In this article ANDY HORTON  examines the feeding cycle in the seas, and discusses some of the edible molluscs and crustaceans found on the shore, and foods available from aquatic shops that are suitable for British sea life kept in aquaria. The basic principles are applicable to the tropical reefs.

DIAGRAM OF THE ORGANIC CYCLE  to be posted at a later date in 1999.

 Real seawater contains phytoplankton, invisible microalgae, single-celled organisms, principally diatoms and flagellates. Phytoplankton provides the food for filter-feeders like the Sea-Mats (Bryozoa), Sea-Squirts (Tunicates), and bivalve molluscs such as mussels and oysters. It is eaten by the zooplankton, comprising 90% of very small crustaceans, and includes the eggs and larvae of fish and invertebrates. If you follow the cycle round, you will find that the Zooplankton is consumed by other invertebrates including the Hydroids and Sea Anemones (Anthozoa), and Crustaceans. Zooplankton is a vital part of the fry and juvenile fish of all species, and the adults of Pipefish and the Sea Horses. All, in turn, are predated upon by larger animals.

 Dead animals and decaying organic waste is scavenged by specially adapted creatures; the whelks, worms and some of the crabs. Even detritus is eaten by the lugworm and certain Cushion Stars like Porania pulvillus, found offshore in Scottish waters. Ammonia is expelled directly by the fish and other animals, and is also released during the decomposition of dead organisms. The final stage is acted upon by bacteria in the 'Nitrogen Cycle', leaving nutrients which are used in the creation of algae, the phytoplankton and larger seaweeds. There are other waste products of which the most important is the Carbon Dioxide respired by all living creatures, including bacteria; and is essential in the photosynthesis process.
 

Porania pulvillus

 Aquarists should try to mimic the conditions found in the oceans. However, in the small confines of a home aquaria, it is not possible to create an identical habitat.

Ubiquitous Feeders

 Most of the species kept are predators an scavengers, often ubiquitous feeders. They will eat everything meaty presented to them. I prefer to feed the various fish and invertebrates what they would eat in the wild. There are two principal reasons. Firstly, their chances to thrive in a healthy condition are improved if they are fed on natural live food. Secondly, one of the major fascinations of keeping British marine life in aquaria, is what can be termed the 'ecology of behaviour' of the varying creatures. As feeding is one of the principal activities, it is interesting to watch how the different fish and invertebrates deal with items of prey. The aggressive Shore Crab, Carcinus maenus, has a particularly varied diet. On the shore, it is frequently found with a polycheate worm in its claws. In aquaria, it demonstrates its ability to tackle mussels and cockles, shrimps (Crangon), univalve (snail-like) molluscs, Hermit and other small crabs. It is generally regarded as undesirable because large crabs will prey upon the more delicate animals. Fish like the Common Blenny, Lipophrys pholis, possess sharp comb-like teeth that enables them to crunch barnacles from rocks and attack hard-shelled crustaceans.

Shore Collection

 Shore collectors will have access to a range of molluscs, worms and crustaceans found in the littoral zone. Tropical aquarists are warned against using most of these foods because of the likelihood of introducing disease organisms. Parasites occur as free-swimming animals. Anemones contain seawater and could be responsible for introducing pathogens into the tank. Gamma-irradicated frozen natural foods from aquarist shops, provide a useful alternative diet.

 My observations on British rock pool fish indicate that parasites are regularly present. Pragmatically, the most sensible procedure is to ensure healthy conditions in the aquarium. Most parasites only become unsightly, and a danger to the fish if it is already weakened by poor husbandry.
 Little and often; this is the general advice for a healthy aquarium. Two feedings a day is an acceptable practice. British fish and crabs are extremely greedy. If you do not feed them enough, they will start eating each other. It is unlikely that you will overfeed the fish. However, it is certainly possible to overfeed the tank, the uneaten food using up oxygen during putrefication, excessive ammonia excretions poisoning the water directly, or causing an explosion in oxygen consuming bacteria. Ensure that you remove food that is not eaten. Watch all the fish and invertebrates closely; the less competitive and slower moving creatures often fail to get their share.

