Assignment 2

The Functional Morphology of Starfish Tube Feet: The Role of a Crossed-Fiber Helical Array in Movement

Authors: R. Skyler McCurley and William M. Kier

Accesses to the paper here

Introduction:

This paper discusses the morphology and mechanics of the tube feet, ampulla, and lateral and radial canals of the water vascular systems of the Echinoderm Luidua clathrata, and the Asteroidea Asteropecten articulatus. The radial canal in most studies is described as being used for extending the tube feet and being capable of accommodating water vascular fluid from contracted tube feet but in this study the radial canal in unlikely to serve in these roles. 

The tube foot wall of sea stars have longitudinal muscles and connective tissue fibers which are arranged in a cross-fiber helical array, with a fiber angle of 67 degrees. The ampulla portion of the sea star tube feet, are bilobed and include circumferentially arranged muscle fibers and connective tissue aligned 90 degrees to the muscle. The lateral canals are short and equipped with one-way flap valves, while the radial canal is thin-walled, nonmuscular, and enclosed in the connective tissue and ossicles of the ambulacrum. The ampulla will contract the feet and antagonize the tube foot musculature. The fiber arrangement of the connective tissue allows protraction and prevents dilation of the tube feet, and limits elongation of the ampulla. 

The water vascular system in many asteroids is essential for locomotion, respiration, and burrowing. The system includes the circumoral ring canal, the radial canals, and the tube feet with their associated ampulla that are connected to the radial canal by the lateral canals. The tube feet move through a hydraulic mechanism in which contraction of muscle displaces water vascular fluid from one portion of the system to another. In order to support this movement of water, the skeletal system is supported with the connective tissue fibers arranged in a cross-fiber helical array as previously mentioned.

Materials and Methods:

L. clathrata and A. articulatus were supplied by the Gulf Specimen Supply, Inc., Panacea, Florida, and maintained in a recirculating artificial seawater system in the Department of Biology, University of North Carolina, Chapel Hill. 

Histology

Segments of the arms of both species were removed and fixed in a solution of 10% formalin in seawater for 24 hours, and the decalcified in a preconstructed solution and then washed in water for 2 hours. The tissues were then cut in segments including four pairs of tube feet each, which were then embedded in paraffin. These blocks were sectioned on a rotary microtome at 10um and stained with picro-ponceay with Weigert iron hematoxylin. After further staining processes the sections and whole mounts were examined by brightfield, phase contrast, and polarized light microscopy. The tube feet and ampulla were dissected from the arm tissue and whole mounts of the ampulla were prepared. 

Computer-assisted three-dimensional reconstruction

A computer program was used to examine the morphology of the valve located between the tube-foot ampulla complex and the radial canal by constructing a 3-D image. A microscope equipped with a camera was used to give an outline of the internal and external surface of the tube foot, as well as the profile of the valve tissue and position of the valve muscle fibers.

Video Recordings

Specimens were recorded to study their locomotion and feeding movements, as well as the mechanism of burrowing by placing the organisms in a glass aquarium with a thin layer of sand on the bottom. The movements were analyzed frame by frame. 

Direct observations of ampulla

The movement of the ampulla was examined under normal movement of the animal, and in response to manual mechanical stimulation of individual tube feet with a dissecting probe.

Results:

Figure 1: Arm from L. clathrata in the region of a
pair of tube feet (T) and bilobed ampulla (A)

Each arm of the sea star has 100 tube feet with colonial ends arranged in pairs along the length of the arm, thus consisting of two parallel rows, one on either side of the ambulacral groove. 

Figure 2: Cross section of an indivisual tube foot
with  epithelium of columnar cells (EP) , nervous
tissue underneath (N), below that there is
crossed-fiber helical connective tissue (CT).
Longitudinal muscle (L) and tube foot
lumen (L) are also visible.

In retracted tube feet, the epithelium is thick and the epithelial surface is highly folded into annular rings. In the protracted tube feet, the epithelium appears thinner and the folding is reduced. The distal conical end of the tube foot is secretory and consists of tall columnar epithelial cells. Under this epithelium, there is a layer of nervous tissue and a dense layer of fibrous connective tissue arranged in a cross-fiber helical array (Figure 2). Internal to the connective tissue there is a layer of muscle fiber arranged longitudinally and unstriated, parallel to the long axis of the tube foot. 

Figure 3: Tube foot wall of L.
clathrata illustrating the crossed-
helical connective tissue array

The ampulla are located within the coelomic cavity of the arm and bilobed in both species. In L. clathrata, the lateral lobe is longer than the medial lobe, whereas in A. articularis the medial lobe is longer. A layer of epithelium covers the ampulla with a nervous layer underneath. Beneath the nervous tissue layer there is a thin layer of dense, fibrous connective tissue (Figure 3), and below this unstriated muscle fibers.   

