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