 Fish and other animals require a diet comprised of proteins, carbohydrates, fats, vitamins and minerals. British shore species are carniverously inclined, and providing the essential amino acids means that a large part of the diet must consist of fishy foods found in their natural environment, or substitute food providing similar ingredients. Wild food can be collected on visits to the coast.

MUSSEL  Mytelis edulis
This bivalve mollusc can be found in dense congregations of millions of individuals on many shores. It attachs itself by the means of byssus white threads, which secures it very firmly in locations where the water movement and the force of the waves would dislodge a less securely fixed animal.
 It ranks as first choice as food. Most rock pool fish and crabs will eagerly feed upon the nutritious flesh.  Pour boiling water over the shells to open them. Scrape out the rich orange flesh and feed whole and in pieces.

 Mussels reproduce in spring and afterwards become depleted of flesh. The small female Pea Crab, Pinnotheres sp. often resides inside. Cemented to the outside are Acorn Barnacles which provide equally attractive food. Whole mussels (up to 6) can be kept alive for a few weeks in aquaria and will provide natural food for the Common Starfish, Asteria rubens, and the Dog Whelk, NucelLapillus. There are at least 8 species of mussel found in British seas, including the Horse Mussel, or Clabachdubh, Modiolus modiolus.

EDIBLE COCKLE  Cerastoderma edule

Cockles can be found buried just below the surface of the sand. They can be prised open by inserting a knife in between the two halves of the shell, and fed raw to the fish and crabs. They will survive for over one month in aquaria. There are numerous other sand burrowing bivalves, Carpet shells, and Tellins found on British shores.

LIMPET  Patella vulgata
On certain shores where Mussels and Cockles are absent, the Limpet proves to be the easiest choice of natural food. At low tide they will clamp themselves firmly to the rock. The collector will need a strong knife to insert underneath the mollusc, which is likely to be damaged in the process. They feed on algae and undamaged specimens (collected on their feeding forays) can be kept for over one year.

 Limpets have tough flesh and are best fed after they have been boiled for over 5 minutes. Relished by Wrasse, but all fish prefer a mussel diet.

PRAWNS Palaemon elegans and P.serratus.
The two commonest species in the south reach lengths of 5 cm (2") and 10 cm respectively. The second species is large enough to make human eating worthwhile. They are easy to keep in a properly established aquarium, where they will scavenge for food.

 Visitors are frequently fascinated by their complex behaviour and feeding habits. Small prawns will be immediately attacked and killed by Wrasse (Labridae), and swallowed whole by young Bass. The Bullhead, Taurulus bubalis, will ambush this crustacean, and diminish a population kept in the same aquarium. Only newly moulted prawns fall victim to Beadlet Anemones.

 Live prawns are the second most important natural food. Many aquarists keep a spare tank to ensure a supply throughout the Winter, when they are found in deeper water offshore. Freshly killed prawn is sometimes  accepted by difficult species that refuse Mussel.

SHRIMP Crangon crangon
Shrimps inhabit sandy shores. They are quickly attacked by a wider range of fish and crabs than the Prawn. Because of this, the introduction of a net full of Shrimps will start an amusing feeding frenzy.
 Young Shrimps are found in sandy pools on mixed shores in the company of young Gobies (Gobiidae). They provide excellent live food.

OTHER SMALL CRUSTACEANS
Numerous small crustaceans are important because because very young fish, and adults of some species e.g. Dragonet, Callionymus lyra, sometimes refuse dead food. The commonest species found are Isopods, Mysids, Sandhoppers and other prawn-like creatures. Isopods are marine woodlice. the largest known as the Sea Slater, or Bilge Bug, Ligia oceanica, is not liked by any of the fish. All other Isopods are eaten. Mysids, or Chameleon Shrimps are readily consumed by young Wrasse and adult Pipefish (Syngnathidae). The amphipod Gammarus locusta is found under rocks amongst large brown seaweeds. It crawls away on its side and is swallowed by small Butterfish, Pholis gunnellus and almost any rock pool fish.

