Opae ula
A small red shrimp from Hawaiian coastal pools that lives in brackish water, feeds on the algal film growing on rock, and survives for decades in closed systems with almost no maintenance. It is the animal sold in sealed ecosphere jars, and it is a far more interesting one than that suggests.
Quick profile
- Scientific name
- Halocaridina rubra
- Adult size
- 1–1.5 cm (0.4–0.6 in)
- Lifespan
- 10–20 years
- Temperature
- 18–28 °C (64–82 °F)
- pH
- 7.8–8.6
- Hardness
- 15–30 °dGH
- Minimum tank
- 10 L (3 US gal)
- Minimum length
- 20 cm
- Swimming zone
- All levels
- Social
- Colony — at least 10
- Diet
- Algae grazer
- Overall confidence
- moderate confidence
What keeping this species involves
It needs brackish water, light and patience, and it does not need a filter, a heater or regular feeding. The difficulty is resisting the urge to intervene.
Anchialine pools: coastal pools with no surface connection to the sea, fed by brackish groundwater, with almost no current and very little food.
Identity and names
- Common name
- Opae ula
- Scientific name
- Halocaridina rubra
- Family
- Atyidae
- Order
- Decapoda
- Class
- Malacostraca
- Also sold as
- Halocaridina rubra, Hawaiian red shrimp, volcano shrimp, anchialine shrimp
Adult size and growth
1–1.5 cm (0.4–0.6 in)
Typical lifespan in an aquarium: 10–20 years. Almost every animal sold in a shop is a juvenile, so the tank has to suit the size in the first line rather than the size in the bag.
Shape, which is what the compatibility engine reads instead of length alone: elongate with a grazing or sucking mouth. Assigned from what is typical of Atyidae, not examined for this species.
Water and tank requirements
A colony of centimetre-long shrimp in a stable brackish volume. A small container is genuinely appropriate here, which is true of almost nothing else in this catalogue, because the animal comes from a small still pool and needs no filtration.
Temperature (°C)
Scale: 0 to 36 °C.
pH (pH)
Scale: 4 to 9.5 pH.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
Why these ranges: Anchialine pool water is brackish, alkaline and remarkably stable, at roughly a third to a half of seawater salinity. Salinity is the parameter that matters and is not captured by hardness alone; the figures here describe the mineral content of correctly mixed water rather than a freshwater band.
Where these figures come from
Habitat class
Brackish and estuarine — Hawaiian anchialine pools, fed by brackish groundwater
Water where fresh meets salt — estuaries, mangrove, tidal creeks and coastal lagoons — with salinity that changes with the tide.
Salinity is a requirement, not a tolerance, for the species genuinely from here — and the commonest way these animals are killed is being sold and kept as freshwater fish.
- Conductivity
- 2000–35000 µS/cm
- Salinity
- 1–20 ppt
- Flow
- variable
- Dissolved oxygen
- moderate
- Seasonality
- moderate
These are figures for the KIND of water body, not measurements of this species. They are AquariumHQ’s summary of the general limnology and are given wide because the water is.
- TemperatureReasoned on this record
- This record states its own reason for this band. See the water section.
- pHReasoned on this record
- This record states its own reason for this band. See the water section.
- General hardnessReasoned on this record
- This record states its own reason for this band. See the water section.
| Requirement | Value | Notes |
|---|---|---|
| Temperature | 18–28 °C (64–82 °F) | Stability matters more than hitting an exact figure. |
| pH | 7.8–8.6 | A stable pH outside the ideal band is usually better than an unstable one inside it. |
| General hardness | 15–30 °dGH | Check your supplier's published figure before choosing species. |
| Minimum volume | 10 L (3 US gal) | A floor, not a recommendation. |
| Minimum footprint | 20 cm long × 15 cm deep | A colony of centimetre-long shrimp in a stable brackish volume. A small container is genuinely appropriate here, which is true of almost nothing else in this catalogue, because the animal comes from a small still pool and needs no filtration. |
| Water flow | still | Match the outflow to this rather than to the filter's rating. |
| Lid required | Recommended | Reduces evaporation and prevents accidents. |
| Structure | Porous rock carrying algal film, brackish water and light. No filter, no heater and no substrate are needed, and a filter actively removes the food. |
Behaviour
- Grazes rock surfaces continuously and can go for long periods with no added food at all.
- Colours from pale pink to deep red with condition and with what it is grazing, so a colony is not uniform.
Telling males from females
Females are slightly larger and carry visible eggs. There is no reliable colour difference.
Feeding
Grazes the film growing on rock in a lit container and needs very little else. Overfeeding is the commonest cause of a failed setup, because a sealed system has no way to process the surplus.
- Diet type
- Algae grazer
- Staples
- Algal film on rock, Occasional powdered algae
Breeding
- Strategy
- Eggs carried beneath the female until fully formed young are released
- Difficulty
- straightforward
- Parental care
- carries eggs
Reproduces slowly, with small broods at long intervals, which matches the very slow life history of an animal that lives twenty years.
