Predators & defenses
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Nudibranch molluscs are the principal predators of sponges, although many other invertebrates and fishes may eat them to one extent or another – either directly, or indirectly as a result of their foraging for prey that live within or near sponges. Shown below are just a few west-coast species that feed primarily on sponges. Defenses considered in this section include spicules, regeneration, and chemistry, all considered in this section.
 

series of photographs of sponge-eating nudibranchs: Diaulula sandiegensis

Diaulula sandiegensis 1X

series of photographs of sponge-eating nudibranchs: Doris ohdneri
Doris (Archidoris) odhneri 0.75X
series of photographs of sponge-eating nudibranchs: Anisodoris nobilis
Anisodoris nobilis 0.3X
series of photographs of sponge-eating nudibranchs: Acanthodoris nanaimoensis
Acanthodoris nanaimoensis 2X
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Research study 1
 
photograph of sponge-eating nudibranch Doris montereyensis nestled within its prey sponge Halichondria panicea The nudibranch Doris (Archidoris) montereyensis preys almost exclusively on the encrusting sponge Halichondria panicea. If the sponge cover is thick, the snail may be so deep within it as to form tunnels. The snail is an annual, while its prey is a perennial, and each year a new crop of Doris larvae must seek out settlement sites close to or directly on a sponge patch; failure to do so means death from starvation. While a good larval settlement of Doris may eventually cause both predator and prey to become locally extinct, large-scale stability of the system is conferred through the patchy distribution of the prey and the “chancey-ness” of successful settlement on it by the pelagic larvae of the predator. Knowlton & Highsmith 2000 Mar Ecol Progr Ser 197: 285.
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Spicules
 

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A sponge skeleton consists of an organic component (collagen protein) and mineral spicules. Spicules are either calcareous or siliceous and come in many varieties. Indeed, there are almost as many names for spicules as there are spicule types (over 50 at last count). Spicule morphology is an integral part of sponge classification.

   
 

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Research study 1

 
Spicules function for support as well as for defense. The texture of some sponges is less firm than others, and many types of spicules appear to function for internal support and for maintaining the integrity of the sponges cells and chambers. The difficulty, without knowing enough about sponge biology, is in knowing exactly what role a spicule is playing in a specific sponge species. What follows is some educated guesswork.
photograph of spicule array of a spongeSpicule array photographed from microscope slide. Magnification approx 50X photograph of a spicule array of a sponge with some possible supportive (structural) spicules highlighted in blue and possible defensive spicules highlighted in redSame array with some possible supportive (structural) spicules in blue and possible defensive spicules in red

Red spicules from left: oxea, regular triradiate, style

 

Blue spicules from left: C- shaped, serrated, unknown, acanthostyle (?), subtylostyle (?)

 

 

Some west-coast sponges, such as Halichondria panicea and Haliclona sp. have only a single type of spicule known as an oxea-type. Oxeas have 2 pointy ends and are likely for defense.

NOTE also known as a diactinal monaxon (2 rays=2 pointy ends, 1 axis)




Squashed Halichondria panicea showing spongin fibers, internal water
canals (clear areas) and oxea spicules. Some spicules are broken
1X



photo/drawing of Halichondria panicea showing spongin fibers and spicules
 

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Research study 2

 

Other sponges, such as Craniella sp. or Tetilla sp., have several different types of spicules. The photo is a section taken through the upper layers of Craniella and reveals tufts of oxea spicules (blueish colour on the right), a scattering of asters (or the tops of anatriaene spicules), and other monaxonic spicules (oxeas)throughout the body of the sponge. The monaxonic spicules are likely for defense; the others, for support.


photograph of a slice through a sponge Craniella, with an inset photo of the intact sponge
Tennis-ball sponge Craniella villosa. Slice taken through the
sponge near the top and a view of a live sponge. Slice magnification,
not known; whole sponge 0.2X.
Photo of slice courtesy Bill Austin, Victoria
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Research study 3
 
photograph of a sponge-eating nudibranch with several hypothetically good features listed for eating sponges

The spicules of a sponge appear to offer little obstacle to feeding by nudibranchs. In fact, the feces of a sponge-eating nudibranch mostly consists of undigested spicules. Given that the spicules of sponges are variously spikey, barbed, needle-like, and jagged, might we expect a nudibranch’s gut to have special modifications that would allow easy (painless) transit of the spicules?

Listed in the illustration are some of the modifications that might be expected to have evolved in sponge-eating species.

 

Now let’s see which, if any, of these modifications actually exist:

MOUTH - no special modifications: lips are soft, no crushing jaws, typical gastropod radula for rasping the tissues of the sponge.

GUT- no special modifications other than the spicules are shunted into the intestine without being directed into the digestive gland.

