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  Tube feet & locomotion
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  Function of the madreporite
  Topics relating to tube feet & locomotion include function of the madreporite, considered here, and LOCOMOTION considered in another section.
   
 

drawing of water-vascular system of a sea star with parts labeledThe tube feet connect to the water-vascular system via lateral canals, radial canals, and ring canal.  From the ring canal a calcified “stone” canal leads to the madreporite (not labeled on the drawing).  The madreporite is visible externally as a porous plate, and is sometimes distinctively coloured.  Because of the physical nature of the connection of the madreporite with the water-vascular system, it has long been thought to act as a conduit for entry of seawater for operation of the tube feet and/or for body-volume regulation.  However, unequivocal evidence for this is known only from relatively recent studies, and the function of water intake via the madreporite is still uncertain.

NOTE one article on sea urchins is included here along with several for sea stars

 
Research study 1
 

Studies on madreporite function in the ochre star Pisaster ochraceus show that individuals with cement-obstructed madreporites do not maintain body volumes as well as normal animals.  The author notes that the osmotic concentration of the water-vascular fluid in asteroids is higher than that of the pervisceral coelomic fluid and surrounding seawater.  Since water would move into the system osmotically from both sources, the question arises as to why there would be need for an auxiliary source of fluid via the madreporite?  The author suggests that while the madreporite is involved with photograph of a madreportie of an ochre star Pisaster ochraceus in close viewmaintenance of general body volume, it is not involved significantly with inflation of the tube feet, where osmotic factors appear to dominate.  In this regard, it is known that the tube feet of a sea star can remain active for days after being isolated from the madreporite system.  Ferguson 1990 Comp Biochem Physiol 95A: 245; Ferguson 1992 Biol Bull 183: 482; Ferguson 1996 Biol Bull 191: 431.

NOTE  good data sets for Pisaster ochraceus, Pycnopodia helianthoides, Luidia foliata, and Pteraster tesselatus (and spotty data for 10 other asteroid species) show slightly elevated, but significant, levels of osmotic concentration in the perivisceral coelomic fluid (1.5 mosmol per kg greater than seawater – the latter of which in the study area is 880 mosmol per kg).  Levels in the water-vascular system for these 14 species average 6.1 mosmol per kg higher than seawater – again, a biologically significant elevation.  In both instances water would move inwards into the 2 systems. Ferguson 1990 Comp Biochem Physiol 95A: 245.

Madreporite of an ochre star Pisaster ochraceus,
nestled amongst the spines on the aboral surface 2X

 
Research study 2
 

photograph of sea star Leptasterias hexactis showing location of madreporiteAfter bathing Leptasterias hexactis in seawater containing fluorescent microbeads (0.2µm dia) for 48h, high concentrations of beads are found in the lumen and lining of the tube feet, the ampullae, and within the Tiedemann’s bodies.  The route taken is from madreporite to stone canal, then radially outwards to ampullae and tube feet.  The flow rate is small (2-3 µl per g animal per h) but, as noted in Research photograph showing close view of the madreporite and other dermal features of Leptasterias hexactisStudy 1 above, is considered by the author necessary for sustaining the volume of the perivisceral coelomic fluid.  Detailed studies on the fine structure of madreporites and stone canals in sea stars suggest that the madreporite is not just a simple sieve-like entrance to the water-vascular system; rather, it has abundant secretory cells, phagocytic coelomocytes, and an extensive complex of nerves.  The authors conjecture that the madreporite may utilise the strong ciliary pumping of the stone canal to cleanse coelomic fluids drawn in from the axial sinus, as well as seawater from outside, and could possibly be a route for pheromone-like substances to reach internal parts and trigger reproductive activities or other functions.  The exact way that this might work is not known.  Ferguson 1990 J Exp Zool 255: 262; Ferguson & Walker 1991 J Morphol 210: 1. Photos courtesy Dave Cowles, Walla Walla University, Washington rosario.wallawalla.edu.

 
Research study 3
 

drawing of cross-sectional view of madreporite, stone canal, and associated features in a sea urchinJust for comparison, the madreporite of sea urchins Strongylocentrotus droebachiensis consists of 300-400 pore canals lined with cilia.  However, the pores are so small that, in total, they represent only about 0.1mm2.   Studies at Friday Harbor Laboratories, Washington using fluorescent microbeads (0.2µm dia) placed for 5d around these openings show inward movement of fluid at an estimated rate of 0.1ml per day for a 200g animal.  Most of the beads are caught up within the stone canal, but a few make it as far as the tube feet.  Is this enough of a flow rate to be functionally significant?  By blocking the madreporite photograph of a test of a sea urchin Strongylocentrotus droebachiensis showing madreporite and gonoporeswith cement, the author is able to show a significant decrease in mass over 27d and credits this to less food being present in the gut.  While diminished feeding could result from gradual loss of tube-feet activity through decrease in volume of the water-vascular system, as suggested by the author, it could also be an effect of the scraping and plugging treatment itself, especially if the madreporite functions in ways other than water-volume regulation.  Ferguson 1996 Biol Bull 191: 431.


Note that the madreporite in sea urchins is perforated by the duct
from one set of gonads and bears the corresponding gonopore. The
other 4 gonopores are visible. The large opening in a living animal is
covered by the fleshy periproct with the anus in its centre 1.3X

 
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