Development, Structure, and Function of Statocysts in Cephalopods
The cephalopod eye, no matter how advanced it is, would be of no benefit to the cephalopod if it were not for their statocysts that help them maintain orientation with respect to the visual field as they move. For this reason, I have included a discussion of cephalopod statocysts and the overall role they play in vision.
For the most part, all vertebrates and invertebrates contain some type of organ(s) that allows them to maintain equilibrium with respect to gravity and movement. In humans, these organs are located within the inner ear and are the utricle, saccule, and semicircular canals. Hair cells inside the utricle and saccule continuously transmit signals through nerve fibers to the brain, indicating position, acceleration, and gravity changes (Campbell et al. 1999). All cephalopods, except the nautilus, have an equally advanced equilibrium organ, that transmits information to the brain, based on movement and changing positions of hair cells (Lane 1960). When a cephalopod moves with respect to gravity, the sensory hair cells in the statocysts move as well, and signals are sent to the brain to help the cephalopod remain orinnted (Williamson 1995). In cephalopods, this organ is called a statocyst. Statocysts are so vital to the cephalopod, that in laboratory experiments in which one or both of them have been removed, the cephalopod can no longer swim and can only walk from place to place in a haphazard, sporadic motion (Wells 1962 and 1978).
Image 8 and 9: The Importance of the Statocyst in Visual Oreintation of the Cephalopod. In figures a-e, the statocysts have not been removed and as a result, the cephalopod's pupils remain horizaontal no matter what position the cephalopod is in. In figures f and d, you can see that the pupil and thus the retina, take the oreintaion of whatever position the cephalopod is in after their statocysts have been removed(Left). The Structure of the Statocyst (Right). (Image used by permission of Harcourt Education; from Wells, M.J. Brain and Behavior in Cephalopods. Standford University: Stanford. 1962. )
Cephalopods' statocysts enable the organism to keep their retinas in continuous alignment with gravity (Williamson 1995). This is well illustrated by the figure above, which shows that the rectangular pupil of the cephalopod remains almost horizontal (a-e), no matter the position of the cephalopod in space (Wells 1962 and 1978). During organogenesis, statocysts are formed from an invagination of the ectoderm at the head end of the embryo (Williams). Each cephalopod has two statocysts, surrounded by cartilage just outside the brain and behind the optic lobes. The interior and exterior of each statocyst is filled with fluid and hair cells and is suspended in location by blood vessels and fibrous strands; hair cells also surround the outside wall of the statocyst, but their function has yet to be determined (Young 1971). At the end of every hair cell, is a nerve fiber that carries signals from the hair cells to the brain. The statocysts themselves can be divided into two parts: the macula and the crista. These globe-like structures have an interior and exterior layer of epithelial tissue with a layer of connective tissue located between them. The macula indicates to the cephalopod changes in gravity and linear acceleration, while the crista indicate changes in angular acceleration (Chrachri and Williamson 1998). The crista can de divided further into smaller units (transverse, longitudinal, and vertical), that are oriented in three planes at 90 degree angles to each other (Young 1971). Each unit is a thin strip of hair cells with a cupula, that run in a line around the inside of the statocyst (Chrachri and Williamson 1998).
Like the eye, the crista of the cephalopod and the vestibule of the vertebrate are analogous to each other with respect to nerve transmission. Both contain primary and secondary sensory hair cells. The primary hair cells have axons that go the brain and the secondary hair cells have no axons that go to the brain (Campbell et al. 1999, Chrachri and Williamson 1998, Young 1971).
Image 10: Circular structure of statocyst during early part of development. The two statocysts are the circular strurtures with small triangle shapes inside of them. (Image used by permission of Catriona Day; from Day, C., J. Wood. 2003. Octopus digueti. Cephbase. http://www.cephbase.utmb.edu/.
Development of the Vertebrate and Cephalopod Eye The Cephalopod Eye 1 The Cephalopod Eye 2 Further Comparisons Vocabulary Links References