Spikekelp
Anatomy
Spikekelp have incredibly thick stipes. Stipes are similar to stems in plants, except stipes do not perform any nutrient transport functions and instead support the blades and fronds of the kelp.
Their stipes, blades and fronds are all a deep blue colour, while their gas bladders are a yellowish-orange. These bright colours scare away animals from consuming them, a form of aposematism.
Along the blades of the spikekelp are thin, sharp spines, that embed themselves in anything that brushes past. The spines contain a fatal toxin that kills anything that it touches, however slow-whales possess skin so thick, particularly around the mouth, that the spines cannot get through.
Toxins & Slow-Whales
Spikekelp release a toxin through their spines when they puncture an animal. This can be when an animal swims past the kelp, and the spines fall off and stick into the animal with a hook on the end of them.
Slow-whales are an animal with extremely thick skin, especially around their mouths. This adaptation allows the animals to consume spikekelp without worrying about the toxic spikes. Slow-whales also help somewhat control the population of spikekelp in the Frozen Volcanoes, as they are the only animal species that consumes them and is the majority of their diet.
Did the moonvine recycle some design parts from spikekelp (or the other way around)? Typos: "...thin, sharp spines, that embed..." shouldn't have second comma, you stated that "slow-whales possess skin so thick, particularly around the mouth, that the spines cannot get through" twice, would appreciate making "Slow-whales also help somewhat control the population of spikekelp in the Frozen Volcanoes, as they are the only animal species that consumes them and is the majority of their diet" slightly clearer.