Coldwater Marine and Tidepool Aquaria: Cooling, Oxygen, Species and Collection Rules
How temperate tidepool aquaria differ from tropical marine tanks: cooling, oxygen, light and algae, which intertidal animals cope with captivity, and why shore collection is regulated.
What a tidepool aquarium models
A tide pool is a shallow pool of seawater left on a rocky intertidal shore, usually a few inches to a few feet deep. Shores are divided into high, mid and low tide zones. Organisms in the high zone face wave action, sun and air exposure; the low zone is uncovered only at unusually low tides and holds the richest plant and animal life, whose members are less adapted to drying and temperature extremes. Inside a pool, temperature, salinity and oxygen all change over the tidal cycle.
Research on rock-pool fishes describes how extreme these swings can be. During daytime low tides, strong sunlight can push pool temperature sharply upward while algal photosynthesis drives the water into oxygen supersaturation (hyperoxia); during night-time low tides, isolated pools can become severely hypoxic. A coldwater marine aquarium does not try to copy these extremes. It aims for the stable, cool, well-oxygenated conditions that animals experience when the tide is in.
Cooling: why a chiller is usually needed
Temperate intertidal animals are not built for room temperature. An Alaska Sea Grant manual on classroom aquaria notes that many intertidal animals can withstand temperatures up to 65 °F (about 18 °C), but most prefer somewhat cooler water, and lists "never let the temperature exceed 65 °F" among its basic maintenance rules. The same manual states that a cooling system is desirable for subtidal fishes, especially northern species. A classroom guide from the Marine Biological Laboratory (Woods Hole) aims for about 65 °F for North Atlantic shore animals that experience roughly 40–70 °F (about 4–21 °C) over a year.
In practice this means a refrigeration unit (an aquarium chiller) for most indoor rooms, sized for the tank volume and the warmest room temperature expected. Both teaching guides also recommend placing the tank in a cool spot, out of direct sunlight and away from heaters, and the Sea Grant manual advises installing any refrigeration unit before the tank is filled.
Oxygen and surface area
Cold water holds more dissolved oxygen than warm water (USGS), which is one reason cool tanks suit these animals; the MBL guide makes the same point and adds that the biological filter bacteria depend on oxygen too. Tank shape matters: the Sea Grant manual notes that a tall, narrow tank absorbs less oxygen because of its small water-to-air surface, while a broad, shallow tank oxygenates well. It treats aeration as essential, warning that without an air pump driving the filter the water rapidly loses its oxygen supply.
Light and algae growth
Some light is needed to keep algae growing, and algae on non-viewing surfaces is generally beneficial food and cover for grazers such as urchins, chitons and snails. The Sea Grant manual recommends controlling the amount of light to limit algal production and removing algae only from viewing panes with a clean sponge, since scrapers mar glass and acrylic scratches easily. Light also matters for some anemones: the aggregating anemone Anthopleura elegantissima hosts photosynthetic symbionts (dinoflagellate zooxanthellae and green zoochlorellae), and individuals living in caves or under docks are pale because they lack them. In one laboratory study these anemones were held in running seawater at about 10.5 °C under screening that let through 45 % of ambient sunlight.
Temperate species
- Sea anemones: the Sea Grant manual notes they have lived in tanks for many decades. Anthopleura elegantissima clones by binary fission and uses specialised tentacles (acrorhagi) to fight neighbouring, genetically different clones, so spacing matters.
- Tidepool sculpin (Oligocottus maculosus): up to about 8.9 cm (FishBase), North Pacific from the Bering Sea to southern California, tolerant of brackish water and warmer than other sculpins. It eats isopods, amphipods, snails, polychaetes and barnacles, can breathe air out of water and returns to its home pool.
- Hermit crabs and other scavengers are listed as hardy starters; the manual suggests conditioning a new tank with only a few hardy animals for the first two to three weeks.
- Sea stars: many Pacific species, including the ochre star Pisaster ochraceus, have been devastated by sea star wasting disease since 2013, and elevated water temperature is correlated with higher incidence and severity. Several public aquaria lost large parts of their collections in 2014.
- Kelps and other large algae are often unsuccessful in small closed aquaria, and very active animals (schooling fish, larger crabs, shrimp) are better left until experience is gained.
Specific gravity of about 1.025, with 1.023–1.027 tolerated by almost all animals, and a pH of about 7.5–8.3 are the Sea Grant manual's working targets; intertidal animals tolerate salinity swings slightly better than deeper-water species. Remove uneaten food after every feeding and take out dead or sick animals promptly.
Collection is regulated
Taking animals from the shore is a form of fishing and is controlled by law. In California, Title 14 of the California Code of Regulations, section 29.05, bans taking invertebrates in tidepools and the zone from the high tide mark to 1,000 feet seaward of the low tide mark except for a short list of species (such as limpets, turban snails, mussels, crabs, shrimp, sea urchins and worms). Sea stars, anemones, chitons and barnacles are not on that list, a fishing licence is required from age 16, collection must be by hand, and take of all invertebrates is prohibited in state marine reserves. In England, Inshore Fisheries and Conservation Authorities make local byelaws under section 155 of the Marine and Coastal Access Act 2009 to manage sea fisheries and protect the marine environment.
Related marine guides cover the macroalgae refugium, the mangrove refugium and seahorse tanks; tropical reef systems are covered in separate reef guides.