Why is growth important?
•Growth indicates health, and physical and ecological stresses experienced
•Growth is plastic: organisms can change relative resource allocation to growth and reproduction
Implications for understanding urchin growth
•Help improve aquaculture methods and increase declining wild populations
•Growth curves can determine maximum size for fishery and the harvesting season
Predator-induced changes in growth
and metabolism of the green sea urchin
Strongylocentrotus droebachiensis
Results: Predator scent reduced growth
Becca Selden, Molly Wright, Amy Johnson, Olaf Ellers
Bertalanffy Growth
The Bertalanffy model defines growth as
dw =a w2/3- bw1
dt
w is weight at time, t
a is the anabolic coefficient (intake)
b is the catabolic coefficient (maintenance)
2/3
Metabolism scales with the 2/3
exponent.
WHAT DOES THIS HAVE TO DO
WITH GROWTH?
Urchins grow more
slowly over time
Therefore, the change in growth over
time is larger for smaller organisms
As urchin diameter increases, growth rate
decreases.
:
Summer Research 2005
Anabolism => uses energy
Catabolism => releases energy
Growth is zero when energy used = energy
gained (x intercept)
Methods:
Flow/no flow metabolism experiment
Ambient
•Oxygen consumption of 40 unfed urchins was measured
using respirometry for 20 minutes in flow conditions and
6 days later no flow conditions (Fig. 1)
Acclimated to 14 degrees C
•Urchins were fed 14 days and 7 days before their
assigned measurement day
•Individual urchins were measured at 13.8 degrees C in
flow and no flow conditions on one of 5 measuring days
•Size-matched urchins smaller than 23 mm exhibited significantly slower growth in the presence of
Cancer borealis scent.
Figure 1. Respirometry equipment: Left
Hansatech, Right: Strathkelvin
•The largest urchins did not show significant growth. If grown at a temperature better for growth, would
differences appear?
Crab/no crab growth experiment
•Grew 56 urchins in the presence or absence of scent of 4 Jonah
crabs Cancer borealis (Fig. 2) in flow through tanks of ambient
seawater. Flow rate~ 4.5-5L/min
Figure 2. Cancer borealis in compartment
separated from urchins
Growth slower than expected
• Boundary layers contribute?
• Urchins rely on flow for
oxygen transport
• Lower flow reduces oxygen
most for the smallest urchins
= slower growth?
Factors affecting growth
Conclusions/ Future Work:
Flow/no flow
•Urchins in lower flow environments consume less oxygen and may have less energy available for growth
•Rotated the individual cages through the positions to control for
position effect (Fig. 3)
•Flow environments should be considered when establishing a site for aquaculture or an urchin fishery to
maximize growth
•Fed ad libitum for 5 weeks
But, the smallest urchins deviate from the Bertalanffy model
•Urchins larger than 23 mm did not show growth differences between treatments
•Future work:
•Compare growth in different flow environments
•Measured weight weekly
Figure 3. Flow-through tank setup in the marine lab
•Compare flows experienced by urchins hiding from predators in rocks to that on a uniform surface
Predation
Results: Higher metabolism in higher flow
•Crabs prey on urchins by flipping and piercing the soft mouth area
•Size-specific reduction in growth
•Large urchins may have size-refuge when they outgrow grasp of crab
•Small urchins may detect cue of given predator as greater risk
Urchins grown in ambient
seawater consumed more
oxygen in high flow
environments
•Mechanism of growth reduction
•Predator scent may decrease foraging time of prey?
•Predator-prey interactions can impact both population dynamics and individual life history traits
•Future work:
•Rerun experiment at 10 degrees Centigrade at which large urchins exhibit growth to detect potential
difference between treatment
Flow
•Patterson (1992): flow impacts metabolism
•Johnson lab (2005): slower growth in small urchins predicted due to decreased rates of oxygen
transport on metabolic rate
•Further test the chemosensory responses to predation cues by quantifying the chemotaxis, cryptic and
aggregation behavior when exposed to predators
Predation
•Reimer and Tedengren (1996):Exposure to crabs slows growth in blue mussels Mytilus edulis
•Palmer(1990); Appleton and Palmer (1988): crabs slow growth in dogwhelks Nucella sp.
•Dumont et al (2004): smaller urchins hide more often in crevices to avoid predation
•We think hiding may reduce the flow they experience
Hypotheses:
Flow
Literature Cited:
Urchins acclimated to 14
degrees-centigrade
seawater consumed 19%
more oxygen in high flow
environments
Appleton, RD, and Palmer, AR (1988). Water-borne stimuli released by predatory crabs and damaged prey induce more predator-resistant shells in a
marine gastropod. Proc. Natl. Acad. Sci. USA 85: 4387-4391
Dumont et al. (2004). Size specific movement of green sea urchins Strongylocentrotus droebachiensis on sea urchin barrens in eastern Canada.
Marine Ecology Progress Series 276:93-101
.
Ebert, T.A. and Russell, M.P. (1993). Growth and mortality of subtidal red-sea urchins Strongylocentrotus franciscanus at San Nicolas Island,
California, USA-problems with models. Marine Biology 117(1): 79-89
Johnson, A.S., Ellers, O., Wright, M, Stranges, B. (2005). Implications of transport processes and 2/3 poser metabolic scaling to Bertalanffy and
bimodal growth in sea urchins. Society of Integrative and Comparative Biology (in press).
•Urchins will have a higher metabolism in high flow environments than low flow environment
Palmer, AR (1990). Effect of crab effluent and scent of damaged conspecifics on feeding, growth, and shell morphology of the Atlantic dogwhelk
Nucella lapillus (L.). Hydrobiologia 193: 155-182.
Predation
Patterson, Mark A (1992). A mass transfer explanation of metabolic scaling relations in some aquatic invertebrates and algae. Science 255: 14211423.
•Urchins exposed to chemical cues from Cancer borealis will grow more slowly than urchins not
exposed to the cue
•Urchins in the smaller size classes will exhibit a stronger response to the predation risk than
urchins in larger size classes
Reimer, O, and Tedengren, M (1996). Phenotypical improvement of morphological defences in the mussel Mytilus edulis induced by exposure to the
predator Asterias rubens. Oikos 75: 383-390.
Therefore, the metabolic rates of all urchin sizes are affected by the flow of the
seawater around them
Acknowledgements:
Special thanks to: Amy Johnson and Olaf Ellers, Allen’s seafood for providing the crabs, Marney Pratt, Molly Wright, Beckman
Foundation, Maine SeaGrant for providing support for urchin tanks, Robert Russell (DMR) for providing larger urchins