Good Algae Make Good Slugs
To possibly belabor a point, the limiting factor in slug husbandry seems to be food. So more, better food = more, better slugs?
Given how hard most aquarists work to eradicate nuisance algae, including the Bryopsis that Elysia clarki find irresistable, it would seem that growing algae should not be all that horribly challenging, right? One just needs good, strong light, plus some nutrients to support growth. The trick is to grow the species one wants, without having the cultures overtaken by undesirables. In my case, the “good” algae are Penicillus, Bryopsis, and, to a lesser extent, Derbesia, Halimeda, and Avrainvillea. The primary nuisance I am trying to avoid is “red slime” cyanobacteria, which can overtake and smother the other species.
Plants require nutrients such as carbon, nitrogen, potassium, and phosphorous in significant quantities, along with smaller amounts of calcium, magnesium and sulfur . Based on my measurements, the desirable algae consumed considerable amounts of nitrate, but adding phosphate did not appear to encourage growth target algae and increased the growth of cyanobacteria. So, along with calcium (Penicillus and Halimeda use it for structural support) and bicarbonate (a pH buffer and source of carbon) I have been using the dosing system to add potassium nitrate in order to maintain nitrate at about 5-10 ppm. For everthing else, I have been relying on the artificial seawater mix to provide an adequate balance. Perhaps not the most systematic approach to feeding the plants.
It has, however, worked reasonably well. In the photos above, there is a dense growth of Penicillus on the right side of the Broodstock tank, and a mat of Derbesia and Bryopsis on the left. The resident E. clarki seem quite happy to spend their days grazing without exhausting the supply. The other tanks are similarly productive. Nonetheless, it might be possible to increase algae productivity, thereby increasing slug capacity and the frequency of mating and egg laying.
There is, of course, an actual literature on the nutrient requirements of aquatic plants. Some commonly used formulas rely on extraction of nutrients from soil to provide the correct balance of elements. I might have tried that had I been able to buy the correct brand of English planting soil. However, based on a number of studies (Kumar et al., 2011 e.g.) Guillard’s f/2 medium (Guillard, 1975) yields excellent growth of macroalgae and does not require purchasing and soaking British soil.
Florida Aqua Farms supplies Guillard’s f/2 medium under the name “Plant Fuel,” providing pre-measured dry ingredients along with a suggested dosing regimen. Seemed worth a try to find out whether more complete supplementation will translate into faster growth of desirable algae. What’s the worst that can happen, aside from massive bloom of toxic, undesirable algae?
The components arrived, I mixed them up, and the dosing system is adding 1 ml of f/2 per 2.5 liters of system volume, distributed over the course of each week. We’ll see…