From the ferocious great white shark (Carcharodon carcharias) to the graceful white-spotted eagle ray (Aetobatus narinari), Elasmobranchii are a diverse group of boneless fishes that are circumglobal, inhabiting a diverse array of habitats, temperature ranges, salinities, and niches in the world’s oceans and rivers.
It is no wonder that these unique creatures, while usually boasting relatively bland coloration compared to the typical teleostei reef fishes, pique the interest of pretty much every hobbyist. This group of fishes definitely has its challenges but, with proper information and species selection, can be kept fairly easily by a moderately skilled aquarist with a generous budget.
Food for thought: nutritional definitions
Nutrition, what does it mean? Proteins, lipids, carbohydrates, vitamins, minerals . . . it gets confusing. The core of a great diet is mimicking what the animal eats in the wild, but it does not end there! Elasmobranchs suffer from some nutritional disorders if care is not taken when determining their diet. In this article, I take a better look at nutritional needs and diet choices in elasmobranchs.
To understand what nutrition means, we first must define the nutrients that encompass the natural diet of elasmobranchs.
Made up of chains of amino acids, proteins are building blocks of tissue and are a fuel source for metabolic processes. Some amino acids are essential; in other words, they cannot be manufactured by the animal, making certain proteins very important.
Lipids are fats; sterols; fat-soluble vitamins; phospholipids; mono-, di-, and triglycerides; and waxes that store energy, build cell walls, and are vital for cellular communication. These can be made of fatty acids, some of which are essential.
Carbohydrates are relatively few and far between in an elasmobranch’s diet. These complex sugars are usually found in vegetable matter.
There are two general types of vitamins: fat-soluble (A, D, E, and K) and water-soluble (B and C). Vitamins have a huge variety of functions, including immune system support, metabolic reactions, and cellular synthesis.
The amounts of minerals needed by elasmobranchs are difficult to judge due to uptake from salt water itself; however, they are needed in metabolic processes and affect the growth of the animal. This is why water changes are extremely important in an elasmobranch system. Certain minerals, like copper and nickel, can be toxic to elasmobranchs at fairly low levels, and minerals such as iodine are needed in their diet to prevent goiter.
Keeping it natural
Elasmobranchs specialize on everything from seals to plankton in their diet. A quick description of the diets of common elasmobranchs is as follows:
- Black-tip reef sharks—teleosts, crustaceans, and cephalopods
- Bonnethead sharks—gastropods, bivalves, crustaceans, cephalopods, echinoderms, and small teleosts
- Bamboo sharks/epaulette sharks—bivalves, crustaceans, gastropods, cephalopods, and small teleosts
- Horn sharks—echinoids, crustaceans, gastropods, cephalopods, polychaetes, small teleosts
- Wobbegongs—teleosts, crustaceans, cephalopods
- Nurse sharks—small sharks, teleosts, cephalopods, crustaceans, bivalves, and echinoderms
- Zebra sharks—bivalves, crustaceans, and small teleosts
- Benthic stingrays—crustaceans, small teleosts, bivalves, polychaetes, and cephalopods
- Cownose rays—bivalves, crustaceans
- Skates—small teleosts, crustaceans, cephalopods, and polychaete worms
- Eagle rays—bivalves, crustaceans, polychaetes, tunicates, cephalopods, and small teleosts
- Electric rays—teleosts and cephalopods
- Guitarfish—crustaceans, bivalves, polychaetes, and small teleosts
It is imperative to note that not all these food taxa are readily available to the average hobbyist. A quick conversion to readily available foods that can usually be found in your grocery store or local fish store is needed to provide the proper diet for your elasmobranch.
- Bivalves: oysters, clams, cockles, and scallops
- Cephalopods: squid and octopus
- Crustaceans: shrimp, krill, crab, and lobster
- Echinoderms/echinoids: starfish are not usually sold in stores; urchins may be available in grocery stores.
- Gastropods: snails and conch can sometimes be found in grocery stores.
- Polychaetes: not sold as a frozen food, this group contains segmented worms and feather dusters.
- Teleosts: silversides, mackerel, sardines, and other bony fish
Because of their unique health issues with an improper balance of nutrients, it is necessary to add a vitamin supplement to the diet of elasmobranchs. Mazuri shark vitamins are a common brand professional aquarists use to supplement vitamins and minerals that are lost when the food is processed. Be aware that a picky ray or shark will spit out the vitamins and should be monitored to ensure consumption.
Portrayed as ruthless eating machines in movies, sharks are actually very particular about what they feed on initially. Be aware of how the shark or ray is feeding at the store before bringing it home. If it refuses to eat in front of you at the store, pass on it and wait for a couple days to see if it will eat for the store then. A shark can go many days without feeding. A ray should eat in front of you whether it ate 20 minutes ago or not.
Ensure the food item is properly sized for the elasmobranch. If the elasmobranch has been in your system a while and stops eating, do a generous water change and see if that helps its appetite. If it doesn’t, a close-up visual inspection for disease must take place. (Disease ID and treatment is coming up in a later installment.)
Rays should consume 4 to 7% of their body weight in food every week, benthic sharks 3 to 5%, and pelagic sharks 10 to 15%. A healthy ray or shark should be fasted for a short period a couple times every few months to prevent fatty liver disease.
Elasmobranchs that are healthy will actively search for food and will even associate you with food. This personable group of fishes will horde around you, and you can monitor each individual’s food intake fairly easily, even in a large group, if you tong feed or, with rays, hand feed them. With some care and planning, a good diet can ensure a long, healthy life for your shark or ray.
Photo credits: Paul Poeschl