Hunger: Mermaids don't get much energy from normal meals.

Eventually, they will pass out, and if another Sim does not splash water on them, they will die. Dehydration will cause the Sim to dry out, giving them a negative moodlet. If The Sims 3: Seasons is installed, they can also hydrate by standing in the rain. This can be satisfied by swimming in the ocean or a pool, using the shower or bathtub, or relaxing in a hot tub.

Hydration: In place of Hygiene, there is a 'Hydration' need, similar to water for a PlantSim.

Mermaids have the same needs as regular Sims, with two main differences: The color of a mermaid's scales and, if female, seashell bikini top can be customized by using the 'Change Scale Color' interaction via mirrors. Mermaids cannot wear socks as the scales will replace socks. Mermaids have their scaly legs from childhood, but their legs won't transform into a tail while in water until the Sim is a teenager. On land, a mermaid's tail will transform into a scaly pair of legs of the same color. Males have a tail with a straight fin like a shark, and go shirtless, while females wear a seashell bikini top and have tails that end in a classic dolphin-shaped fluke. While in water, Mermaids have a colorful scaly fish/dolphin tail in place of legs. And because it is a life state it can't have another life state, for example, it can't become a zombified mermaid, despite being a secondary life state that can be combined with other major life state. If both parents are mermaids the child will be 100% mermaid. There is a 50% chance a child will be a mermaid for each mermaid parent. However, you can still share households with normal sims and at least one piece of “ Mermadic Kelp” in a Sim’s inventory to instantly turn them into mermaids. Mermaids cannot be created manually or shared on the exchange, since EA wanted them to be an exclusive life state. They have to become normal Sims before they can turn into a mermaid.

Note that other life states cannot become mermaids even if they eat the mermadic kelp, unless mods are used. A Sim can become a mermaid by ingesting mermadic kelp, which can be obtained either by becoming good friends with a mermaid and using the 'Ask About Fish Parts' interaction while both the Sim and the mermaid are underwater, or purchasing the “ Mermadic Kelp” lifetime reward for 25,000 lifetime happiness points.

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