Ostracod

Ostracods are tiny benthic crustaceans (about 0.75 mm) enclosed in a bivalved carapace -- they look like miniature clams with legs poking out. In the model, they fill the role of a generalist benthic scraper and detritivore, crawling over substrate and surfaces to consume biofilm, settled organic matter, and whatever small organisms they encounter. Freshwater ostracods (family Cyprididae) reproduce parthenogenetically, allowing rapid population growth without mating, similar to Daphnia but somewhat slower due to their smaller body size and lower ingestion rate.

Feeding. Ostracods are non-selective scrapers: they use their appendages to rake across surfaces, ingesting periphyton (preference 1.0, 65% assimilation), settled detritus (preference 0.8, 35% assimilation), cyanobacteria (preference 0.7, 55% assimilation), and bacteria (preference 0.6, 50% assimilation). They also incidentally consume suspended detritus near the substrate (preference 0.4, 30% assimilation), benthic ciliates (preference 0.3, 55% assimilation), planktonic algae that settle near the bottom (preference 0.2, 55% assimilation), and nanoflagellates (preference 0.15, 45% assimilation). Their maximum ingestion rate is about 0.72 mg C per day per mg body C (half-saturation at roughly 0.36 mg C/L), making them moderate feeders -- slower than Daphnia but with access to food sources that Daphnia cannot reach. The most ecologically important aspect of ostracod feeding is their strong access to settled detritus (access 0.75), a food source that most other consumers largely ignore. This makes ostracods a key link in the decomposition loop: they physically break down and assimilate dead organic matter sitting on the bottom, recycling nutrients that would otherwise depend entirely on bacterial decomposition. Fecal pellets have an elevated C:N ratio (10.0 versus body C:N of 5.5), with 85% settling to the bottom and 15% staying suspended.

Surface access. Because ostracods crawl directly on surfaces, their access to periphyton and bacteria varies by surface type. They scrape smooth glass very efficiently (90% access) but have reduced access on rough gravel (50%) and sand (55%), where their small body can reach some crevices but not all. Periphyton on surfaces with deep refugia (thick EPS matrix, pore protection) is partially protected from ostracod grazing -- a minimum of 2% of surface biomass is always accessible, but the rest is protected behind a half-saturation threshold.

Habitat dependence. Ostracods have the strongest biofilm maturity dependence of any consumer. They need a structurally complex EPS scaffold to shelter and brood in — a bare surface with abundant food is a poor habitat. At M = 0.3 (roughly 2 months into a new tank), their habitat factor is only 0.09 — the population can barely grow. At M = 0.7 (roughly 5 months), the habitat factor reaches 0.49 and the population begins expanding rapidly. This produces the characteristic lag-then-emergence pattern observed in real aquaria, where ostracods appear months after tank setup.

Benthic lifestyle. Ostracods spend most of their time on the substrate or crawling on surfaces, not swimming in the water column. This benthic habit has several consequences in the model. Only 15% of the population is removed during water changes (most stay on the substrate where siphoning is less effective). When ostracods die, 90% of their biomass settles to the bottom rather than staying suspended. Population density is regulated by crowding mortality based on substrate area density rather than volume-based concentrations -- at roughly 4e-7 mol N per cm² of benthic area, crowding mortality reaches half its maximum rate. Ostracods are moderately nocturnal, with nighttime activity at 70% of daytime levels.

Tolerances. Ostracods are reasonably hardy. They tolerate temperatures from 5 to 28 degrees Celsius without stress (lethal below 0 or above 33 degrees), prefer very low salinity (optimal 0.5 PSU, lethal above 15 PSU -- these are strictly freshwater animals), and handle a wide pH range (stress outside 5.5-9.5, lethal outside 4.5-10.5). Their hypoxia tolerance reflects their benthic lifestyle: they become stressed at about 1.3 mg/L dissolved oxygen and die below about 0.3 mg/L, which is somewhat better than Daphnia but worse than the truly hypoxia-adapted Neocaridina shrimp. Ammonia toxicity follows the same pattern as other crustaceans, with stress at moderate unionized ammonia levels and lethal effects at high concentrations. Specific dynamic action (the metabolic cost of digestion) consumes 18% of assimilated food energy.

Body composition. Body C:N ratio is 5.5 (slightly higher than most soft-bodied crustaceans like Daphnia at 4.5-5.0), reflecting the calcified bivalve carapace. The model does not explicitly track calcium uptake for carapace construction -- ostracod calcium demand is implicitly small relative to larger shell-builders like snails and shrimp. Body N:P ratio is 20.0 (molar), similar to copepods.

Resting eggs. Unlike Daphnia and copepods, ostracods are tracked with only an active body pool plus a dormant resting-egg pool — there is no juvenile/adult split, because the eight ostracod instars are functionally a continuous size series with no sharp ecological discontinuity. The dormant pool, however, is biologically essential: ostracod resting eggs are among the most durable of any freshwater invertebrate, with viable eggs recovered from sediments after decades of dry storage (Brendonck & De Meester 2003; Horne & Martens 1998). The dormant pool is metabolically inert — it does not feed, respire, or grow, and is bypassed by every active-pool mortality kernel (Cu²⁺, hypoxia, NH₃, NO₂⁻, temperature stress, pH stress, H₂S) — and it is exempt from the extinction-threshold check. This means an ostracod population that loses its entire active stage to an acute event (anoxia, copper pulse, severe ammonia, dry-out) can survive in the egg bank and re-seed the system once conditions normalise. A 5% constitutive background allocation routes a small fraction of new biomass to the dormant pool even under good conditions; cued allocation rises to 50% under combined food-limitation, crowding, low-temperature, and short-photoperiod stress. Hatching back into the active pool is gated by rewarming above 10 °C and Monod-saturated food availability.

References:

Grant, I.F., Egan, E.A. & Alexander, M. (1983). Grazing rates of Cyprinotus carolinensis on blue-green algae. Hydrobiologia, 106, 199-208.
Heip, C. (1976). The life-cycle of Cyprideis torosa. Oecologia, 24, 229-245.
Roca, J.R., Baltanas, A. & Uiblein, F. (1993). Adaptive responses in Cypridopsis vidua to food and shelter offered by a macrophyte. Hydrobiologia, 262, 127-131.
Brendonck, L. & De Meester, L. (2003). Egg banks in freshwater zooplankton: evolutionary and ecological archives in the sediment. Hydrobiologia, 491, 65-84.
Horne, D.J. & Martens, K. (1998). An assessment of the importance of resting eggs for the evolutionary success of Mesozoic ostracods. Geological Society Special Publications, 142, 105-120.