Sea Lettuce (Ulva lactuca): The Most Underappreciated Macroalgae?

The use of macroalgae in marine aquaria has steadily increased over the years. Ever since they have become more widespread in the aquarium hobby, they are increasingly being cultivated for specialized purposes. This specialization has generated the demand for a more diverse selection of “macros” with each species being preferred for a particular application.

Some of these species are indeed well-suited for their special role. Even so, others are highly desirable as all-purpose seaweeds. These types of macroalgae are most appealing for aquarists who have smaller refugia or desire simplicity. They should additionally be hardy, easy to grow and simple to maintain. The green algae Ulva lactuca most certainly fits this bill.

Ulva and General utility

Aside from being attractive and satisfying to keep in their own right, seaweeds can additionally benefit the aquarium system environmentally:

• They improve water quality and compete with nuisance algae by sequestering (or, as some say, “absorbing”) excess nutrients.
• They provide a biogenic substrate upon which various organisms (e.g. copepods) can hide, rest and reproduce.
• They provide a nutritious live food source for various herbivorous fish and invertebrates.

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Ulva can be effectively used to accomplish all of the above! Let’s start with its ability to take up nutrients.

Sea lettuce is very commonly encountered in shallow, rocky, nutrient-rich coastal marine habitats. It is physically delicate and contains no chemicals to deter herbivory; therefore, it is heavily grazed upon by fish, snails, sea urchins, etc. Despite this pressure, it often dominates all other macroalgae in areas. It can even form dense, extensive meadows. This is due to its high growth rate. It owes its high productivity to its ability to very efficiently utilize the bright light and fertile waters of its native habitat.

As marine aquarists know all too well, providing dissolved nutrients such as nitrate and phosphate is rarely a problem. Usually, ugly benthic algae are evident to some degree and are indicative of excess nutrient concentrations. These algae might be found on the glass, the rock, the sand or (in the worst case) on the skeletons of stony corals. When kept in a brightly illuminated refugium, sea lettuce rapidly assimilates nutrients, thereby putting the squeeze on undesirable algal forms. And, who wouldn’t prefer a lush, bright green patch of sea lettuce to a slimy turf of hair algae?

Particularly when cultivated in a refugium, sea lettuce creates a welcoming environment for a host of beneficial organisms. It provides an especially hospitable refuge for tiny crustaceans such as copepods and amphipods. When growing densely, the thin, flat, fan-like architecture of this plant provides a huge amount of habitable surface area and a variety of microhabitats for pods to make a home in. Here in a refugium—where environmental conditions are ideal and there are few predators—pod populations can remain large and stable. Pods love to eat many forms of “bad” benthic microalgae; therefore, the presence a sea lettuce-based pod oasis can indirectly contribute to algae control.

As the patch of sea lettuce grows, it stores nutrients. These nutrients are truly exported from the system only when plant biomass is harvested and physically removed some the system. Harvested material can be thrown in the trash and forgotten. However, the best use of freshly harvested sea lettuce is as an aquarium food. This kind of recycling reduces the amount of material you will have to introduce into the system as “new” food. Because it is so palatable, sea lettuce is eagerly accepted by a wide range of aquarium herbivores from tangs to emerald crabs. Why buy expensive prepared foods for these creatures when a natural, wholesome, live food source can be farmed right there on the spot?

Growing your own Lettuce

Ulva lactuca is quite hardy. Since it is adapted to the unstable and sometimes harsh conditions of the intertidal zone, it can tolerate wide swings of salinity and temperature. So long as it receives intense light, it requires very little care.

Because of its soft, delicate form, it is best used in open refugia or planted tanks (as opposed to reactors) with moderate water flow. With just the right sort of water circulation, the mass can be made to tumble; this increases the maximum amount of plant material that can be adequately exposed to light. While happy to float at surface, it can attach to hard substrates (e.g. rubble rock). Small chunks of rock came be continuously exchanged between the refugium and main tank to ease seeding/harvesting/feeding out of plant biomass.

How much sea lettuce should one use? Actually, the size of your standing crop should always be in flux. This is because (1) the patch will always be growing and (2) you will be periodically harvesting portions of the crop. The optimal amount of plant material to use in a particular system depends upon many factors including bioload (i.e. consumer waste output). Just remember that the bigger its growing space is, the bigger a patch of sea lettuce can grow. And, a bigger patch means a higher capacity for nutrient management. Net growth ceases after plant material completely fills the growing space. Thus, the patch size ideally is maintained near, but not at, the maximum capacity as dictated by growing space. In this way, the patch size is as large as possible yet is always growing (i.e. sequestering nutrients).

Being as this seaweed provides such a good home for all kinds of small creatures, it is worthwhile to obtain starting stock that is free of any pests. That is why the best choice is Clean Sea Lettuce (Ulva lactuca) from AlgaeBarn’s innovative Cleanmacro™ Series. In selecting Clean Sea Lettuce, aquarists greatly reduce the risk of inadvertently introducing unwanted hitchhikers (Aiptasia anemones, fish parasites, all sorts of “bad” worms, etc.) into their systems.

Clean Sea Lettuce is one of the most easy-to-keep macroalgae. Given its many great attributes, it’s a bit surprising that Ulva is not yet in widespread use amongst reef aquarists. Perhaps its days of glory are yet to come!

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