The radial canal is lined with simple squamous epithelium, lacks musculature, and surrounded by connective tissue. No valaves or sphincter muscles were observed along its length. The ridges of the radial canal are formed by transverse ambulacral muscles (Figure 5). 

Discussion: 

The tube foot-ampulla complex of L. clathrata and A. articulatus relies on the hydraulic mechanism as force transmission will result in localized muscle contraction that displaces fluid from one portion of the system to another. This was determined upon the contraction of the muscle causing a decrease in the volume of the lumen, displacing water vascular fluid from the lumen into the tube foot. 

Figure 4: Valve structure of L. Clathrata illustrating the
lateral canal (LC) extending for the radial canal (R). The
valve falps (V) project into the tube foot lumen (T) on either
side of the lateral canal.   

The connective tissue of the ampulla played a role in controlling its shape, as there is a pressure difference between the lumen and the lumen of the lateral canal causing the valve to close to prevent water vascular fluid in the tube foot from backing into the radial and lateral canals (Figure 4). The tube foot is therefore elongated by fluid displaced from the ampulla causing an increase in volume. The tube foot shortens by contraction of the longitudinal muscles of the tube foot wall causing a decrease in volume, and water vascular fluid moves into the ampulla, which expands causing the valve to close preventing water vascular fluid leakage as previously mentioned.  

Figure 5: Radial canal (R) of  L. clathrata surrounded
by connective tissue (CT) of the ambulacrum. No muscle
fibers. A portion of the valve (V) is visible and radial (N)
in the center, between the two tube feet (T). 

In the species studied there was no evidence that the radial canal had played a role in protraction, as other studies have shown. It completely lacked musculature and there was no evidence for sphincter muscles or other structures that might allow the radial canal to be partitioned along its length (Figure 5). Also, it was wrapped with connective tissue and calcite ossicles preventing expansion. It is concluded, that the tube feet and ampulla for these species have little or no fluid entering or leaving that system during movement. Also, the radial canal in tube foot elongation is not a universal feature of the water vascular system of asteroids.   

The crossed-fiber helical array of connective tissue fibers for the species under study had reinforced the wall so that an increase in pressure causes an increase in length, rather than diameter. There were no circular rings of the connective tissue seen in any of the material from the species examined in this study. When the fiber angles were large, an increase in volume caused an elongation, and a decrease in the fiber angle caused the tube foot to shorten. These fibers resist both an increase in length and an increase in diameter. Therefore, the crossed-fiber helical array thus determines the shape change that results from an increase in volume of the tube foot. This arrangement allows length change, smooth bending without kinking, and resistance to torsion about the long axis. 

In conclusion,the study showed that the tube foot-ampulla complex functions as an autonomous unit during normal activity. Also, significant flow of water vascular fluid in and out of the radial canal during normal movement appears unlikely. 

Critique: 

This paper is useful in describing the histology of the sea star tube feet and how the various structures are important in contributing to locomotion. It provided insight as to how the cross-helical array of the connective tissue enables the tube feet to maneuver in various positions without running into any problems such as kinking. It also provided evidence of how L. clathrata and A. articulatus do not have musculature around the radial canal and do not contribute to the water vascular systems equilibrium throughout the body. 

The paper states that the specimens were collected from Gulf Specimen Supply, Inc., but does not give information as to how many of these specimens were used. It is important to have a large sample size when doing research to ensure that the results collected are consistent throughout all specimens. Also, the specimens were only collected from one location in Florida. It is possible that these species may have evolved differently then others due to a different environement, therefore, the specimens of each species should have been taken from various locations.  

The paper also includes refereneces as to papers that discussed how the radial canal is most normally used as a part of the water vascular system in equiliberating the water throughout the body and how it is highly musclarized. It then gives evidence as to how this is not the case in the specimens studied. However, the paper does not get into detail as to reasons as to why these specimens do not have this feature for their radial canal. 

The labratory techniques used in this study were all efficient, and overall, the experiment was well organized and described in an understandable manner. It provided new imformation on the radial canal of the two specimens discussed and provided insight on the purpose and structure of the connective tissue in sea star tube feet.

References:

McCurley, R.S., Kier, M.W. (1995). The function morphology of starfish tube feet: The role of a crossed-fiber helical array in movement. Biology Bulletin, 188(2). 197-209. Retrieved from: http://www.journals.uchicago.edu/doi/pdfplus/10.2307/1542085 

Assignment 1:

The Sea Star Tube Feet!

Figure 1: Sea Star tube feet retrieved from http://www.gettyimages.ca/photos/tube-feet?sort=mostpopular&excludenudity=true&mediatype=photography&phrase=tube%20feet  

Sea Stars are echinoderms that are related to sand dollars, sea urchins, and sea cucumbers. They live among a variety of ocean floors, including tropical and cold habitats, as well as in brackish waters. Sea stars live do not live in freshwater environments as they require salt water for survival. The five-arm sea stars are the most common but some species can have up to 10, 20, and even 40 arms! Sea stars have bony, calcified skin that protects them from most predators, and come in a variety of wear striking colors that camouflage them or scare off potential attackers.

