Update on Scuds (Shrimp, not Missiles!)

Photo by Matt Hill of EcoAnalysts, Inc., our go-to expert on invertebrate identification, of one of our male scuds.

One of the livefoods for fish we raise in our hatchery is scuds, also known as Gammarus. We recently sort of got our scuds identified. See this blog for details: Update on Gammarus Identification.

First, in this blog, I’ll provide general information about scuds, then I’ll discuss how we raise them and suggest ways hobbyists could successfully raise them.

General Information:

Scuds are shrimp-like crustaceans or amphipods. Scud species live worldwide in a wide range of aquatic environments, including freshwater, brackish environments, and full marine conditions. The suborder which scuds belong to has a huge list of families. Here’s a link to the listing of families in the suborder Gammaridea:  https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=93295#null. Each of those families have dozens of genera which in turn each have dozens of species. Gammaridea are very successful organisms. Our scuds, which we used to call “Gammarus sp.” (meaning unknown species in the genus Gammarus), are now identified as “Hyalella sp. (meaning unknown species in the genus Hyalella). “According to ITIS, there are six species in the genus Hyalella, although if you read the blog above, you know that one of those six species, Hyalella Azteca, actually consists of dozens of undescribed species of scuds.

Scuds all have a laterally compressed body, and most are about 1 cm (0.4 in.) in length. Scuds have a cephalothorax (head/thorax), seven thoracic segments (each with a pair of legs), a six-segmented abdomen, and a tail. The front three pairs of legs are used for swimming and the rear pairs are used for walking along substrates. They have two eyes and two pairs of antennae.

Our scuds are nocturnal and usually hide away during daylight. While our scuds will swim when disturbed, they usually crawl along rocks, sticks, and stems (substrates) and remain close to the bottom in cover to be safe from predators. Lots of things like to eat scuds.

While able to tolerate temperatures as low as 0°C (32°F) and high as 35°C (95°F), our scuds prefer tropical aquarium temperatures ranging from 20-30°C (68-86°F) for reproduction. pH and water hardness are not important.

Scuds breed year-round, but most reproduction occurs during spring, summer, and fall when water temperatures are relatively high. A mating male carries the female on his back prior to copulation and they feed and swim together sometimes for a week or so until the female molts. When the female sheds her exoskeleton, the male releases the female and they mate. The female has a brood pouch between her legs where she holds the fertilized eggs, which hatch after one to two weeks. The young remain in the pouch until their mother molts once again in a week or two. Once on their own, the young scuds grow rapidly. Under good conditions they can begin mating in less than a month.

Scuds are detrivores (scavengers) that graze on decaying plants, algae, fungus, animals, etc. There is little they don’t eat. Using their anterior (front) legs, they continuously sweep up water and debris and extract anything edible. Our scuds will feed directly on both plant and animal materials.

We originally collected scuds in the late 1990s from Coleto Creek in south Texas near where Goliad Farms™ is now. At the time, our hatchery was in Santa Fe, NM. We reared those original scuds in 55-gallon vats inside our greenhouse. They thrived and provided an excellent food for the rainbowfishes we were raising at that time. When we moved to Texas in the early part of this century (2000-2001), we brought along those scuds. Thinking the warmer Texas climate would be conducive to outdoor rearing, we put the scuds into 300-gallon vats outside the greenhouses. We hadn’t counted on the myriad species of dragonflies and the voracious appetites of their nymphs. When we went to harvest scuds, we got a bumper crop of dragonfly nymphs, a great food for large cichlids, but deadly to small fish and, apparently to scuds. Unfortunately, no scuds survived the dragonfly onslaught. We live six miles from Coleto Creek, but I never got around to collecting them again.

Fortuitously, in December 2008 I visited Dr. Caitlin Gabor at the Texas State University campus on the beautiful San Marcos River in central Texas. Dr. Gabor studies Poecilia formosa, an all-female livebearing fish species commonly known as the Amazon molly. While giving me Amazon mollies, she offered me some scuds, which she called Gammarus, from her molly rearing pools. Those scuds were indistinguishable, at least I couldn’t distinguish them, from those I’d collected at Coleto Creek a decade ago.

At the hatchery, I dumped Dr. Gabor’s scuds Gammarus into a 55-gallon vat, added some hornwort (Ceratophyllum demersum), a nice, rootless, floating plant, and forgot about them. A few weeks later, scuds began showing up in vats throughout the greenhouse. We use a recirculating system and all the vats share a common recirculating water source. Scud larvae (very tiny) had found their way through pumps and pipes to colonize many other vats. From my perspective this wasn’t a problem. Adult livebearers eagerly eat scuds of all sizes, while fry and juveniles eat the smaller ones. Low populations of scuds persisted in the vats by finding refuge in the floating hornwort and the sinking guppy grass (Najas guadalupensis) we have in the vats. While this provided a continuous source of livefood to the fish, I decided once again to actively culture scuds in larger numbers.

