Body Structure & Water Flow
 


The sponges that we see commonly in tidepools or under rocks at low tide contain a highly vacuolated rubbery protein material known as spongin that provides support for a variety of different types of cells, some of which are scattered loosely, others of which are arranged in layers or epithelia.  No tissues or organs are present, nor are there nerves or sensory organs.  Spicules, in a variety of shapes, provide protection and structural support.

NOTE  in a recent overview of cellular organisation and  integration in sponges, scientists at the University of Alberta and University of California, Berkeley propose that sponges actually have at least 6 differentiated epithelia, and that these function like, and are homologous to, the epithelia of “higher” animals.  Although evidence to justify such beliefs is thus far lacking, with respect to the first idea, the authors forecast that distinct physiological roles will eventually be identified.  Leys et al. 2009 Integr Comp Biol 49: 167.   

Haliclona permollisHaliclona permollis showing multiple chimneys of a single individual sponge 2X spicules under microscope
Miscellaneous spicules photographed from a microscope slide 8X
  drawing of a choanocyte chamber of a spongeSeawater is pumped continuouslythrough a sponge by the beating activity of flagella on choanocytes that line the internal cavities.  Water is drawn into the sponge through many small intake openings (also known as inhalent or incurrent pores) located on the outside surface and is released from exhalent openings on chimneys and vented away.

NOTE lit. “funnel cells” G.

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Research study 1
 

Demosponges have either a solid type of construction, such as in the branching red sponge Clathria prolifera, or a hollow type of construction, such as in Haliclona permolis and Halichondria panicea. In the first type, Clathria, water flow is in and out basically through the same tissue mass; thus, the flows are opposing.  In the second type, Haliclona, water flow is unidirectional, entering through myriad ostia on the outer surface and exiting through large oscula (chimneys). Interestingly, despite the difference in construction the two types have similar density of choanocyte chambers (1-1.8x107 chambers . cm3) and number of choanocytes per chamber (57-95).  Reiswig 1975 J Morph 145: 493.

Clathria water flow
Water flows (blue arrows) in Clathria oppose one another
water flow in Haliclona
Water flow in Haliclona is unidirectional

NOTE formerly Microciona prolifera, introduced from the Atlantic coast

NOTE it is now uncertain what the species name is for this common west-coast Haliclona; however, permolis is retained here because because many authors continue to use it to describe their specimens

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Research study 2
 

drawing of a flagellated chamber in an hexactinellid sponge Rhabdocalyptus dawsoniHexactinellid or glass sponges, such as Rhabdocalyptus dawsoni, live in deep water in British Columbia, a habitat possibly favoured owing to the fragility of their body structures. The major component of the body of an hexactinellid is a multinucleated syncytium, called the trabecular syncytium.It connects through cytoplasmic bridges to various cells in the sponge, such as choanocytes and archaeocytes. The origin of the syncytium is from fusion of early embryonic cells. The syncytium is cytoplasmic, lacks cell walls, possesses multiple nuclei, and extends through the entire body of the sponge.

 

 

 

drawing showing cytoplasmic streaming within the cyncytium of an hexactinellid sponge Rhabdocalyptus dawsoniIt is the largest example of a syncytium known in the animal kingdom. Cytoplasm within the syncytium flows bidirectionally. If a cytoplasmic stream is impeded, as by a cut (drawing on left: 1) the flow temporarily reverses itself (2-3) until communication is re-established with the original cytoplasmic track (4-5). The author proposes that food products may be distributed through the sponge via the syncytium and not via cellular transport as in other sponges. Evidence that the embryos are cellular until gastrulation suggests to the author that hexactinellid sponges may have evolved from cellular sponges and that a syncytial construction may not be a primitive trait.  Leys 1995 Biol Bull 188: 241; Leys 2003 Integr Comp Biol 43: 19.

NOTE the hexactinellid sponge Rhabdocalyptus dawsoni is described as having a “flimsy” construction consisting of thin, perforated sheets and filamentous strands draped around a scaffolding of spicules, much like a three-dimensional cobweb. The strands are the ramifications of the trabecular syncytium. The syncytium makes up 75% of the organic matter in the sponge.  Leys & Mackie 1997 Nature 387: 29; for a detailed description of the histology of R. dawsoni see Mackie & Singla 1983 Phil Trans Roy Soc Lond B 301: 365.

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Research study 3
 

In Rhabdocalyptus dawsoni large (1.3mm diameter), highly branched incurrent canals carry water into the sponge and large but less branched excurrent canals carry water out of the sponge.  Along the excurrent canals are many small (60µm in length) flagellated chambers.  The syncytium or trabecular reticulum that makes up the bulk of the sponge is bilayered.  One layer, the primary reticulum, encloses and supports the collar bodies and the cells that produce them (the choanoblasts), while another layer, the secondary reticulum, branches from the primary reticulum and forms a kind of barrier around the collars of the collar bodies.  Nuclei are scattered within the two retucula. Water is drawn through openings, the drawing of water flow through a bilayered trabecular reticulum of an hexactinellid sponge Rhabdocalyptus dawsoniprosopyles, and moves through the microvilli of the collar bodies and thence into the excurrent canals to the outside.  Leys 1999 Invert Biol 118: 221.

