With at least 3 broad classifications of seafolk, knowing how to read other species' body language will save you and other people a lot of trouble. Here is a generalized chart (not size accurate) comparing some common emotions between seafolk!
Smiling is a key component in relieving tension and communicating mood to others. Inkfolk and finfolk both generally share this trait. Fangfolk stick out in this regard however, as they lack the facial muscles for (at least well-intentioned) smiles. Instead, they signal peace by squinted eyes and down-turned fins.
The first sight of worry or danger is pretty uniform across species- and even folk gaps, with a general readiness for action. Species with the ability for rapid color change will often bolden their patterns to look more decked out.
In both nature and society, creatures will generally depend on size and strength to intimidate attackers and rivals. Many tricks are used to convey size (even when it is lacking), which includes raising of the fins, spreading the limbs to take up more horizontal space, and taking an upright posture. Species that are capable of rapid color change often darken their colors or bolden their patterns to increase perceived fitness and perform a deimatic display. Threat displays between folk vary, but are generally recognizable, and most involve showing the mouthparts. Finfolk and fangfolk are both known to flare out the gill covers to increase the apparent size of their head, whereas inkfolk reach a similar conclusion from raising their mantle and inflating it with air.
Inkfolk are peculiar for their ability to spray ink from their backs (hence the name of the class). While this ink is typically sprayed at predators to flee a confrontation, aggressive inkfolk may also ink to confuse challengers and obscure the environment, especially for predators which depend on chemical cues to track their prey and react poorly to the disruption from a lingering ink cloud.
Fangfolk have varying threat displays and are unique among seafolk in that they have virtually no instant color changing abilities between any of the existing types. Still, threat displays may include a "white head" display, which is present in some fangfolk due to blood flow being concentrated elsewhere in the body to prepare for pursuit or full-body combat. Fangfolk will often use their jaws and tails to fight off competitors, but will adapt a signature wavy hunched-back position to signal immediate aggression or agitation, which is often enough to settle conflicts with no scars.
Distress reactions vary among individuals but are general reactions to extreme emotion. Among seafolk, fangfolk lack a "disruptive" distress reaction and may simply be compelled to isolate upon feeling unwell, particularly among less socially bonded species where weaker individuals may just be eaten. Inkfolk and finfolk are generally more tight-knit with their social groups and convey distress very similarly by blanking (losing color) and being generally unresponsive. Between seafolk, droopy fins are a telltale sign of foul mood.
Finfolk experience a tear-based crying response seemingly similar to that of humans. Although all seafolk and most land fauna with similar eye structures possess tear glands, finfolk are the only ones for which the tears serve a purpose of venting excess mood-affecting chemicals and relieving immediate tension.
Inkfolk, like species of finfolk with rapid color-change, may experience blanking and lose their color either partially or completely. Inkfolk also commonly experience uncontrolled movements and curling of the limbs, which is usually most obvious in the crown. Similar to the tears of finfolk, inkfolk will often ink as a response to extreme emotions in any direction, and this is often uncontrolled (and sometimes called 'smoking'). Science has shown that stress and strong emotions stimulate the production of ink and general ink sac activity, and similarly, releasing ink reduces chemicals and hormones such as adrenaline. (However, inhaling lingering ink clouds or remnants has also shown to make fellow inkfolk more distressed and likely to also ink.)