How We Raise Scuds:

Scuds are drop-dead easy for us to culture. We raise them in 55-gallon vats on our recirculating systems, and they thrive in the high-quality water provided by our plant filters (for a description of our systems, see this blog Plant Filtration). We don’t aerate the scud vats, instead we allow a steady drip of water to provide oxygenation. We don’t use as high a water flow as we do with our fish because scuds don’t require as much oxygen as fish, so the continuous water drip provides adequate aeration. Scuds eat most anything, but we have an abundance of hornwort which they seem to relish. We keep hornwort in most of our 55-gallon fish rearing vats. It grows very fast and we harvest 20-30 pounds of hornwort daily to keep it from overgrowing the vats and interfering with feeding. Most of the surplus hornwort goes into our litter worm beds (for information about our litter worms see blogs: Lumbricus rubellus – Litter Worm [by the way, our worm was originally misidentified, see the next blog for an update] and Litter Worm Update), but some is added to the scud vats as food for them. The scuds graze on the microscopic organisms growing on the hornwort and, when crowded and hungry, directly eat the hornwort leaving only stems.

Collecting scuds to feed to our fish is simple; we sweep through the hornwort with a 10-inch aquarium net to yield hundreds of them. Feeding them to fish is also simple, just shake some from the net into the vat or dump them in a bucket of water and pour that into the vat. Any scuds not immediately consumed survive our systems’ conditions indefinitely so they can be fed in abundance without fear of water fouling.

Hobbyist Culturing:

 Scud culture by hobbyists can be almost as easy as what we do. Any container that can hold water can be used. I’ve found plastic garbage cans are inexpensive rearing tanks. The hobbyist could take a clean plastic trash can (rinsed a few times to remove manufacturing chemicals such as formaldehyde) and then follow these simple instructions:

• Fill the container with aged aquarium water.
• Place about 5 cm (2 in.) of dampened dried leaves into the container. Most tree leaves are good, but you should avoid oak leaves since I’ve found them to be too acidic. Dried mulberry leaves are excellent. Horse quality hay is great, but make sure it isn’t insecticide laden. If you have a source of excess hornwort, by all means put some of it in the container.
• Aerate the water. Large bubbles are better than very fine bubbles, which can get under their carapaces and make them float and eventually die.
• Keep the water near or above room temperature. Scuds can survive winters outside in most of North America, but reproduce best at 20-30°C (68-86°F), so try to maintain that temperature range.
• Add a starter culture of scuds; a few dozen will be enough.
• Scuds will feed on the rotting leaves and microorganisms take grow on any surface. Periodically add more leaves. The hobbyist could feed the scuds hornwort like we do.
• Since scuds need surface area to feed on, provide adequate surface area by placing rolled up plastic screening in the culture container. In Santa Fe I used plastic-coated water-cooling pads, but it seems that today’s manufacturers add anti-mildew chemicals that aren’t good for scuds; so, I don’t recommend their use anymore. Another good choice is aquaculture netting like that we use to make breeding cages for our fish.
• There should be enough scuds to begin harvesting scuds in four to six weeks. Harvest by netting them with a fish net or by picking up the plastic screening and shaking over a bucket.
• Keep an eye on the number that can be harvested daily or weekly without reducing the scud population. That is the amount you can harvest safely each period.
• Periodic, partial water changes are necessary. Anytime the water becomes grayish-cloudy is an indication of a past-due water change.
• Cultures are long-lasting and sub-culturing is necessary only if production declines, which if you maintain your culture properly is never. Nevertheless, it is wise to maintain a replicate culture in case of a disaster. We always keep multiple cultures; you never know when a pesky dragonfly will lay eggs and her rapacious offspring will decimate or eliminate your scuds.

Raising scuds is an easy, almost labor free way to get your fish to reward you with more vibrant colors, longer lives, and more and healthier fry.

We offer scuds to hobbyists and others. The others including seahorse and newt breeders, aquaponics and hydroponics operations, and fishpond owners. Apparently, both seahorses and newts (actually, the aquatic larval newts) love scuds. Aquaponics and hydroponics growers both use scuds to keep their sumps clean. The scuds breakdown and process solid waste into micro-particles and plant nutrients and prevent clogging of water pumps. It’s important that such growers harvest scuds to keep their population under control otherwise they will begin to eat plant roots. For aquaponics growers, the scuds make excellent fish food. fishpond owners stock scuds to provide a continuously reproducing food source for their fish.

A word of warning: since scuds will eat plants if they don’t have adequate amounts of other foods, I’d be reluctant to place them in a prize-winning planted tank unless there are enough fish in the tank to control the scud population.

Good fishkeeping!