Naval Shipworm (Teredo navalis)
Family: Shipworm Family (Teredinidae)
IUCN Conservation Status: Unassessed
Despite what their name and worm-like bodies suggest, shipworms like the Naval Shipworm are actually small clams adapted to burrowing; their shells, made up of two halves called valves, are located on the front of their body and act like powerful jaws used to bore into surfaces (although there is no mouth behind these "jaws",) while their siphons (a pair of muscular tube-like structures that take food and water to the gills and release waste and gametes into the water, and which are protected by the shell in most other clam species,) are located on the opposite end of their body (labelled d and e in the image above.) Like most shipworms, the Naval Shipworm builds its burrows in submerged wood such as driftwood, and is particularly infamous for its habit of boring into hulls of wooden ship, threatening the ship's structural stability and allowing the species to spread to as the ships travel - today members of this species can be found in non-polar seas and estuaries worldwide, leaving it unclear where exactly they originate from. After settling in a piece of wood young Naval Shipworms construct a strong inner casing around themselves using mucus and calcium carbonate filtered from the water around them, and if left to their own devices they will remain in their burrows for the rest of their lives with only their siphons exposed, gaining nutrients from algae and wood shaving taken in through their inhalant ("intake") siphons (with their gills both filtering algae from inhaled water and housing mutualistic bacteria that secrete enzymes capable of digesting wood, which the shipworm could not do on its own) and periodically releasing dung, gametes or larvae from their exhalent ("outgoing") siphon. All Naval Shipworms are born as males, but some individuals develop into females as they get older - mature females take in gametes released by males to fertilize their eggs internally, and after hatching the larvae remain in their mother's gills where they are protected and given access to the nutrients she takes in until they are old enough to be released into the water, after which they (now known as "veligers") undergo further development as free-swimming, algae eating plankton before settling on a suitable piece of wood and developing into adults.
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Image Source: https://www.inaturalist.org/taxa/209057-Teredo-navalis
Cool diagram of a Naval Shipworm burrow, plus some more information: https://animaldiversity.org/accounts/Teredo_navalis/pictures/collections/contributors/Grzimek_inverts/Bivalvia/Teredo_navalis/