Locomotion
  Topics relating to locomotion include tube feet, and ARM NUMBER, considered in subsections here, and RIGHTING RESPONSE, and FUNCTION OF THE MADREPORITE considered elsewhere.
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Tube feet

 

drawing of the water-vascular system of a sea starLocomotion of sea stars is via multiple tube feet that are part of a larger system of hydraulic ducting known as the water-vascular system.  Hydraulic movement of fluid from storage sacs or ampullae to the tube feet (podia), accompanied by relaxation of longitudinal muscles, act to extend the tube feet.  On contact with the substratum the tube feet attach momentarily to the substratum by means of a sucker. Contraction of postural muscles at the proximal regions of the tube feet moves the body in relation to the tube feet for locomotion.  Contraction of the longitudinal muscles in the tube feet pull them closer to the substratum for anchoring, or pull prey closer to the arms for feeding. There is evidence that the madreporite acts as a conduit for at least some of the fluid present in the water-vascular system.

The nervous system is sited close to the elements of the water-vascular system. It consists of a ring around the mouth and radial nerves running down each arm, with smaller nerves running to each tube foot. There are no ganglionic clusters and nothing equivalent to a brain. Other than moving in the same direction during locomotion, there is no coordination of stepping movements of the tubefeet.  

 

photograph of a sunflower star crawling towards the camera
The number of tube feet in a sea star is large (estimated in an adult sunflower star Pycnopodia helianthoides to be about 15,000), so precise control may in any case be beyond the capacity of the nervous system.  It is common to see fast-moving sea stars, such as Pycnopodia, pulling themselves along with the tube feet on the leading half of the body and letting the arms on the back half of the body stream out behind, with little or no contact of tube feet with the substratum.

 

 

This fast-crawling P. helianthoides is moving towards
the camera with its back arms trailing (not visible). Not
atypically, the arms at each side are held up and/or curled,
indicating that they are not participating in the action 0.5X

 
 
photograph showing close view of ambulacral area of a forcipulate sea star showing tube feet and pedicellariae taken from a video

CLICK HERE to see a video of movement of tube feet in the ambulacrum of a forcipulate sea star. Also visible are pedicellaria clusters that are able to move up and down on the spines. Dermal papulae (branchiae) for gas exchange are abundant. Note the accordian-like nature of the epidermis part of the tube foot, permitting great length changes with relatively little change in diameter.

NOTE  the video replays automatically

 
Research study 1
 

How do the tube feet attach to the substratum? Early thoughts were that suction is created by special levator muscles raising up the centre of the terminal disc of the sucker in combination with secretion of sticky mucus, but focus is now mainly on chemical adhesion by fast-acting glues.  Microscopic examination of tube-foot epithelium of Leptasterias spp. reveals 3 types of cells: adhesive cells, large-granule secreting cells, and monociliated cells thought to be sensory in function. A sea star moving across a clean surface, like glass, actually leaves tube foot-prints, marks left by secretions.  A sea star photograph showing close view of tube feet of sea star Pisaster ochraceus anchoring to rock surfaceattaching and detaching to surfaces naturally filmed with bacteria and diatoms has clean tube feet, indicating that adherent material does not accumulate on the tube-foot surfaces.  Nor do the tube feet stick to, or pull on, the substratum as they are lifted away. Through the nature of proteinaceous secretions from the adhesive and large-granule secreting cells the entire surface of the sucker end of the tube foot becomes negatively charged, which also may be involved in attachment to the substratum.  Tube feet attach best to charged substrata, and attach poorly or not at all to uncharged surfaces such as Parafilm, dental wax, epoxy resin, and the like.  Detachment may be effected by secretion of other chemicals, releasing the tube feet cleanly from substrata to which they have attached, but leaving footprint residues behind.  From the results of their study, the authors do not discount the involvement of suction, especially on solid surfaces, but suggest that this is a secondary adjunct to adhesion established by a protein glue.  Thomas & Hermans 1985 Biol Bull 169: 675.

