Are the shells that I see for sale collected in such a way that it endangers the animals, or harms the environment?
Your questions are good ones, and not easily answered. The truth is we often don’t know how the shells were collected. Some shells are collected empty (or dead) but probably most are collected live and the animals within die. This is simply because if one collects only the empty shells, they are often faded and not colorful or attractive as they are when the animal is still alive inside.
In the case of mollusks collected for food, such as clams, mussels, etc. the market for shells is a by-product of food collecting and shouldn’t be a problem. But sometimes the commercial shell dealers buy in huge lots, and may pay fishermen to dredge or trawl for shells in very large quantities that could be depleting the stock. In other cases, there may be bare-subsistence fishermen who need to sell these shells to feed their families, or who keep the shells to sell as by-products of the hauls they make for fish and edible mollusks. They may save the shells to sell to a middleman to try to earn a bit more money.
It is difficult if not impossible to say what the specific conditions are in each case, when you purchase shells from a store. Also, we are not guilt-free, either. We often do things that result in destroying mollusk habitat ourselves by dredging out areas to replenish sand for beaches, or dredging harbors for ships, or even dumping waste in the ocean.
As a general rule, however, the Museum is wary about the sale of any kind of specimens, such as fossils, shells or rocks, simply because it can serve to encourage people who are money-hungry and have no appreciation of the value of organisms other than for profit. They may collect and deplete the natural source of these specimens, in some cases in such a way as to threaten the species. This was the case for some mollusks such as Florida snails that were over-collected to make jewelry and are now endangered. Like many environmental issues, this is one that is a hard call as to what is ethical and what is not.
There are many different barnacle species within the Museum's collections. Read about a few species, including the rabbit-eared barnacle, and methods the Museum uses to preserve its specimens.
When you first pick up a shell at the beach, if it is not obvious that the shell is still occupied by its living inhabitant, it is still possible that the animal is inside and has withdrawn back into the shell for protection. To determine if it is still alive, you will need to observe it for awhile. It is best to place it in a container of seawater, or in a small tidepool, leave it alone and watch for a few minutes. Usually the animal will feel safe enough after a bit to begin to extend out of the shell and explore its surroundings and at that point you need to immediately release it back where you found it.
The only really good time to find shells is at low tides. Check the tide times on the internet or in the local papers or dive shops. Also, California laws make it illegal to collect live mollusks intertidally without a license--that includes making sure there are no hermit crabs in the shells.
What sounds like ocean wave noise when you hold a shell up to your ear is actually just the movement of air across and through the shell. You could hear similar sounds if you were to hold any kind of bowl or container up to your ear. Try just cupping your hand up to your ear and you can see what I am talking about. In other words, you are really just hearing ambient noise within a resonant cavity. The sounds you hear will vary depending on the shape, size, and any convolutions over which the air flows to produce sound.
One of the best areas for finding shells would be at the north end of Tourmaline Surfing Beach in the Pacific Beach section of San Diego -- which also has a nice parking lot. Starting at Crystal Pier in Pacific Beach and walking north there is a beautiful long sandy beach and at the very northern end there is a large rocky tidepool area with a large diversity of sea life, particularly if you look under the turnable rocks (remember to turn the rocks back if you do look). The sides of the cliffs adjacent to the beach are a good area to find fossil shells, but be careful the cliffs are unstable and occasionally collapse. Do not set your blanket near the cliffs for sunbathing!
No intertidal collecting of mollusks (living shells) is permitted in California without a fishing license. With a fishing license, certain groups of mollusks such as clams and "top shells" may be taken. Consult the current California Fish and Game regulations. Diving to collect shells is permitted 1000 feet from shore. However, there are "bag" limits and seasons for some mollusks such as abalone. Consult the current California Fish and Game regulations. In general, there are no restrictions against collecting empty shells from California beaches. However, on some beaches, empty shells may not be collected. Two examples are the La Jolla Ecological Reserve in San Diego County and Point Lobos State Reserve in central California, where it is forbidden to remove live animals, empty shells, and even rocks. When collecting empty shells from a beach where this is permitted, make sure you check the inside of the shell to be sure there is not a snail, hermit crab, or other animal hidden deep within.
It's very simple to care for these shells, provided they are empty inside -- which beach shells usually are. If there is still an animal inside, you will soon know because they will begin to smell just awful. Just wash them well in soap and water in a big bowl and set them on paper, aperture side down, to dry. If some of them seem dull, you can use silicone grease or jelly, which is a sealant and also will bring out the color. Apply just a tiny dab on your finger and rub in all around. Then buff dry with a soft cloth or towel. Don't use any acid or it will gradually eat through the shell. If there is foreign matter on the shells, soak them in a dilute bleach solution for awhile, and the discoloration will come off. It will also loosen any attached calcareous matter. Enjoy your shells. They are a wonder of nature!
Although your shells appeared "empty", there were clearly some animal remains deep within the shells. If a bit of animal remains in the shell after cleaning, the odor will get quite strong.
There are several things that can be done.
At most any dive shop and some sporting goods stores.
Contact a shell club or start on your own with a good reference book from our of recommended reading.
Many mollusks prey on other species of shells by means of drilling. This means that they use specialized mouthparts and enzymes to excavate a circular hole through the shell and then insert their proboscis to eat the soft body parts of the clam or other shell. Sometimes octopi also drill holes in shells to get to the tasty animal inside.