Tankmates and compatibility
Assessments are derived from stored traits by the compatibility engine, with the reasoning shown. A verdict is a statement about probability, not a guarantee — individual temperament can override it.
| Towards | Risk | What it means |
|---|---|---|
| Small fish | none | Anything that fits in the mouth is at risk once this species reaches adult size. |
| Adult dwarf shrimp | none | Cover reduces losses but rarely prevents them where the risk is high. |
| Shrimplets | none | Determines whether a shrimp colony can grow rather than merely persist. |
| Snails | none | Applies to snails you wanted as well as snails you did not. |
| Long or trailing fins | none | Fin damage is an entry point for secondary infection. |
| Plants | grazes-algae | Tough species such as Anubias and Java fern survive most grazers. |
Any pair in the catalogue can be assessed with the compatibility checker.
Welfare considerations
Points where the way this species is commonly sold or kept differs from what it actually needs.
- Anchialine pools are a rare and threatened habitat, damaged by coastal development, groundwater extraction and introduced fish. Captive-bred stock is the responsible source.
- The sealed-jar trade frequently uses containers too small to hold a stable colony and sells them as maintenance-free. A larger open container with light and rock is a better home and costs no more effort.
Problems to watch for
Most problems in a tank containing this species are general rather than species-specific. Start with the troubleshooting index and work from the observation rather than from a guess at the cause.
What AquariumHQ does not yet hold
Species-level disease susceptibility is modelled in the graph and is not yet populated. Health pages will not be published until each condition has veterinary sourcing, because health content written without evidence is the most harmful thing this product could publish. The gap is recorded in the coverage report.
Where it comes from
- Native range
- Hawaii — anchialine pools on the islands of Hawaii, Maui and Oahu
- Habitat
- Anchialine pools: coastal pools with no surface connection to the sea, fed by brackish groundwater, with almost no current and very little food.
What the checklists say
3 distribution statements retrieved from GBIF checklists. Establishment means is reproduced from the source where it states one, and left unstated where it does not — a record of presence is not a record of origin.
| Status | Places | What this means |
|---|---|---|
| Introduced | 1 | A checklist states the species arrived here with human help. That is not a statement that it is established, harmful or illegal. |
| Recorded as present, establishment means not stated | 2 | The source records the species as present and does not say whether it is native or introduced. AquariumHQ does not fill that in. |
No checklist AquariumHQ holds positively calls this species native anywhere, so the native range above rests on AquariumHQ’s own reading of the literature rather than on a cited checklist.
Evidence
Every stored value with the kind of support behind it and where to check it. Evidence type and confidence are separate axes: an editorial position held confidently and a measurement held tentatively are different things.
| Claim | Value | Evidence type | Confidence | Source |
|---|---|---|---|---|
| scientific name | Halocaridina rubra | Observed / measured | high confidence | Catalogue of Life; GBIF — Global Biodiversity Information Facility |
| native range | Hawaii — anchialine pools on the islands of Hawaii, Maui and Oahu | Observed / measured | moderate confidence | GBIF — Global Biodiversity Information Facility |
| adult size | 1–1.5 cm | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| lifespan | 10–20 years | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| temperature | 18–28 °C | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| ph | 7.8–8.6 pH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| general hardness | 15–30 °dGH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank volume | ≥ 10 L | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank length | ≥ 20 cm | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum group size | ≥ 10 | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
Last reviewed 2026-08-10. Values labelled as an aquarium-literature consensus are AquariumHQ editorial positions rather than measurements — see how evidence is handled.
Why we say this
What each figure on this page rests on, and how far that is from being independently established.
Every figure on this page has a provenance. Open any claim to see what AquariumHQ holds for it, which sources say what, and what a claim of that kind actually needs. Different claims need different evidence: an accepted scientific name is settled by one authority, while a legal restriction needs a current primary instrument. A single source is often the right answer rather than a thin one.
scientific nameAquariumHQ's own position, externally checkableBased on a verified authoritative source.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: accepted scientific name and classification — 1 authoritative source in an appropriate context. Checked against gbif on 2026-08-29. One authoritative backbone is what this class requires.
What would improve this: Go and read a source. This is reachable and has not been reached.
native rangeAquariumHQ's own position, externally checkableBased on a verified authoritative source.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: where the species occurs — 1 authoritative source in an appropriate context. 3 distribution statements held from GBIF checklists — introduced in 1 country. Establishment means is reproduced from the source, never inferred from occurrence.
What would improve this: Go and read a source. This is reachable and has not been reached.
adult sizeAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: size a well-kept specimen reaches — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
lifespanAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
temperatureAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
phAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
general hardnessAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
minimum tank volumeAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: minimum tank size — no external source is required, provided AquariumHQ shows its reasoning. No authority publishes this. AquariumHQ derives it, and the derivation is recorded with its inputs and assumptions.
What would improve this: Keep the rationale current. There is no source to find.
minimum tank lengthAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: minimum tank size — no external source is required, provided AquariumHQ shows its reasoning. No authority publishes this. AquariumHQ derives it, and the derivation is recorded with its inputs and assumptions.
What would improve this: Keep the rationale current. There is no source to find.
minimum group sizeAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: group size — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Keep the rationale current. There is no source to find.
Guides relevant to this species
Chosen from this record's own traits rather than listed on every page.
- Schooling and shoalingThis species is recorded as colony, and the number it is kept in decides how it behaves.
- Wild conditions and aquarium recommendationsThe water band on this page is an AquariumHQ position with a stated reason, not a measurement of a river.
Group size and social behaviour
A colony animal with no aggression at all. Numbers matter for breeding and for the colony being visible rather than for social behaviour.