NOTE  although the authors state that the “distal surface of the tube foot is coated by a negatively charged surface which somehow attaches to the substrata”, the way that this might work is not made clear.  Perhaps it will be the subject of a future study 

Ochre star Pisaster ochraceus anchored to a rock with its tube
feet during low-tide exposure. The attachment is so stong that
the the tube feet may be ripped away when force is applied to
the body, leaving the torn-off portions hanging to the rock 2X

 
Research study 2
 

photograph of arm tip of a sunflower star showing eyespotMany species of sea stars curl up the tips of their arms when crawling, as shown here for an arm of a sunflower star Pycnopodia helianthoides.  The behaviour can also be induced by a variety of mechanical, chemical, and photic stimuli.  Not only does the behaviour provide maximum exposure for sensory tube feet located at the arm tips, but the light-sensitive eyespot is also exposed.  Sloan 1980 J Nat Hist 14: 469.

NOTE  also known as the compound ocellus or optical cushion.  Information on light effects on the morphology and physiology of ocelli of Patiria miniata, Leptasterias pusilla, and Henricia leviuscula can be found in Eakin & Brandenburger 1979 Zoomorph 92: 191.

 

 

Arm tip of a sunflower star Pycnopodia heliathoides showing the
red eyespot nestled within a protective array of spines. Other
features to note are the long, chemotactile tube feet, clusters
of pedicellariae, and the sac-like dermal branchiae 2X

 
Research study 3
 

Usually we assume that sea stars either have pointed/non-suckered tube feet (categorized as PNS) useful in mud-bottom habitats as in Luidia spp. in the Order Paxillosida, or they had flat-tipped/suckered tube feet (FS) useful in hard-bottom habitats, as in Pisaster spp. in the Order Forcipulatida. A study by researchers at the University of Alabama, however, shows that this simple notion needs to be reconsidered, as there is considerably greater variation in tube-foot morphology in asteroids than was previously thought.  The authors examine 45 world species in 7 orders and 19 families, with 16 west-coast species being included.  Their observations  require the creation of 4 new categories of tube-foot morphology, including semi-pointed/non-suckered (SPNS), flat-tipped/non-suckered (FNS), semi-flat-tipped/suckered (SFS), and semi-flat-tipped/non-suckered (SFNS).  The authors also find that a consistent relationship exists between  tube-foot morphology and taxonomic order.  Thus, all Forcipulatids have flat-tipped/suckered tube feet (FS), all Velatida have semi-flat-tipped/suckered ones (SFS), all Valvatida have flat-tipped/non-suckered ones (FNS), and so on.  See examples below for a few west-coast species.  Notwithstanding this taxonomic consistency, the authors remark on the variability in tube-foot design in asteroids and suggest that what is needed now is a better idea of how each morphology suits a particular locomotory, feeding, and anchoring need.  Vickery & McClintock 2000 Amer Zool 40: 355. Photograph of Henricia pumila courtesy Dave Cowles, Walla Walla University, Washington.

 
photograph of sunflower star Pycnopodia hellianthoides with closeup of tube foot, latter courtesy Vickery & McClintock 2000 Amer Zool 40: 355
Sunflower star Pycnopodia helianthoides: Order Forcipulatida: flat-tipped/suckered FS
photograph of tube feet of sea star Patiria miniata
Bat star Patiria miniata Order Valvatida: flat-tipped/non-suckered FNS
photograph of tube feet of sea star Pisaster brevispinus
Pink star Pisaster brevispinus: Order Forcipulatida: flat-tipped/suckered FS
photograph of slime star Pteraster tesselatus with close view of tube foot, latter courtesy Vickery & McClintock 2000 Amer Zool 40: 355
Slime star Pteraster tesselatus: Order Velatida: semi-flat-tipped/suckered SFS
photograph of upturned sea star Luidia foliolatum showing tube feet
Sea star Luidia foliolatum: Order Paxillosida: pointed/non-suckered PNS
photograph of blood star Henricia pumila courtesy Dave Cowles, Walla Walla University, Washington
Blood star Henricia pumila: Order Spinulosida: flat-tipped/suckered FS
 