NOTE   the author describes the trabecular reticulum as “multinucleate amoeba strung out as a cobweb”

NOTE   because the flagellated cells in hexactinellids lack nuclei, the name collar bodies is applied to them.  The cells producing them, the choanoblasts, are nucleated

Orientation of bilayered trabecular reticulum in a flagellated chamber of an
hexactinellid sponge. The primary reticulum supports the collar bodies, while the
secondary reticulum is perforated to allow the collars to poke through. Water
flow from the incurrent canal mainly passes through openings, prosopyles, and
is drawn through the microvilli collars of the collar bodies as shown by the
blue arrow on the Right. Some water passes directly through the prosopyles
into the flagellated chamber as shown by the blue arrow on the Left

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Research study 4
 

drawings of cytoplasmic streaming in an hexactinellid sponge Rhabdocalyptus dawsoniCytoplasmic streaming can best be seen in a hexactinellid sponge like Rhabdocalyptus dawsoni by growing a small piece between microscope coverslips. The studies are done in Victoria, British Columbia on samples collected in Saanich Inlet and Barkley Sound, British Columbia. Within a few hours of preparation, growth is 2-dimensional and cytoplasmic streaming is evident. Flows are bidirectional at about 2µm . sec-1. Note in the figures the increased amount of streaming and regularity of pattern from 2:40h to 3:10h after preparation of the culture. Note also the formation of a tissue bridge between the two adjacent cytoplasmic masses. By attaching polyclonal antibodies bearing fluorescent particles selectively to tubulin, and by use of microtubule inhibitors, the authors determine that the streams flow along the tracks of microtubules abundantly distributed through the cytoplasm. The authors propose that streaming is part of maintenance and growth, and an essential part of regeneration.  Leys & Mackie 1994 p. 417 In Sponges in space and time (van Soest, ed.) AA Balkema, Rotterdam

  photograph of a boot sponge Rhabdocalyptus dawsoni
A boot sponge Rhabdocalyptus dawsoni in Barkley Sound, British Columbia, tagged for growth studies. A small wolf eel Anarrhichthys ocellatus is resting in the sponge's osculum 0.2X.
Photo courtesy Sally Leys, University of Alberta, Edmonton.
 
Research study 5
  Sponges lack nerves, but studies on cloud sponges Rhabdocalyptus dawsoni  disclose a mechanism by which electrical signals are propagated through the sponge.  The signals are conducted mostly within the unique syncytium found in these and other “glass” sponges.  The significance of the impulse transmission is that within about 30sec of its initiation, all pumping stops in the sponge.  Since stoppage can be induced experimentally by adding particulate material to the water near a cloud sponge, it is thought to be a mechanism, present in other filter-feeding organisms such as sea squirts and mussels, to prevent or minimise the entry of unsuitable material into the sponge’s internal filtering chambers. The sponge will start and stop its pumping several times, as though testing for water quality.  As pumping may commence on sides of the sponge not affected by particulates, there may also be a kind of “back-flushing” involved as well.  The authors note that theirs is the first report of electrical signalling in a sponge.  Leys & Mackie 1997 Nature 387: 29; Leys et al. 1999 J Exp Biol 202: 1139: see review by Leys & Meech 2006 Can J Zool, Lond 84: 288; see also Lawn et al 1981 Science 211: 1169 and Mackie et al 1983 Phil Trans Roy Soc Lond B 301: 401.
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Research study 6
 

Seawater flowing over oscula draw water out of themIn addition to the flow caused by flagellar beating, water is passively drawn out of the chimneys by currents of seawater flowing overtop. This is an example of Bernoulli’s principle, where the passage of the water current creates a negative pressure.  It is calculated that this passive flow can equal or surpass the active pumping flow even at low current velocities (e.g., as low as 6cm . sec-1)Vogel 1974 Biol Bull 147: 443; Vogel 1977 Proc Natl Acad Sci USA 74: 2069. Photo courtesy Sally Leys, University of Alberta, Edmonton.

Seawater flowing over this sponge's oscula will tend to draw
water out of them and thus enhance water flow through the sponge

 
Research study 7
 

schematic showing effects of mechanical stimulus and sediment application on pumping in hexactinellid spongesA laboratory study at the Bamfield Marine Sciences Centre, British Columbia confirms that hexatinellid sponges Rhabdocalyptus dawsoni and Aphrocallistes vastus arrest pumping in response to mechanical stimuli and sediment, doing so by propagation of electrical signals through their syncytial layers. The signals are thought to be generated by membrane depolarisation following contact with sediment or mechanical stimulus such as a glass probe, leading to calcium influx into the choanocytes accompanied by cessation of beating. 

photograph of in situ application of fluorescein dye to a cloud sponge Aphrocallistes vastus with subsequent out-pumping via the osculumThe two species differ in their sensitivity to suspended sediment (<25µm), with pumping (confirmed by fluoresceine dye) generally resuming immediately in A. vastus, but only after sometimes prolonged periods in R. dawsoni. Greater than 4h duration exposure to sediments causes gradual reduction in pumping in both species, with recovery taking up to 25h.  During recovery, both species exhibit frequent arrests, and these arrests have a precise periodicity indicative of some sort of pacemaker involvement.  The authors suggest that the different pumping patterns in the 2 species may reflect specialisations for coping with different sediment loads, although what the ecological implications of these might be are not known.  In situ observations show the sponges will stop pumping in the presence of SCUBA-divers, but natural stimuli are likely to be fishes and sediment-disturbance by fishes. Tompkins-MacDonald & Leys 2008 Mar Biol 154: 973.

NOTE  the sponges are collected from locations in Barkley Sound, British Columbia either by SCUBA for boot sponges R. dawsoni (30m depth) or by special manipulator arms on a remotely operated submersible (Canadian Scientific Submersible Facility, Sydney, British Columbia) for cloud sponges A. vastus (160m depth).  The sponges are transported in water collected from depth to special holding facilities at the marine lab

NOTE  the sediment is collected along with the sponges and stored for later use in these experiments.  Before use the sediment is filtered to <25 µm

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