Food in water - % of inflowPumped - L/sLagoon filtered every -Clock -fps -

Drag to orbit, wheel or pinch to zoom. Current flows left to right. Yellow dots are food particles (their number follows the real concentration); each sponge pulls them in and breathes clean water out of its top as a jet. Particle motion is schematic: speeds are exaggerated so you can see them.

Sponge Reef Filter-Feeding Lab

Sponges are pumps. Each one drives water through thousands of tiny chambers, keeps the bacteria, and expels clean water through its osculum. How many does a lagoon need to stay clear?

Time-lapse

Sponges

Water

Food particle

Live readouts

Food concentration vs time (solid: simulation, dashed: closed form)

Steady clarity vs number of sponges

Experiment: how many sponges halve the food?

Integrates the lagoon balance numerically for different sponge counts and compares it with the closed-form prediction.

Independent numerical checks

Controls

Drag / one finger: orbit; wheel / pinch: zoom.
Every slider recomputes the pumping budget at once. Time-lapse only speeds the clock of the chemistry; it never changes the physics.
The water-clarity colour follows the real food concentration.

The science

Continuity at the osculum. A sponge of tissue volume V_s pumps Q = p·V_s, where p is the specific pumping rate (mL of water per mL of sponge per second; typical values are of order 0.01 to 0.5 s^-1, so this is a dial, not a measurement). All that water leaves through the osculum of area A_o, so by conservation of mass v_o = Q / A_o. The jet Reynolds number is Re = v_o d_o / ν with ν ≈ 1.0×10^-6 m²/s.

Retention. Collar cells (choanocytes) trap particles; the fraction of a particle type removed from the water that passes through, η, is high (typically 0.8 to 0.99) for bacteria and small algae. The values in the particle menu are rounded typical figures, not measurements of one species. The clearance rate is F = η·Q.

Lagoon balance. A well-mixed lagoon of volume V is flushed by the tide with residence time τ_f: dC/dt = (C_in − C)/τ_f − (η Q_tot/V)·C. With k = ηQ_tot/V the solution is C(t) = C* + (C₀ − C*)·e^−t/τ, 1/τ = 1/τ_f + k, C* = C_in / (1 + k τ_f). The page integrates the equation with a 4th-order Runge-Kutta stepper and compares it with this closed form.

Temperature and silt. Pumping follows a Q10 law, p(T) = p₂₀·2^(T−20)/10 (Q10 = 2 is a common rule of thumb). Suspended sediment makes sponges close their pores and stop pumping; here 1/(1 + (S/30 mg/L)²), an illustrative curve of the right shape, not a calibrated one.

Simplifications. The lagoon is one well-mixed box, so there are no concentration gradients between sponges, no re-filtration of the exhaled jet, no growth or death of sponges, no bacterial growth, and a constant inflow concentration. Sponge tissue volumes come from simple geometric estimates of the three modelled shapes (tube 0.04, vase 0.07, barrel 0.12 L per (height/10 cm)³). The 3D particle motion is schematic and exaggerated; only the numbers on this page are physical.

Please spot-check against primary literature before citing any constant: p, η, Q10, the carbon per cell and the silt curve are typical or illustrative values.