Green Marine Algae: Friend or Foe?

Although this one small ball of Bornetella sphaerica green marine algae looks like a cluster of Valonia sp., it is just one singular mass. Thankfully, it is not very common in reef tanks.

When I started in the reefkeeping hobby, Bryopsis algae was one of the worst plagues, especially of all the green marine algae. Practically nothing eats it in large quantities, it takes over everything, and it can grow in just about any aquarium, regardless of nutrient and light levels. Thankfully, there are now some very effective treatments.

Bryopsis looks like long feathery strands of green algae, and short clumps of it can sometimes be confused with green hair algae.

Caulerpa nummularia is no exception. Although it has very unique “mushroom-shaped” cups, I recommend this only for macroalgae aquariums or “natural” reef aquariums.

Since this algae was quite unhealthy, the distinguishing serrated edge is difficult to see. Caulerpa brachypus looks quite similar to Caulerpa prolifera with the addition of small points along the leafy frond edges. Once again, this species of Caulerpa follows the typical behavioral characteristics of its parent genus and is probably best kept out of a typical reef aquarium.

Caulerpa serrulata has blade-shaped fronds and is also known for “going sexual” in an aquarium. Again, the holdfasts can grow deep into rockwork, and manual removal usually ends in breakage.

Caulerpa taxifolia is a fern-like species on the Federal Noxious Weeds list and is nicknamed the “killer alga” due to its invasive and toxic nature. Use caution, as “No person may move a Federal noxious weed into or through the United States, or interstate….” (See the Federal Noxious Weed Act for full regulations.) This “weed” has outcompeted native seagrasses in the Mediterranean and off the coast of California. It is very tolerant of poor conditions and has few predators. Best method of eradication is manual removal. Caulerpa mexicana is very similar to Caulerpa taxifolia but has more tightly-segmented fronds.

Although very similar to Caulerpa toxifolia, this may be Caulerpa ashmeadii or Caulerpa sertulariodes (this was a very unhealthy sample, so a proper identification is difficult). Due to its similarity in appearance and behavior, it is probably not an ideal reef tank macroalgae.

Similar to Caulerpa brachypus, Caulerpa prolifera (in the front right of the photo) has tall, leafy green fronds extending from a wandering root structure. However, Caulerpa prolifera is missing the hallmark characteristic of the serrated edges of Caulerpa brachypus.

Chaetomorpha is one of the most commonly encountered reef tank macroalgae. It grows in long spaghetti-like masses and readily utilizes nitrates and phosphates.

I do not have a great identification on the algae in the photograph, but a species of Cladophora (maybe C. pellucida?) is currently my best guess. This algae is more wiry and less fern-like than Bryopsis, which made it easy to remove from the rockwork.

Green Hair Algae (“GHA”) is just what it sounds like…green algae that looks like a hairball. Hermit crabs, snails, etc. may eat it, as long as it doesn’t get out of control. Cleanup crews usually will not touch massive mats of GHA. I recommend manually removing as much GHA as possible to allow the cleanup crew to effectively remove the remnants. Limiting nutrients through reduced feeding, skimming, running granular ferric oxide (GFO) and activated carbon (AC) may help slow GHA’s growth. Adding a refugium with “good” macroalgae, such as Chaetomorpha, can help outcompete the GHA.

This alga can be found in new, cycling aquariums, although I do occasionally find it growing in my tank whenever I know it’s time to replace my cleanup crew. Typically, it only grows to about two inches (about five centimeters) in length, although I have seen a few pieces grow to over 10 inches (about 25 centimeters). It is a cylindrical type of algae in the Ulva family (very palatable algae), and it is apparently quite delicious to my cleanup crew. Similar to the more leafy type of Ulva, it prefers cooler water (typically 77 degrees Fahrenheit or below) and higher nutrient water.

Although it’s not the most visually appealing algae, it’s a sign of good things to come in a new aquarium (and a sign of some issues in an established tank.) These microalgae films usually appear during the first couple of months of a new tank cycle, and they can cover the glass/ acrylic, rock, and sand. For a new aquarium, this is a critical part of the cycle: this algae feeds and helps establish a pod population! If it becomes a nuisance, it can be removed manually by scraping it or by utilizing a cleanup crew. It can also be reduced by limiting the nutrients in the aquarium.

As a calcareous type of marine algae, Halimeda grows slowly and does not become invasive. It requires a stable supply of calcium and seems to prefer lower nutrient levels. This algae can make a beautiful backdrop for macroalgae tanks, decorative refugiums, or even “natural-looking” reef tanks as it is not a target for most clean-up crews.

Not to be confused with Brown Clove Polyps, this green marine algae grows green flower-like fronds supported by a calcified stem.  They do not appear to proliferate much in home aquaria; in fact, these only appeared for about a month on a shell, then vanished.  They are typically found in shallow, warm water (above 78 degrees Fahrenheit). The most crazy part about this alga is that it is unicellular; the nucleus is found at the base of the stalk.

Ulva grows in large, floating sheets, and in the wild it tends to grow nearer to the shore. Its natural habitat suggests it prefers higher (but gentle) flow, high light, and high nutrients. This makes it a difficult algae to cultivate in a reef aquarium as most aquarists place macroalgae in low flow, low light areas with competitive algae (such as Caulerpa and Chaetomopha.) Additionally, this algae is a great food source for…well…just about everything. Herbivorous fish love it, as do snails, pods, and even bristleworms up to a point. The algae shown was in a 10g aquarium with about 65gph turnover rate and a 150W metal halide lamp over it.

Update [6/6/2023]: As one of the readers shared in the comments below, if/when Ulva “goes sexual,” it turns into a nuisance, with growths nearly everywhere. Interestingly, according to various posts, nothing seems to eat it (or eat it fast enough) at this point. While I haven’t dealt with this personally, I would assume Fluconazole would take care of it.

Although green bubble algae won’t choke out a reef quite like Caulerpa, Valonia macrophysa is extremely invasive. Very few reef-safe inhabitants eat it, but some emerald crabs will (as always, just because some do, doesn’t mean all will.) The best prevention for this algae is a good quarantine system and manual removal outside the aquarium. There are some products available now that have varying degrees of success with eradicating it (although they come with their own risks of use).

Here is another example of green bubble algae, possibly Valonia aegagropila (although it is missing the characteristic branching segments, but this colony may be to small for branching yet). Fortunately for my aquarium, there wasn’t enough present for a good identification.

This calcium-based algae is always a treat for me to see.  It is not often found in home aquaria, and when it does appear, it seems to require a narrow set of parameters to thrive (I’ve only seen one case where these started to reach nuisance levels).

And then, there are the unidentified green marine algae, like this one shown in the foreground (note the Halimeda and Padina, along with some other species). With over 1500 known species of green marine algae, there’s no way this blog can cover them all. What neat species have you found? Post up in the comments!