent in summer -coastal jet remains near coast -upwelling increases production -upwelling doesn’t prevent larvae from getting to shore -upwelling positively affects recruitment of feeding larvae California region: -continuous upwelling, strong in summer -upwelling pushes coastal jet and larvae offshore -nearshore production may be lower -relaxation events are important for recruitment