Filter feeder gap between the two rows of baleen plates lets the water in together with the prey, while the baleens filter out the water. Rorquals such as the blue whale, in contrast, have smaller heads, are fast swimmers with short and broad baleen plates. To catch prey, they widely open their lower jaw — almost 90° — swim through a swarm gulping, while lowering their tongue so that the head's ventral grooves expand and vastly increase the amount of water taken in. Baleen whales typically eat krill in polar or subpolar waters during summers, but can also take schooling fish, especially
Baleen have exploited new resources using filter feeding while not abandoning their previous prey strategies. The result of this stepwise transition is apparent in modern-day baleen whales, because of their enamel pseudogenes and their "in utero" development and reabsorbing of teeth. If it is true that many early baleen whales also had teeth, these were probably used only peripherally, or perhaps not at all (again like Dall's porpoise, which catches squid and fish by gripping them against its hard upper jaw). Intense research has been carried out to sort out the evolution and phylogenetic history of mysticetes, but much debate surrounds