SHORE CRAB  Carcinus maenas
During late Spring and Summer, small Shore Crabs can be found in their thousands hiding underneath almost every rock and scampering over the sand. Young crabs will be eaten whole by Blennies and Wrasse. Expect to find this crab emerging in your tank without deliberate introduction. Young multi-coloured crabs conceal themselves in small rock crannies and amongst the weed.

RAGWORM  Nereis diversicolor
Ragworms are found in sand and mud on the shore and in estauries. They are readily taken by almost every fish. They are an important food because fish from sandy shores, small Flatfish and Dragonets may refuse other items. 

 Other worms can be tried. Detritus feeders like the Lugworm, Arenicola marina, tend to foul the water and are best avoided.

PERIWINKLES  Littorina littorina
Boil this abundant mollusc and extract the flesh with a pin. It is a popular food. Empty shells can be placed in the tank for the smaller Hermit Crabs.

 Live winkles may be responsible for the introduction of the parasite Cryptocotyle lingua which encyst to form black blisters on shore fish such as the Butterfish and the Rocklings.
 It does not seem to be worth collecting Topshells and other Gastropod molluscs as regular foods, because of the small amount of flesh on each animal.

OTHER FOODSTUFFS
GAMMA-IRRADICATED FROZEN FOODS
A large selection is available including Brine Shrimp, Mussel, Squid, and Crab.
Even people who live beside the shore may use these foods in the display tanks. If used regularly, it becomes an equal to any of the collected foods, with the exception of raw Mussel.

RAW FISH
Raw whitefish like Cod and Plaice, from the fishmongers, can be fed to the larger fish. Avoid oily fish like Herring and Mackerel which will pollute the water and leave an film on the surface. Cooked and frozen prawn is often refused.

GARDEN FOODS
Some aquarists feed the fish and invertebrates on earthworms and woodlice. I have never found this satisfactory. They justify their decision because this reduces the possibility of introducing disease. I believe that these foods lack certain ingredients necessary to keep marine life in a healthy condition.

BRINE SHRIMP EGGS
Brine shrimp eggs are available from aquarists shops which can be hatched out in a special hatchery fitted in the corner of the aquarium. Temperature should be above 20ºC. These minature brine shrimps will be fed on directly by small Pipefish. They provide a substitute for some of the natural zooplankton. Plumose and Sagartia  Anemones benefit from this live food as it seems to stimulate their catch-tentacles to search for larger prey and consume dead food such as Mussel.

 Many filter-feeders consume exclusively phytoplankton and keeping them alive can be difficult. Hatchery researchers have fed the larvae of Oyster on cultures of minute green flagellates and diatoms. Marine Rotifer cultures are now becoming available through specialist aquarium retailers.

ADULT BRINE SHRIMPS (LIVE)
Adult brine shrimps are available in marine aquarist shops on one or two days during the week. They are eagerly pounced upon by the Sea Stickleback,  Spinachia spinachia, which eats exclusively live food in the form of small planktonic animals.

MYSIDS

MARINE FLAKED FOOD/DRIED FOODS
Although Prawns and young Grey Mullet, Chelon labrosus, will feed ravenously on dried and flaked food, most shore life forms will need their diet supplemented by flesh foods; raw fish etc. 

BIBLIOGRAPHY/FURTHER READING
Marine Breeding: Rotifers (Zooplankton), Algae cultures etc. Alan Hale in Mari-News (Journal of the British Marine Aquarist's Assoc. 1989)
The Living Seashore by Joan M Clayton (Warne 1974)
Plankton - Cartoons by Bob Foster-Smith in the Journal of the Marine Conservation Society.
Marine Aquarium Keeping by Stephen Spotte (Wiley-Interscience 1973)  Chapter 9 Nutrition & Feeding
Basic Marine Biology  by A.A. Fincham (Cambridge University Press 1984)
British Shells by Nora McMillan (Warne 1968)