Amphipod (Scuds)

Amphipods are small (3-8 millimetre) benthic crustaceans found in nearly every established freshwater system — established planted aquaria, stream margins, leaf-littered ponds, lake sediments. Hobbyists call them "scuds." The simulator parameterises Hyalella azteca, the EPA freshwater ecotoxicology reference organism, because the literature on its copper, ammonia, nitrite, oxygen, calcium, pH, and temperature thresholds is unusually deep. Functionally amphipods occupy the shredder guild, which is otherwise absent from the model: unlike snails and shrimp (which scrape biofilm off intact surfaces) or filter feeders (which capture suspended particles), amphipods grip coarse settled detritus — leaf fragments, decaying plant tissue, fecal pellets — between their gnathopods and physically tear it apart. Some of the resulting fragments are ingested, but a substantial fraction lifts off the substrate as fine suspended particles that bacteria, protists, and filter feeders can then access. This coarse-particulate-organic-matter-to-fine-particulate-organic-matter conversion is the canonical "shredder pathway" of stream ecology (Wallace & Webster 1996; Graça 2001) and a key reason mature aquaria with leaf litter mineralise organic matter faster than equivalent bare tanks: without amphipods, settled detritus only converts to fine particles via slow microbial decomposition, especially in cool water and in sealed sediments where surface-area-to-volume is unfavourable.

Mechanistically the simulator runs two parallel fluxes when amphipods are present: a normal Consumer feeding pathway (settled detritus is the primary food at 25 per cent assimilation, with periphyton, biofilm bacteria, and benthic cyanobacterial mats as secondaries) and a separate first-order shredding flux that drains settled detritus into suspended detritus at a rate proportional to amphipod biomass and Monod-saturating on detritus availability. The shredding rate is calibrated so that at saturation roughly one third of amphipod-detritus interaction is mechanical fragmentation and two thirds is biological feeding (shred_rate_per_h_per_C ≈ 0.0028 per hour vs. Imax_C_per_h_per_C of 0.005 per hour) — matching the canonical stream-ecology figure that shredders convert about 30-40 per cent of CPOM they process into FPOM via fragmentation rather than assimilation. Mass balance is preserved by construction: every C, N, and P atom shredded leaves the settled pool and arrives in the suspended pool at the prevailing detrital stoichiometry, with no element conversion and no creation of mass.

Amphipods are direct developers — females carry fertilised eggs in a marsupium until release as miniature adults — so the model uses a single active C/N/P pool with no juvenile/adult morphological split and no resting eggs (contrast Daphnia, which has ephippia, and copepods, which have resting cysts). Like other crustaceans they molt continuously, consuming Ca²⁺ from the water column for cuticle synthesis (about 14-day intervals at 22 degrees Celsius) and incurring a brief soft-shell mortality window after each molt. The Hyalella cuticle is much less calcified than a shrimp exoskeleton, so per-molt Ca demand is set at roughly one third the shrimp value (0.003 vs 0.010 mol Ca per mol body C) and the amount of CaCO₃ deposited to substrate on death is correspondingly smaller. They are notably Cu-sensitive — 96-hour LC50 around 25-35 micrograms per litre free Cu²⁺ — and the Cu toxicity half-saturation (1.5×10⁻⁷ mol per litre) places amphipods between Daphnia and hydra on the freshwater invertebrate Cu-sensitivity ladder, consistent with the macroinvertebrate ecotoxicology consensus (hydra > amphipod ≈ Daphnia > cherry shrimp > pulmonate snails). They are also moderately sensitive to ammonia and nitrite, prefer cool water (optimum 18-24 degrees Celsius, stress above 26, lethal above 32), tolerate cold well (overwinter under ice in the wild), need at least 0.5 mg Ca per litre to molt successfully (chronic toxicity threshold, Borgmann 1996), and are obligate freshwater organisms (some tolerance to slightly brackish water but not the estuarine span of marine gammarids).

Visible-population behaviour: amphipods scuttle through leaf litter and biofilm, are nocturnal (the simulator sets their night_activity parameter above 1.0 to reflect more activity at night than during the day), and reach densities of several hundred per litre in well-established planted tanks with detritus accumulation. They are a hobbyist bioindicator for two states at once: their presence usually indicates that the tank has accumulated enough organic matter to support a shredder community, and their disappearance after a copper or ammonia spike is one of the earliest visible signs of water-quality damage. They do not eat plants and are not removed during water changes (they hide in substrate and leaf litter); like cherry shrimp they are functionally permanent residents of established systems.