Research study 4
 

bat stars Patiria miniata courtesy Kevin Lee, Fullerton, CaliforniaAmong most animal taxa, regardless of mode of locomotion, absolute locomotory speed is predicted to increase as body size increases, but is this true for asteroids where locomotory speed is a function of tube foot length, stepping frequency, and tube-foot attachment area?  This is tested for bat stars Patiria miniata by researchers at the Bamfield Marine Sciences Centre, British Columbia comparing absolute and relative escape-response1 speeds in different-sized individuals.  Results show, interestingly, that both absolute and relative2 crawling speeds actually decline with increasing body size (see graph for absolute data).  Note that maximal velocity for a small Patiria (50g live mass) in this study is about 2.5mm per sec and, for a large Patiria (300g), about 1mm per sec.   Comparison of 5- and 6-armed individuals, moreover, shows that arm number does not significantly affect crawling speed nor, incidentally, does leading arm differ significantly from random in the 2 types.  The explanation for the results is unclear but, after considering a range of possibilities, the authors suggest that the explanation may relate to scaling relationships of body mass and total cross-sectional area3 of the tube feet.  Thus, while  body mass scales as the cube of linear dimension, tube-foot cross-sectional area scales only as the square.  With increasing size, then, the sea star would eventually outgrow the propulsive capacity of its tube feet.  Montgomery & Palmer 2012 Biol Bull 222: 222. Photograph courtesy Kevin Lee, Fullerton, California diverKevin. graph comparing absolute locomotory speeds of bat stars Patiria miniata

NOTE1  to standardise the assays, the test Patiria are squirted on an aboral between-arm surface 180o from a randomly chosen “arm-to-lead” with 10ml volume of effluent seawater from tanks containing predatory sea stars Solaster dawsoni, which encourages the Patiria to move away at presumed maximal velocity

NOTE2  relative to body size, measured as body mass, arm length, and ambulacral and oral-surface areas

NOTE3  the authors did not measure this, nor did they measure number of tube feet, and if not already being investigated would make a useful research project for someone

 
 

Arm number

 
Research study 1
 

What about number of arms?  This seems fundamental to any consideration of locomotion in sea stars, yet only rarely is the condition of multi-rays (= multiradiate) even considered in textbooks from an evolutionary standpoint, let alone from a standpoints of efficacy in locomotion, or feeding, or whatever.  The evolutionary aspect is considered in a scholarly article by Prof. F.H.C Hotchkiss, Harvard, Mass., long a student of “armedness” in asteroids and other echinoderms.  The article deals broadly with world representatives of the group, but includes several west-coast asteroids.  Of 34 asteroid families in the world, 20 are strictly 5-rayed, while the remaining 14 have multiple rays (9 of the 14 have both 5- photo/diagram showing different arm insertions in multiradiate sea starsand multi-rayed species).  Thus, only 5 families are exclusively multiradiate.  The professor considers several ideas for the origin of the multiradiate condition.  Basically, in 5-armed families the 5 “primary” rays develop as a unit (= en bloc pathway), and this pattern remains intact through metamorphosis.  One idea for multiradiate families is that the en bloc condition holds for only a brief time during development, then the supernumerary rays develop.  Another idea proposes that the “primary-arms en bloc-pathway” has no heritable variation and cannot be “co-opted” for the production of extra arms. However, according to the Professor, the timing and pattern of development of the extra rays in the different families is actually so variable as to suggest recurrent multiple origins in the evolution of multiradiate sea stars.   Hotchkiss 2000 Am Zool  40: 340. For more on numbering of arms and symmetry of sea stars see SEASTAR: SYMMETRY.

NOTE  rays or arms?  Either is good.  The first is more commonly used in the UK and areas of the east coast of North America, while the second is used most anywhere

NOTE  an interesting side-remark in the paper is that selective breeding for aberrant arm number is unsuccessful, as only 5-armed offspring are produced.  Thus, genetic control of “aberrant” arm number seems to be ruled out

 

 

Three west-coast species Pycnopodia helianthoides, Solaster dawsoni, and Crossaster
papposus
shown in both diagrammatic (oral view) and photographic (aboral view) views.
In all 3 species, metamorphosis leads to initial formation of 5 arms, with other arms
being added later. Note: 1) that the madreporite is in the same relative position for each species,
2) that the later addition of arms differs in location for each species (black arrows), and 3) that the diagrams and photos are mirror images of one another because the first is an oral view and the second, an aboral view

 
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