AQUARIUM INVERTEBRATES  by ROB TOONEN, Ph.D.

Green-Lipped Mussels

I have recently seen a number of shops offering blue- or green-lipped mussels (P. viridis) for sale, and thought I would take some time to discuss them this month.  I was particularly motivated to write this article after overhearing a local petshop employee telling a potential customer that they were a hardy addition to any reef aquarium.  “In fact,” he bragged, “we haven’t lost one of these guys in the shop yet!”  Of course the fact that the animal was almost certain to die within months of being introduced to the home aquarium was never mentioned…

Figure 1

Reproduction & Growth:

Sexes are separate and both sperm and eggs are freely spawned into the water column where fertilization takes place externally.  Spawning can occur year round, although in many parts of the world there are pulses of spawning in the early spring and late fall each year.  Fertilized eggs develop into larvae called viligers which feed in the water column for approximately two weeks before they become competent to settle as juveniles (Rajagopal et al. 1998).  The juvenile mussels can grow very quickly, reaching a size of about 2-3cm long in as little as 3 months, at which point they also usually reach sexual maturity (Rajagopal et al. 1998).  Individual growth rates vary by location (depending on a variety of factors), and studies have shown that individuals can reach about 5 cm in their first year in Hong Kong, and as much as 10 cm in India (Lee 1985, Rajagopal et al. 1998).  Overall, the maximum size of these animals is usually somewhere between 8-10 cm long (although individuals as large as 16.5 cm have been reported).  These mussels typically live for about 2-3 years (Lee 1985, Rajagopal et al. 1998), and if the mussels are large when you buy them, they are probably already at least a year or two old, making them already quite old (at least by mussel standards).

Natural Habitat:

There are three species of mussels in the genus Perna: P. viridis, P. perna, and P. canaliculus, and these species differ from all other mussels by the absence of an anterior adductor muscle and the brightly green or blue-tinged periostracum (the hard chitinous outer covering of the shell).  Perna perna is native to Africa and South America, and is unlikely to be confused with either of the other species because of the uniform brown color, whereas both P. viridis and P. canaliculus have a bright blue or green fringe on the edge of the shell (Siddall 1980).  However, P. canaliculus is native to the cold waters of New Zealand, and should not be offered for sale for a tropical reef aquarium.  However, if there is any doubt, Perna viridis can be distinguished from the other species in this genus by the presence of a kidney-shaped posterior adductor muscle (see this National Introduced Marine Pest Information System document for a diagram).  P. viridis is the most likely species to show up in the hobby trade, and these mussels form dense aggregations on a variety of hard surfaces throughout their range (including the bottoms of ships, wharves, pilings, buoys, mariculture equipment and other man-made substrates, which is why they are frequently considered a pest).  Aggregations of P. viridis may reach densities as high as 35,000 individuals per square meter in some locations (Lee 1985)!  Although these mussels have a very broad range of temperatures (7-37.5°C or ~ 45-100°F) and salinities (0-80 ppt) at which they are able to survive for short periods of time, they do best in temperate estuarine habitats (e.g., Lee 1985, Rajagapol 1998, Segnini et al. 1996, Shafee 1976).  The conditions at which they are found most commonly throughout their range are essentially brackish waters (18-33 ppt) and sub-tropical temperatures (11-32°C or ~ 52-89°F).  Although these mussels are frequently sold as a tropical reef species, and are certainly capable of surviving under the conditions at which most coral reefs are found, none of the Perna species are really a tropical reef animal, and are not particularly well-suited for the habitat we are trying to recreate in a coral reef aquarium.       

Feeding & Aquarium Care:

These mussels appear to be generalist suspension feeders that filter tiny particles of phytoplankton, zooplankton and organic detritus from the water that passes over them. However, having said that, I believe that the single biggest problem with keeping these beautiful animals in an aquarium is quite simply starvation.  It is rare that they are provided with sufficient and appropriate food in a reef tank.  The bottom line is that mussels need to be fed in order to survive in captivity.  These animals tend to specialize on tiny planktonic foods that are rare in reef aquaria, and unless provided with sufficient food of the appropriate size, these animals will starve to death in a matter of months.  If you simply provide standard fish food (flake, frozen, etc.) to your aquarium, then it will certainly not be possible to keep any mussels for long.  On the other hand, if you are in the habit of feeding planktonic foods regularly (i.e., at least once daily), then it is possible that a mussel may be able to survive and even thrive in your aquarium.  Although most larger bivalves can and will eat small zooplankton of the size of rotifers or possibly even as large as newly-hatched baby brine shrimp, the vast majority of their diet is still typically composed of smaller particles in the size range of phytoplankton (usually on the order of 2-20 micrometers (μm)).  These mussels will do best with at least daily feedings of tiny planktonic foods, and the most nutritious alternative for feeding these foods is to maintain live cultures at home.  There are a number of great articles on the culture of phytoplankton, rotifers, ciliates and copepods in the archives of the Breeder’s Net column, and Joyce Wilkerson has a great discussion of how to culture rotifers in her 2001 Clownfishes book.  Furthermore, if your mussels are small, they will not be able to eat foods as large as newly hatched brine shrimp, and these tiny (2-20 μm) particles are even more critical to allow the animals to survive in an aquarium.  If you are to have any chance of keeping these animals alive in your aquarium, you will need to plan on feeding some sort of tiny plankton supplement to your animals on a regular (at least daily, if not several times per day) basis, or they are doomed to slowly starve to death in the aquarium.