Figure 2: Sea Star with many legs retrieved from
https://en.wikipedia.org/wiki/Starfish

Beyond their distinctive shape, sea stars are known for their ability to regenerate limbs, and in some cases their entire body. This is accomplished by housing all of their vital organs in their arms. Some species of sea stars require the entire central portion of the body to regenerate, but others can regenerate an entirely new sea star just from a severed limb.    

Surprisingly, the sea star is a carnivore, as they normally feed on clams or oysters. The way sea stars feed is remarkable. They are able to consume prey outside their bodies by expelling their sac-like cardiac stomach from their mouth onto their prey. The stomach will then envelop the prey to digest it and finally withdraws back into the body. The sea stars open shells of the oysters and clams through the use of their tube feet on the bottom of their arms.

Fun Facts: 
Sea stars have no brain and no blood. Their nervous system is spread through their arms and their “blood” is actually filtered sea water called hemolymph. 

Also, sea stars have eye spots at the end of each arm. It is a very simple eye that does not see much detail but can sense light and dark.

 

Figure 3: Retrieved from
 https://s-media-cache-ak0.pinimg.com
/736x/06/75/e1/0675e138049703e4a2353fd18968db21.jpg

Starfish can change their gender depending on the temperature of the ocean or the availability of food. Usually, sea stars are born male and produce sperm, then part way through their life they transform into females and produce eggs. In fact, one female can have up to 1.3 million eggs. 

Structure

Figure 4: Sea Star arms anatomy retrieved from http://tolweb.org/Asteroidea

Sea star tube feet are essential for locomotion, feeding, and respiration. They have hundreds of these tube feet and they are located on their underside. Tube feet are short, hollow, elastic, thin-walled, closed tubes that extend through a gap, called ambulacral pore, which lies between two ambulacral ossicles.The radial canal runs down the entire length of the sea star arm and receives water from the annular, which is then passed into the tube feet. 

Figure 5:Structure of the Sea Star tube foot retrieved from https://www.britannica.com/science/tube-foot

Tube feet consist of three parts, the ampulla, the podia, and the sucker. Ampulla, are rounded sac-like structures situated above the ambulacral ossicle that projects into the coelom and containing both circular muscles and longitudinal muscles. When these muscles contract it allows water to enter into the tube foot, allowing it to extend, and when it dilates the foot retracts. The podia are the middle tubular portion extending through the ambulacral groove and are covered externally by ciliated epithelium and internally with peritoneum. Between these two layers lie connective tissue in a crossed-fiber helical array and longitudinal muscle fibers. The fiber angle of the connective tissue allows protraction and prevents dilation of the tube feet, it also limits elongation of the ampullae. Lastly, the sucker is at the lower end of the podium and is flatted forming a cup-like structure.  

Histology and Anatomy
The tube feet epithelium is covered by a thin cuticle that is continuous with the covering of adjacent areas of the ambulacrum. When the tube feet are retracted, the epithelium is thick and the epithelial surface is highly folded into annular rings. When tube feet are protracted, the epithelium will appear thinner and the folding will be reduced. 

Figure 6: Starfish tube feet sec, 7um H&E Microscope Slide Retrieved from http://www.carolina.com/animal-microscope-slides/starfish-tube-feet-sec-7-um-h-e-microscope-slide/308218.pr  

The distal conical end of the tube feet have an epithelium made up of tall columnar cells with intensely staining cell inclusions. Underneath the epithelium layer lies nervous tissue containing many nerves and neurons. Below the nervous tissue, there is a dense layer of fibrous connective tissue with the fibers arranged in a cross-helical array. The connective tissue is of great importance for the mechanism of the tube feet as it reinforces the wall so that an increase in pressure causes an increase in length, rather than diameter preventing large swelling or stretching. Below the dense connective tissue sheet, there is a robust layer of muscle fibers which are arranged in circumferential bands around the lumen if the ampullae. The internal lumen of the tube feet is also lined with an epithelial layer.  

The radial canal as previously mentioned runs down the entire length of the sea star arm and receives water from the annular, which is then passed into the tube feet. The connective tissue of the ambulacrum will surround the radial canal preventing it from expanding. There are no muscle fibers encircling the radial canal.

Figure 7:  Reproduced from McCurley et, al. 1995. Biology Bulletin

Figure 5 is a summary of the arrangement of the tube feet (T), ampullae (A), and the lateral (LC) and radial canals (R), showing the trajectories of the connective tissue fibers (CT), longitudinal muscle (L) of the tube feet, and the circular muscle (C) of the ampullae. 