INTESTINE - yes, here we find some useful features. The intestine is mucousy and contains a special secretory area where spicules and other undigested material are packaged in mucus-covered bundles (fecal pellets). However, these features are common to all nudibranchs, not just spongivorous ones.

ANUS - no special modifications: the slippery fecal pellets are released from the anus and tumble off the snail’s body onto the sea floor

We have to conclude that a sponge-eating nudibranch’s gut is not particularly well adapted to eating its spiculey prey.

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  Regeneration
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Research study 1
 
graph showing regeneration times for wounds inflicted on a sponge Halichondria panicea Sponges readily regenerate wounds caused by predators. In that a predator, such as a nudibranch, generally crops non-lethally, then regeneration could be considered a kind of defense. Let the predator take its fill while the rest of the sponge goes about its business, and regenerate the missing bits later. For example, studies in Alaska on regeneration of experimental woundings (simulating a predatory attack by the nudibranch Doris montereyensis on Halichondria panicea) show about 70% recovery within the first month, and virtually complete recovery after 4mo. It is not known whether sponges containing zoochlorellae symbionts heal quicker than ones without, but it should be readily testable.  Knowlton & Highsmith 2005 J Exper Mar Biol Ecol 327: 36.
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Research study 2
 

photograph of boot sponge Rhabdocalyptus dawsoni with a photo of a potential predatory sea star Pteraster tesselatusBoot sponges Rhabdocalyptus dawsoni live at 30-60m depth and grow at about 2cm per year. An average-sized individual is estimated to be 35yr old, and larger specimens possibly as much as 200yr old. Experimental woundings of 5 cm2 in size take about 5mo to regenerate, but this rate is 40 times the normal growth rate. The sponges slough off their outer spicule coat during winter, perhaps as a way to rid themselves of accumulated debris. Leys & Lauzon 1998 J Exp Mar Biol Ecol 230: 111.

NOTE are there any predators of boot sponges? During 150 dives over a 5yr period, researchers in British Columbia observe only 5 predatory attacks, all by the slime star Pteraster tesselatus

 

 

 

Boot sponge Rhabdocalyptus dawsoni
with slime star Pteraster tesselatus
 

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Research study 3
 
photograph showing possible competition between 2 sponges Halichondria panicea

The crumb-of-bread sponge Halichondria panicea is another species that regularly cleans itself of debris and sediment. Studies in Germany show that its outer layer is sloughed off every 3 weeks at 15°C, and this may explain why this species generally remains free of fouling organisms. Barthel & Wolfrath 1989 Oecologia 78: 357.

 

 

Two individual sponges Halichondria panicea
appear to be competing for space on the rock 1X

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Chemistry
  The literature on west-coast sponge “chemistry” is huge and is well represented by investigations of nudibranchs and their sponge prey. Rather than splitting the presentation, go to LEARN ABOUT NUDIBRANCHS: DEFENSES: SECONDARY METABOLITES.
   
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Research study 1
 
photograph of a purple sponge Haliclona sp.Haliclona 1X photograph of a sponge Halichondria sp.Halichondria 1X Investigation into the “biological activity” of extracts of local sponges reveals considerable anti-bacterial and some anti-fungal properties. Strongest anti-bacterial activity is found in the purple sponge Haliclona, and less-strong activity in the crumb-of-bread sponge Halichondria, the boring sponge Cliona, and the tennis-ball sponge Tetilla (Craniella). Thompson et al. 1985 Mar Biol 88: 11.
photograph of a boring sponge Cliona sp.
Cliona 5X
Tetilla (Craniella) 0.2X NOTE the method involves soaking bits of sponges in alcohol (methanol) and then evaporating the solvent. The resulting powder is then used to test growth of bacteria and fungi in agar cultures
 
 
Research study 2
 

Sponges are rarely overgrown by other organisms and often have bare zones surrounding them, suggesting the presence and/or release of biologically photogaph of sponge Haliclona sp. competing with colonial tunicateactive metabolites. As a follow-up to Research Study 1 above, tests of naturally released exudates from healthy, undamaged sponges Aplysina fistularis from around La Jolla, California show behavioral effects on a limpet (foot retraction) and sea star (tube-foot retraction), inhibition of settlement and/or metamorphosis of a polychaete and gastropod, but are not toxic to 3 species of dorid nudibranchs known to eat sponges.  Active substances are likely the dibromotyrosine-derived metabolites aerothionin and homoairothionin.  Thompson 1985 Mar Biol 88: 23.

NOTE  the test sponges are collected and transported to the lab without exposure to air

NOTE  the author’s definition of an “active response” from the dorid nudibranchs to the exudates is death, but is it possible that they could exhibit more subtle reactions to the exudates (e.g., lack of feeding or inactivity) that are either not observed or not reported by the author?

Clean-looking sponge Haliclona sp. competing
chemically with a colonial tunicate 0.3X

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