Hydra

Hydra are small (5-15 millimetre) freshwater cnidarians — close cousins of corals and jellyfish — that attach to plant stems, glass, and other substrates by an adhesive basal disc and deploy a ring of nematocyst-laden tentacles to ambush passing prey. Unlike filter feeders (Daphnia, rotifers) that generate their own feeding currents, hydra are sit-and-wait predators: they wait for zooplankton to brush a tentacle, fire stinging nematocysts to paralyse the prey, and engulf the immobilised animal through a single mouth. This passive encounter-rate feeding mode means hydra capture is strongly gape-limited: juvenile cladocerans (newly-hatched Daphnia under 0.5 millimetres), copepod nauplii, and rotifers are accessible, but adult Daphnia, copepodites, ostracods, and snails are too large or armoured to be handled. Hydra reproduce primarily by asexual budding (a small daughter polyp grows from the parent's body wall and detaches), enabling population doublings of 2-4 days when prey is abundant; sexual reproduction with resting embryos occurs under stress in some species but is not modelled. Hydra are textbook indicator organisms for clean, well-cycled aquaria — they are exceptionally sensitive to dissolved copper (96-hour LC50 around 5-30 micrograms per litre free Cu²⁺, among the lowest of any freshwater invertebrate; the model uses an element_half_sat["Cu_tox"] of 1×10⁻⁷ mol per litre, roughly half this LC50), to ammonia (lethal above 0.05-0.1 milligrams NH3-N per litre), and to nitrite. They tolerate temperature poorly above 28 degrees Celsius (brown hydra collapse) but handle cold well, and require strictly freshwater (intolerant above 2 PSU). The "hydra appear when water quality is good" hobbyist observation is not a hardcoded rule but an emergent property of these tolerance kernels — drop hydra into a cycling tank with elevated NH3 and NO2 and the active population decays before it can establish; drop them into a mature tank with abundant juvenile zooplankton and they bloom while leaving adult cladocerans largely untouched, since gape-limited predation cannot reach the adult refugia. Modelling-wise, hydra are functionally sessile: they live on substrates but are tracked as a single planktonic-style C/N/P pool rather than per-surface, with low maximum ingestion (0.6 per day, far below filter feeders) and continuous tentacle deployment (no diel feeding rhythm). They produce compact regurgitated pellets and detach as sinking polyps on death, so most lost biomass settles rather than staying suspended.