Top ocean predators have evolved multiple solutions to the challenges of feeding in the water1, 2, 3. At the largest scale, rorqual whales (Balaenopteridae) engulf and filter prey-laden water by lunge feeding4, a strategy that is unique among vertebrates1. Lunge feeding is facilitated by several morphological specializations, including bilaterally separate jaws that loosely articulate with the skull5, 6, hyper-expandable throat pleats, or ventral groove blubber7, and a rigid y-shaped fibrocartilage structure branching from the chin into the ventral groove blubber8. The linkages and functional coordination among these features, however, remain poorly understood. Here we report the discovery of a sensory organ embedded within the fibrous symphysis between the unfused jaws that is present in several rorqual species, at both fetal and adult stages. Vascular and nervous tissue derived from the ancestral, anterior-most tooth socket insert into this organ, which contains connective tissue and papillae suspended in a gel-like matrix. These papillae show the hallmarks of a mechanoreceptor, containing nerves and encapsulated nerve termini. Histological, anatomical and kinematic evidence indicate that this sensory organ responds to both the dynamic rotation of the jaws during mouth opening and closure, and ventral groove blubber7 expansion through direct mechanical linkage with the y-shaped fibrocartilage structure. Along with vibrissae on the chin9, providing tactile prey sensation, this organ provides the necessary input to the brain for coordinating the initiation, modulation and end stages of engulfment, a paradigm that is consistent with unsteady hydrodynamic models and tag data from lunge-feeding rorquals10, 11, 12, 13. Despite the antiquity of unfused jaws in baleen whales since the late Oligocene14 (~23–28 million years ago), this organ represents an evolutionary novelty for rorquals, based on its absence in all other lineages of extant baleen whales. This innovation has a fundamental role in one of the most extreme feeding methods in aquatic vertebrates, which facilitated the evolution of the largest vertebrates ever.
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A new sensory organ discovered in rorqual whales
If you've learned a new term today, it makes me happy. Rorqual whales (Family Balaenopteridae) are baleen (toothless) whales that have pleated throat grooves that expand tremendously when the whales feed, allowing them to take huge gulps of water that they then force through their short baleen (baleen is basically a keratin filter system) to trap the organisms that were in the engulfed water. Examples are the blue whale, humpback whale, Minke whale, Sei whale. "Rorqual" is Norwegian for furrow. In this BBC video you can see a Sei whale feeding on a fish school.
One of the greatest mysteries of nature is how these massive animals manage to consume enough tine fish and other marine organisms to not only survive, but grow to their massive size. Their feeding style, known as lunge feeding, is a feat of synchronization and power. When a rorqual finds a dense patch of krill or fish, it accelerates with an open mouth. The amount of water that flows into the whale's mouth can almost double its weight! The throat pleats expand and the normally sleek giant looks temporarily like a bloated monster, until it expels the water out forcefully. Rorqual whales can lung feed for hours at a time. The skull and mandibles of rorquals are very specialized: they have unique jaw joints made from dense elastic matrices of fiber and cartilage, that allows the jaw to be opened at 90 degrees. The lower jaw has a flexible joint in the middle, which allows the two sides to rotate. The tongue can be turned inside out, creating more space for water. But how does the whale coordinate all these movements? Researchers the Smithsonian Institution describe a sensory organ, highly vascular and highly enervated, embedded in the lower jaw joint of several rorqual whale species. The organ is evolutionarily derived from ancient tooth sockets and it contains papillae that appear to work as mechanoreceptors, in other words, an organ to sense stretching and pressure. The organ is an evolutionary novelty for rorqual whales, because it was not found in any other group of baleen whales.
- Nature 485, 498–501 (24 May 2012) doi:10.1038/nature11135
Tags: baleen, evolution, feeding, organ, receptor, rorqual, sense, whale
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