Mechanism of Locomotion 

Figure 8: Seastar using its tube feet to maneuver
across the sand retrieved from
http://0.tqn.com/d/marinelife/1/0/2/5/-/-/seastar-tubefeet-
wildcatdunny-flickr-500x375.jpg

During movement 1 or 2 arms act as leading arms and all the tube feet extend in the same direction in a coordinated manner. Water first enters through the madreporite to different canals such as the stone canal, the ring canal, and the radial canals. From the lateral canals water enters into the ampulla of the tube feet, and the body is moved by a stepping action of the tube feet which are alternately adhered and raised from the surface. As one or two arms are lifted in the desired direction of movement, the vertical circular muscles of the ampulla of the tube feet of these arms contract and the valves of the lateral canals close. This will increase the hydrostatic pressure within the tube feet, and they will consequently elongate and extend forward. The podial suckers will then adhere to the surface by suction force as well as the adhesive secretory products of the tips of the tube feet. Then by muscular activity, the tube feet will assume vertical posture, dragging the body forward. Lastly, the tube feet will then shorten by contracting their longitudinal muscles, forcing the water back into the ampulla and causing the suckers to release their hold on the surface.

Here is a short video of the sea star's tube feet at work!
https://www.youtube.com/watch?v=T1pQe9dWXuQ 

Function
Tube feet serve many functions to the sea stars survival. They are extremely helpful in locomotion as previously mentioned. Without the use of the tube feet, sea stars would not be able to maneuver around the ocean floors in the hunt for prey. The tube feet also help the sea star to burrow within the mud and sand by bending away from the ambulacrum toward the sides of the arm. The tube foot bends laterally into the sediment and then retracts and bends back toward the ambulacrum to repeat the cycle until the sea star is burrowed beneath the sediment.  

Figure 9:Retrieved from
 http://disney.wikia.com/wiki/Peach

Of course, what most people think of about sea stars are their ability to adhere to strange surfaces vertically or horizontally. The tube feet are what allow the sea star to accomplish this. This is due in part to suction and in part to the secretion of mucus. The mucus provides a sticky surface for the tube feet to attach to, and the sucker is fashioned so that the median part of the disk may be withdrawn from the surface of contact, with the resultant production of a vacuum. The disk on the sucker has an arborescent system of connective tissue and fibers extending from the basal plate to the outer limit of the ectoderm. By means, of this system, the pull initiated by contraction of the longitudinal musculature of the podium is transmitted to the ectoderm sucking disk, the central part of which is thereby lifted up. Some sea stars actually lack a well-defined sucker, and therefore, lack the arborescent system of fibers and rely completely on their intrinsic stickiness to adhere to surfaces.     

The sea star tube feet are also helpful in feeding. They help the starfish to hold onto food or pry clams open. They are also responsible for the direct uptake of organic nutrient material directly across the tube feet epithelium due to the secretion of mucus and reabsorption. Lastly, the tube feet are in involved in conveying food down the length of the ambulacral groove to the mouth. As a particular food approaches a given tube foot, it bends toward the particle and protracts until the tip adheres to the food. Once attached, the tube foot retracts, pulling the food toward the mouth.   

Figure 10: Seastar eating an anchovy using its tube feet. Retrieved from https://www.reddit.com/r/pics/comments/1xiigs/starfish_eating_an_anchovy/ 

References

Kennedy, J. (2016, April 25). 10 Facts About Starfish (Sea Stars). Retrieved from http://marinelife.about.com/od/invertebrates/tp/seastarfacts.htm

McCurley, R.S., Kier, M.W. (1995). The function morphology of starfish tube feet: The role of a crossed-fiber helical array in movement. Biology Bulletin, 188(2). 197-209. http://www.jstor.org.qe2a-proxy.mun.ca/stable/pdf/1542085.pdf

National Geographic. (n.d.). Starfish (Sea Star). Retrieved 
from http://animals.nationalgeographic.com/animals/invertebrates/starfish/

Smith, J. E. (2009). The structure and function of the tube feet in certain echinoderms. Journal of the Marine Biological Association of the United Kingdom, 22(1), 345-357. Retrieved from https://www.cambridge.org/core/journals/journal-of-the-marine-biological-association-of-the-united-kingdom/article/the-structure-and-function-of-the-tube-feet-in-certain-echinoderms/4051CAB92EEDFCEC8CBF2F9A05E57BAA 

Ursadhip. (2011, September 3). Tube Feet of Echinoderms. Make Easy Zoology. Retrieved from http://ursadhip.blogspot.ca/2011/09/tube-feet-of-echinoderms.html 

Wikipedia The Free Encyclopedia. (2016, October 18). Tube Feet. Retrieved     from https://en.wikipedia.org/wiki/Tube_feet