In the past, it has been difficult enough for the average aquarist at home to provide a continuous supply of rotifer-sized particles to an aquarium, let alone smaller particles such as invertebrate larvae or phytoplankton.  However, today there is a rapidly expanding selection of planktonic food products available in the pet market.  Traditionally, commercial invertebrate foods were based on a mixture of pea flour and yeast, which, although appropriately sized were rarely successful at maintaining obligate suspension feeders in aquaria for extended periods of time.  Sadly, despite years of trying, few aquarists have had much success with suspension-feeding marine invertebrates kept with these traditional invertebrate foods as the sole source of nutrition.  Although these traditional pea flour and yeast-based products were easy to obtain and feed to an aquarium, they have proven to be rejected by a variety of suspension-feeding invertebrates, and if the animals do not actually eat the product, then it is not really food for them.  These pea flour and yeast-based products have proven to be a poor substitute for the type of particles that suspension-feeders (like mussels) filter from natural seawater.  Fortunately for us as hobbyists, however, a wide variety of alternatives to such traditional invertebrate foods are now becoming available to hobbyists.      

I want to take a slight aside here to explain what I mean by the animals reject the food, because I think it is important.  Most aquarists believe that if a suspension-feeding marine invertebrate reacted to a given food, then they must be eating it.  We now know that even if an animal appears to be feeding, the active filtering of particles from the water by animals such as mussels or clams does not necessarily mean that they are eating to the food.  In many filter-feeding marine invertebrates, the action of feeding is done by the gills, and therefore filtering and respiration (breathing) are intimately linked: an animal cannot really isolate one activity from the other.  Furthermore, many suspension feeding marine invertebrates filter all particles of the appropriate size out of the water column, but then discard the particles they do not find tasty as “pseudofeces.”  Pseudofeces is a fancy word that means quite simply “fake poop” - the animals filter the particles and to the naked eye, they appear to eat it, but for some reason (typically the taste, surface texture or particle organic composition) they decide not to actually ingest the filtered particles.  Instead, the animals ball up the filtered particles before they are eaten, wrap them in mucus and spit them out as fake poop.  These fake poop particles are very hard to distinguish from the real thing, and unless you are able to observe the animals under a microscope, it would be very hard to tell whether your animals are really eating the food you are providing or not.  

I wanted to explain this because I often hear from people who feed such products that “my animals eat the stuff, and therefore it must be good for them.”  I do not agree with this argument at all.  Aside from the fact that it is difficult to determine whether an animal is actually eating the product or simply rejecting it as pseudofeces, it is impossible to determine whether or not a food product is “good for them” without some idea of the relative growth rate of animals fed on various diets.  For example, a number of researchers have now used tiny polystyrene (plastic) beads in feeding experiments with bivalve molluscs (e.g., Beninger and Veniot 1999; Hawkins et al. 1998; Shimeta and Koehl 1997; Tamburri and Zimmer-Faust 1996; Ward et al. 1998) and shown that they eat these particles quite happily.  It varies by species whether or not the animals appear to be able to distinguish between unflavored plastic beads and real plankton of the same size, but most species of mussels, clams and oysters seem to happily eat the unflavored and completely non-nutritious plastic beads until their guts are full.  Obviously, without the ability to digest and gain nutrition from such particles, the animals can feed until no more plastic beads will fit in, and still starve to death on this worthless food.  Without some controlled comparison of how the animals do in aquaria with and without the food item, I would say that seeing a suspension-feeding animal eating something does not necessarily tell us anything about whether or not the food is “good for them.”  I have done such feeding experiments with a number of the planktonic food products available on the market and shown that relative growth rates can vary considerably depending on the food and the animal to which it is fed (Toonen et al. 2002).

Similar experiments have not been done for these green mussels, so I cannot say for sure whether or not these animals will ingest any particle of the correct size or whether they are capable of selecting particles based on taste and rejecting distasteful particles.  I can tell you, however, that experiments with the common mussels found along the coast of North America (Mytilus species) suggest that these animals are not very discriminating in terms of swallowing particles provided that they are of the size range (roughly 2 – 20 μm) of phytoplankton (Beninger and St-Jean 1997; Beninger et al. 1999; Duggins and Eckman 1997).  In experimental feeding trials, Mytilus mussels appear to ingest unflavored beads or particles that are chemically defended at about the same rate as they eat the tiny phytoplankton that they filter from natural seawater, despite the fact that eating such particles can significantly decrease their growth rates.  When we tried to use a pea flour and yeast based invertebrate food for a student feeding demonstration in our invertebrate zoology class at UC Davis, the students quickly realized that the mussels were taking the stuff into their shell, but were later disposing of it as pseudofeces rather than actually eating it.  Given that these same animals would happily eat plastic beads, it would seem that the pea flour and yeast-based foods must be outside the correct size range for these animals to eat, or the product must taste even worse than plastic in order for the animals to reject it...