Different types of shellfish
Biologically speaking, shellfish are aquatic invertebrates rather than fish. They can be divided into two main groups:

1.Crustaceans
(e.g. crab, lobster, crayfish, shrimp, prawn)

2.Molluscs:
(a) Bivalves (e.g. mussels, oysters, scallops, clams)
(b) Gastropods (e.g. limpets, periwinkles, snails)
(c) Cephalopods (e.g. squid, cuttlefish, octopus)

People who have reacted to one type of shellfish (e.g. crab) are likely to react to other members of the same group (in this case, other crustaceans). If you react to crab, avoid that and the rest of the crustacean group. If you react to squid, avoid that and the rest of the cephalopod group. Shellfish from the other groups may not necessarily present a problem, although they may do for some people. It is advisable to avoid them anyway. A special reason for being cautious is that there is a relatively high risk of cross-contamination among different types of seafood.
Kosher food is shellfish-free. Always read the labels and check with food companies if you are unsure.

Should people with shellfish allergy avoid fish, and vice versa?
Fish and shellfish are biologically distinct. People who are shellfish-allergic may be able to eat fish, and vice versa - unless they have both allergies. But as stated above, be aware of the risk of cross contamination in restaurants, markets and open fish counters. If the same pair of tongs is used to handle different types of fish and shellfish, or if you suspect one type of fish or shellfish may have spilled over and had contact with another, you should avoid buying or consuming.

People have been known to go into anaphylactic shock after breathing in airborne particles of shellfish or fish allergen in open fish markets.

Avoiding shellfish and fish in restaurants
In restaurants, inform staff of your allergy. Find out what your food is fried in, and whether the oil has been used for anything else. Check the ingredients of all stocks and soups. Ideally, your companions should avoid eating fish/shellfish in your presence, as there is a small risk that breathing in the cooked food may cause a reaction.

Pre-packaged foods
As they are relatively expensive, shellfish and fish are rarely an undeclared or unexpected ingredient in pre-packaged foods. Always check the label for the type of shellfish/fish to which you are allergic. Some supermarkets (e.g. Publix, Sainsbury's) will have a "CONTAINS" bar, which will indicate the presence of fish and shellfish, but policies vary from store to store and brand to brand. Be vigilant when choosing stocks, soups and highly processed foods, which may contain shellfish or fish extract to add flavour.

Shellfish and fish allergy: dishes to look out for
Dishes to look out for include paella, bouillabaisse, gumbo (a Tex Mex dish), frito misto (a mixed fried fish dish from the Mediterranean coast), and fruits de mer (seafood). Oriental food tends to contain lots of different kinds of fish at once, and chopped pieces can be difficult to spot.

Surimi (a processed seafood product) is usually made from white fish but may contain shellfish extract. Surimi can be present in processed foods such as pizza toppings.

Caesar salad dressing normally contains anchovies. Worcestershire sauce may.

Caponata, a traditional sweet and sour Sicilian relish, can contain anchovies.

Kedgeree is made with rice and fish.

Fish sauce is a common ingredient in the Far East. Terms to look out for are Nuoc Mam and Nam Pla.

Fish sauce can be made with shellfish as well as fish.

Patum Peperium (Gentleman's Relish) is a spread made with anchovies.

Check the ingredient lists on ready-made Oriental sauces, pastes and prepared meals. In fact check the ingredient lists of all food for unexpected ingredients.

The above list is not exhaustive. If you are not sure of any product, question catering staff or shop staff.

Iodine
People with shellfish allergy are sometimes warned against iodine, an element present in a wide range of items including shellfish, seaweed, cleaning products, and X-ray dyes. However, iodine allergy is unrelated to shellfish allergy. The allergen present in shellfish is not iodine but muscle protein in the flesh.

Shellfish shell and skeleton derivatives
Although it is the flesh of shellfish that contains the allergen, people with shellfish allergy are advised to avoid shellfish shells and skeletons.
1. Glucosamine, used in the treatment of arthritis, is derived from the skeletons of shellfish and is unsuitable for people with shellfish allergy.
2. Chondroitin is a shellfish-free alternative.
3. Chitin, derived from shellfish shells, is used in commercial "fat absorbers" such as Chitosan HD, and should be avoided.
4. Moisturisers can also contain shellfish-derived chitin.
5. Some calcium supplements may contain ground oyster shells.

The cod worm
Some people who think they are reacting to seafood are actually having an allergic reaction to a worm-like parasite called Anisakis (also known as the cod worm). This parasite, relatively common in Spain, can cause urticaria, gastrointestinal upset or even anaphylaxis when present in fresh cephalopods, or hake, anchovy or cod. If you react to a particular fish on one occasion, but subsequently eat it with no problem, you should consider the possibility that the cod worm was responsible. Speak to your doctor.

Latex allergy
On rare occasions, people who think they react to seafood (or other food for that matter) may be reacting to the latex gloves used in preparation. They should consider the possibility that they may be latex allergic and speak to their doctor.

Non-allergic reactions
Adverse reactions to seafood are not always symptomatic of genuine allergy.

1. Histamine, sometimes present in spoiled fish (especially tuna and mackerel), can cause a condition not unlike allergy called scrombroid poisoning. Unlike an allergy, this would affect everyone who consumed the offending food.

2. Shellfish sometimes absorb poison from toxic algal blooms (red tide), which appear in the waters at certain times of year. This can cause illnesses known as amnesic, diarrhetic, paralytic and neurotoxic shellfish poisonings. These toxins will affect everyone who eats the shellfish.

ALWAYS CONSULT YOUR DOCTOR OR PHARMACIST FOR FURTHER ADVICE.

I hope this information was of help?