A crab's shell doesn't grow, but the crab does. To solve this dilemma it must molt as it grows, shedding its old exoskeleton and forming a new, larger one. The old shell loosens as a new one forms beneath it. When the old shell splits, the soft animal crawls out. Before its new shell hardens, the crab absorbs water and expands to a size larger than before the molt. While the new shell is hardening, the crab hides from predators. Once they've reached sexual maturity, decorator crabs stop molting. 

The decorator crab recycles its living decorations during the molting process. It removes the anemones, sponges and other decorations from its old shell and uses them to adorn its new shell.  

Christmas tree worm - Spirobranchus giganteus

A tropical tube worm that burrows in corals. Christmas tree worms are filter feeders that use their "fir needles" on their "branches" to collect food that is passed down to their digestive tract

They also use them to collect sediment to continue building their tubes. These tubes are also used for respiration

Sea Angels - Clione limacina & Clione antarctica

They are extremely small, with the largest species reaching only 5 centimeters long

As adults, sea angels are soft bodies creatures, but when they are initially born, they reside within a shell. As they age, they undergo a metamorphosis, much like a caterpillar transforms into a butterfly. During the transformation the shell is shed. The closely related sea butterfly, however, keeps its shell throughout life. Built of calcium carbonite, both the sea butterfly's shell and the sea angel's larval shell may have trouble forming as ocean acidification due to climate change becomes a greater threat. 

Main prey are sea butterflies - a planktonic shelled snail. Some species are ambush predators, lying in wait until prey passes by, while other species actively attack. Once the sea angel gets hold of its prey, it uses two specialized eating appendages known as buccal cones to extract sea butterflies completely out of their shells. The buccal cones have numerous hooks and a toothed radula that enable a quick and efficient meal. From capture to consumption, it can take as little as two minutes!

Sea angels in Antarctic waters have evolved to produce a chemical compound that deters fish. This is a pretty extraordinary feat for a mollusk-it is the only example in which a chemical defense is produced by the mollusk and not absorbed from a food source. The tiny crustacean called an amphipod even seeks out sea angels to benefit from their chemical protection by hitching rides on their backs.

Specifically, they are protandrous hermaphrodites, which means they start out male and turn female throughout the course of their lives. 

Marine Snow  

As plants and animals near the surface of the ocean die and decay, they fall toward the seafloor, just like leaves and decaying material fall onto a forest floor. In addition to dead animals and plants, marine snow also includes fecal matter, sand, soot, and other inorganic dust.

The decaying material is referred to as "marine snow" because it looks a little bit like white fluffy bits. The "snowflakes" grow as they fall, some reaching several centimeters in diameter. Some flakes fall for weeks before finally reaching the ocean floor.

This continuous rain of marine snow provides food for many deep-sea creatures. Many animals in the dark parts of the ocean filter marine snow from the water or scavenge it from the seabed. NOAA scientists and others have measured the amount of useable material in marine snow and found that there is plenty of carbon and nitrogen to feed many of the scavengers in the deep sea.

The small percentage of material not consumed in shallower waters becomes incorporated into the muddy "ooze" blanketing the ocean floor, where it is further decomposed through biological activity. About three-quarters of the deep ocean floor is covered in this thick, smooth ooze. The ooze collects as much as six meters every million years. 

Jingle Shells - Anomia simplex

Jingle shells have thin, translucent shell halves that look like frosted nail polish, they have also been nicknamed "Mermaid's Toenails", "Saddle Oyster", or "gold shell". Jingle shells were given their common name because of the sound they make when strung together. Beachcombers often use jingle shells to make jewelry or wind chimes.

Jingle shells appear all year and are found in shallow waters, beaches, oyster beds, and mollusk shells as far north as the coast of Nova Scotia, and all the way down south to Brazil.

The jingle shell is a bivalve mollusk, similar to mussels, oysters, and scallops, which all have two separate shells or "valves". The upper valve is rounded and movable. The lower valve is typically flat and forms to the object which it is attached. It has a hole in the top where tufts of filament, the animal's byssal threads, grow out to attach it to another surface (i.e., Jingle shells are epifaunal just like bromeliads).

The raw meat of the jingle shell is sharply bitter to the taste.

The jingle shell can reach up to 1-3 inches.

The lower valve of the jingle shell remains white, while the upper valve ranges from shiny lemon yellow, golden, brownish, silvery black, or pale buff. The shiny iridescence of the jingle shell is retained even after death.

Jingle shells take in water and filter nutritious plankton and other food through ciliated gills.

Fishermen have been known to disperse jingle shells over oyster beds in a process known as "shelling" to create a habitat for oysters can settle.  

Cookie Cutter Shark - Isistius Brasiliensis

Cookie Cutter Sharks grow to 10 -20 inches and live in the deep ocean

A parasite - doesn't kill it's prey just takes pieces out of it (which is how it got it's name)

Several species - including bluefin tuna, great white sharks, spinner dolphins, and other large predators - have been observed with one or more scars caused by these sharks.

cookiecutter sharks apparently purposely swallow the teeth that they lose. Some scientists believe that to be a result of them living in the nutrient-poor deep water column. By swallowing the relatively large teeth, they may be able to recycle the calcium and other materials important in tooth development.

These sharks are covered with light organs, likely used for either communication or camouflage. Cookiecutter sharks feed closer to the surface at night and in deeper water during the day, so they are almost always in the dark.

Though this species lives in the open ocean, there has been one confirmed case where an individual bit a person. The circumstances that led to that incident, however, are extreme. The person was a distance athlete, swimming a very long distance between islands in Hawaii, at night, surrounded by boats with lights that attracted prey. That swimmer was bitten on the calf, leaving a gruesome scar but otherwise not causing permanent damage.

Honorable Mentions

Full Source List

• https://www.montereybayaquarium.org/animals/animals-a-to-z/decorator-crab
• https://marinemadness.blog/2020/01/04/decorator-crabs-dressing-up-to-avoid-attention/
• https://oceana.org/marine-life/christmas-tree-worm/  

• https://ocean.si.edu/ocean-life/invertebrates/angels-sea

• https://oceanservice.noaa.gov/facts/marinesnow.html   
  

• https://ocean.si.edu/ocean-life/invertebrates/jingle-shells
  

• https://beachchairscientist.wordpress.com/2012/12/05/jingle-shells-jingle-shells-jingle-all-the-way/
  

• https://oceana.org/marine-life/cookiecutter-shark/