José Luis Acuña-University of Oviedo, SpainAngel López-Urrutia, Spanish Institute of Oceanography, Gijón, Spain

Sean Colin, Roger Williams University, US

Faking giants: The evolution of high clearance rates in jellyfishes

Why gelatinous?

-Probably NOT to be untasty and transparent and evade predation:

Many predators love eating jellies.Many jellies live in the deep ocean, where being seen is NOT an issue.Many of those jelliesGZ are bioluminiscent.

-Jellyfishes must have some trick to endure low food:

They do NOT use lipidsThey do NOT migrate in timeThey withstand prolongued degrowth

-A possibility is that large, water ladden bodies allow for large food capture surfaces (e.g. Harbison 1992).

-A cost-benefit analysis of their predation mechanism supports this idea.

Fishes and jellyfishes are cruising predators…

S’

U

Modified from Kiørboe 2008

-Fishes remove prey from a volume which can be calculated as the visually perceptive surface times the swimming velocity.

Volume cleared=βVolume Perturbed

adapted from Kiørboe 2008

-Jellyfishes swim to force the water in front of the bell (in the figure, in red) to flow accross the bell margin, where a pressure drop causes the water to rotate in a vortex around the tentacles, where prey capture occurs (in the figure, in green). We will assume that:

or else

Clearance rate=βSU-Where β is a search efficiency expressing the ratio of volume cleared to volume perturbed.

-Intuitively, β>>1 in fishes but <1 in jellyfishes. Is this true?

…with widely different searching efficiencies.

-We can solve the above equation for β and combine it with published measurements of clearance rates, S and U to arrive at estimates of β.

Clearance rate=βSU

β=Clearance rate/SU

Net Energy available

for production

Gross energy obtained by predation

Metabolic cost of

swimming

Basal respiration

𝐻=𝑒𝑃 𝛽𝑆𝑈− 12𝜂 𝛼𝐶𝐷 𝜌𝑆𝑈 3−𝑅𝑏

TOTAL RESPIRATION

= --

Searching efficiency

Food assimmilation efficiency (Uye & Shimauchi 2005)

Prey concentration

Bell cross-surface

Swimming velocity

Propulsive efficiencySahin et al.2009

Energy conversion coefficient

Drag coefficient(Mc. Henry & Jed 2003)

Water density

Basal respiration

Thus we can apply Ware’s (1978) cost-benefit model

Acuña, López-Urrutia and Colin 2011

Daniel 1983

𝐻=𝑒𝑃 𝛽𝑆𝑈− 12𝜂 𝛼𝐶𝐷 𝜌𝑆𝑈 3−𝑅𝑏

Thus we can apply Ware’s (1978) cost-benefit model...

0 10 20 30Swimming velocity, U (cm/s)

-40

0

40

80

H (m

g C

/day

)co

sts,

bene

fits R

b (m

g C

/day

)

0

100

200

300P=100 μg C/L

P=50 μg C/L

P=25 μg C/LP=12.5 μg C/LP=6,1 μg C/L

-The costs increase as the cube of the swimming velocity

-The benefits increase only linearly.

-Benefits-costs generate a dome-shaped curve.-If we subtract the basal respiration, the curve is slightly shifted downwards.

-The slope of the benefit function decreases with P. This shifts the H function to the left and the optimum velocity becomes slower. This is Ware’s basic prediction.

-There is an optimal foraging velocity for which the scope for growth is maximum.

-However, we know that jellyfishes do not adjust their swimming velocities (e.g. Titelman & Hanson 2006).-What kind of fixed swimming velocity should be favored by natural selection in this case?

-The one allowing net benefit with less prey.bene

fits-

cost

s (m

g C

/day

)

-The basal respiration is constast.

30

...to analyze the evolution of structure instead of behavior.

H(mg C d-1)

-When H is plotted against logP and logU, the 0-growth isocline reaches a global minimum which also corresponds to a single, and slow, global optimal swimming velocity

𝑈𝑜𝑝𝑡=( 𝜂 𝑅𝑏

𝛼𝐶𝐷 𝜌𝑆 )13

Uop

t

-Aurelia’s actual velocity is slightly slower, and remains within the region where degrowth rates are mild.

Ure

al

-Reducing the bell surface, i.e. by eliminating the excess water from the body, allows for a faster global Uopt, but brings the isocline up, causing starvation at higher food concentrations

-Last, increasing β has no effect on Uopt, although it lowers the threshold, making the sistem more effective and less sensitive to optimization. This is the fish strategy

-Here we propose that jellyfishes have evolved from B to A (increasing S), while fishes have evolved from B to C (increasing β).

So are they so good at clearing prey?

-In a carbon basis Jellyfishes clear and respire just as much as fishes.

-However, both jellyfishes and fishes clear 10 times as much as their putative prey, the crustaceans.

-What is going on here?

Not more but not less than fishes

¿Son las medusas capaces de procesar mayores volúmenes de agua?

Some help from terrestrial ecology

-In terrestrial systems, predators have only 10% of prey biomass than their prey. This is why they use an area 10 times larger than their prey

-For that very same reason, we should expect that, in aquatic systems, predators should use a volume ten times larger than their prey.

1st trophic level

2nd trophic level

3rd trophic level

BIOMASS

¿Son las medusas capaces de procesar mayores volúmenes de agua?

Wrap up

-As predators, both jellyfishes and fishes suffer prey dilution.

-The fish strategy: increase β.

-The jellyfish strategy: increase S.

-This is not a wide range, but it is essential: it envelopes the typical range of turbulent velocities in the upper ocean layer.

-Thus, this is probably why jellyfishes are counted among the plankton not necton, and why they depend on ocean currents and fronts for sex encounter and to recruit into benthos.

-This has probably had a cost in terms of swimming velocity, from a potential of tens of centimeters per second to just a few centimeters per second.

-However, this trick has allowed jellyfish to bear up in the competitive game, and to replace their fish competitors when overfished.

THANK YOU