This thread is quite old, but the rules say to post here if I have a new suggestion for this, so...
I initially wrote this as its own thread (hence the length, and some of the redundancy with things previously proposed here) before deciding to post it here.
HABITATS / STRUCTURESThere are quite a few possibilities for natural aquatic structures.
- Coral reefs: Diverse marine ecosystems, covered with corals and teeming with wildlife.
- Estuaries: A partly enclosed body of brackish water, freely connected to the sea.
- Hot springs: Bodies of water that are warm enough to go without freezing, providing comforting warmth to your dwarves. Few animals can survive in such conditions, however.
- Hydrothermal vents: Found at the bottom of the ocean, hydrothermal vents provide much needed nutrition to the animals living this deep.
- Icebergs: Large chunks of ice, broken off of glaciers or ice shelves. An infamously dangerous obstacle to ships.
- Kelp forests: Massive underwater structures, filled with kelp, and many organisms that feed off of it.
- Lagoons: Shallow bodies of water cut off from larger ones by islands or reefs.
- Mangrove coastlines: Mangrove swamps already exist in Dwarf Fortress, yes, but they can be expanded on. Such areas would be full of both aquatic and terrestrial life.
- Marine caves: Caves found in the ocean, whether at the surface or far deeper, leading into the caverns. The aquatic animals found in the caverns would flourish and exist closer to the surface in such areas.
- Seagrass meadows: Seagrasses grow in marine environments, providing habitats for many aquatic animals, and food for many others, such as manatees, dugongs, turtles and crabs.
- Shipwrecks: Not all boats have smooth sailing. Many sink deep beneath the waves, or are dashed up against the coasts. Such shipwrecks would be overtaken by marine life... or, perhaps, the crew doesn't know quite how to die.
- Trenches: The deepest reaches of the sea, reaching down all the way to the innards of the world. Despite this, trenches are teeming with life. Some may reach a bit too deep, however...
EXPANDED AQUATIC LIFEBoth mundane...
- Dolphins: Famous for its intelligence and curiosity, the bottlenose dolphin is a brutally efficient pack hunter. River dolphins are found in freshwater, while other dolphins are found in all sorts of oceans - including the infamous orca.
- The King Crab: A large marine crustacean, found in the arctic ocean.
- The Man o'war: A large cnidarian creature that closely resembles a jellyfish. It delivers a painful sting.
- The Megalodon: The largest predatory animal of all time - a shark of unparalleled size and ferocity.
- Rorquals: The largest animals to have ever lived - great marine mammals that filter plankton and other small animals into their mouths. Humpback whales breach the water majestically, while the immense blue whale swims peacefully.
- Sea turtles: Venturing on land only to lay their eggs, sea turtles are graceful marine mammals found the world over. There are various types, too - green sea turtles and leatherback sea turtles are just two of many.
- Sirenians: Manatees, dugongs, and sea cows. Peaceful marine mammals that graze in mangrove swamps and seagrass meadows.
- Among many other interesting options!
... and fanciful.
- Kraken: Gigantic marine monsters, neither squid nor crab, but something in between. They use their tentacles to pluck sailors from boats and either drown them or crush them within their pincers.
- Leviathans: The forgotten beasts of the sea; brutal, massive monsters that lay siege to coastal towns and devastate boats, with some being powerful enough to cause tsunamis when they surface.
- Merrow: Not all merfolk are friendly. Merrow are vicious marine predators that hunt in groups, terrorizing coastal villages and slaughtering the crews of whatever boats dare infringe on their territory.
- Sirens: Mysterious women that call out to sailors with their lovely song, drawing them off of ships and to a watery grave.
DEPTH LEVELS AND OCEAN ZONESThe ocean is a very large place, with all kinds of different organisms living at all sorts of depth levels. At the surface, or hundreds of miles below. As it is now, ocean life Dwarf Fortress exists at all levels with no real inclination to any depth, while the ocean itself isn't particularly deep.
Basically - each oceanic creature and/or plant would be found at various depths in the ocean. (Kind of like the different cavern layers.) Some are found near the surface; others are found only in the deepest reaches. Of course, the surface animals would be seen the most - the ones that live deeper would be far more rare and reclusive.
The deepest parts of the sea are dark, with no light, being much like the caverns in that respect, while the creatures found in these locations are just as unnatural and alien as they are on Earth. Giant squids, sperm whales, goblin sharks... and even deeper, vampire squids, blobfish and anglerfish.
Some marine animals are found near the coast, while others live far into the ocean. Coastal and more open areas would have vastly different populations of animals.
EXPLORING THE OCEANIn Fortress Mode, harbors could be set up and boats constructed. Using these boats, one could catch smaller fish en masse, and even attempt to catch the larger aquatic animals. Others could be armed to defend the coastlines from aquatic invaders, whether they be megabeasts or merrow. With the army arc, ships could be sent abroad, for trade, military, or just to get more fishing done. Of course, a ship would need a crew - beds, cooks, a militia, and a captain.
In Adventure Mode, ships could be used to travel the oceans. On occasion, they could be attacked - by merrow, military ships, marine megabeasts, or pirates. Pirates would be a threat even in Fortress Mode, striking out from the waves to steal supplies, slaughtering dwarves in the process. Another threat are the ghostly ships, with skeleton crews. Of course, you could always join the ranks of the pirates, or the undead...
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