Defenses
 
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Seaweeds hot buttons for seaweed defenses part of BCCR hot button for structural defenses of seaweeds part of BCCR hot button for toxic chemicals of seaweeds part of BCCR hot button for growth strategies of seaweeds part of BCCR
Seaweed defenses relating to toxic chemicals are considered here, while those relating to STRUCTUE and GROWTH STRATEGIES can be accessed via the icons.
 
 

Toxic chemicals

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of seaweeds on a Caribbean reef "Lots of seaweeds have structural defenses, but lots of them also contain chemicals that are bad-tasting to herbivores. " - St. Thomas 2008
 
 

photograph of red alga Laurencia sp.
To give an idea of the potential level of chemical defeses in a seaweed, and also of our lack of knowledge of the precise functions of such chemicals, species of red algae Laurencia collectively have over 300 different secondary metabolites, most of which are terpenoids. The function is known or surmised for only a few.

NOTE chemicals in an organism that have no known function but are commonly thought to be involved in defense

NOTE a large class of naturally occurring chemical substances built on a 5-carbon molecular structure. Turpenoids are highly aromatic (e.g., ginger, cinnamon, cloves). The anticancer drug taxol is a terpenoid

 

 

 

Red alga Laurencia sp. 1X

 

photograph of green alga Halimeda in close viewSpecies of green algae Halimeda are doubly protected. Heavy calcification, up to 70-90% of their dry mass, makes them hard to ingest, and chemical defenses, most notably halogenated terpenoids, make them unpalatable to herbivorous fishes, although apparently not to sea urchins who readily eat them. Paul & van Alstyne 1988 Proc 6th Int Coral Reef Symp3: 133.

NOTE presence of bromine in different chemical forms in seaweeds (other halogens are fluorine, chlorine, and iodine) is commonly associated with increased chemical activity and toxicity of the compounds


Green alga Halimeda copiosa with what appears to be bites taken from two
of the segments on the left-hand side. Note also what appear to be scratchings
on the uppermost segment which, like the bites, reveal the white calcium-
carbonate skeletal structure. The first could have been caused by a sea
urchin, but probably not the second, unless it was quite a tiny individual 2X

 

Here is a list of possible hypotheses generated by this observation, some worthy of further experimentation, others not. Take a moment to consider the value of each for further testing, then check the second list below for some other opinions. Ideas from Paul & van Alstyne 1988 Proc 6th Int Coral Reef Symp 3: 133.

1. Halimeda is not eaten by the fishes because they are more sensitive to Halimeda chemicals than are sea urchins.
2. Fishes don't like green-coloured food.
3. Halimeda is hard for fishes to digest, but not for sea urchins.
4. The tough holdfast of Halimeda makes it difficult for the fishes to pull up the plant to eat it, while sea urchins nibble from the top.
5. The fishes actually would like to eat Halimeda, but they can't see green wavelengths.
6. Halimeda has low nutritional value for fishes but not for sea urchins.
7. Calcium carbonate is toxic to fishes but not to sea urchins.

   
  Now for the merits of testing these hypotheses:
 

1. Yes. This is inferred by the observations and would be interesting to follow up with further experiments, if not done already.
2. No. This is not worth pursuing. Herbivorous fishes commonly eat other green-coloured algae, so colour alone is unlikely to be involved.
3. Yes. Although we think we know this already because of the high content of calcium carbonate in Halimeda, it might be worth investigating from the standpoint of possible differences in digestibility of Halimeda between fishes and sea urchins.
4. No. This is not worth investigating for the reason that herbivorous fishes commonly crop algae without destroying the holdfast. Such a strategy is beneficial to both "players", in that it allows the plant to re-grow, while at the same time producing a steady supply of food for the fishes.
5. No. This would be really NOT worth pursuing. Not only can fishes see green colours, but many species eat green algae, and at least some of these are identified by sight.
6. Yes. This would be a good research project. Even though it is widely assumed that Halimeda is not a good food for fishes, this is based not on a knowledge of nutritional content, but on knowledge of the alga's high content of calcium salts and on the presence of terpenoids. A better question is: might some herbivorous fishes be tolerant or resistant to the terpenoids, but still find Halimeda deficient in required nutrients? Also, what about sea urchins? Can they maintain good health and growth on a strict diet of Halimeda?
7. No. This would not be a productive line of research. Calcium carbonate is generally harmless to animals unless, of course, it is eaten to the total exclusion of any other food.

 
 

Most work on chemical defenses in tropical seaweeds seems to have been done on species of Halimeda. For example, in vitro studies on terpenoids extracted from Halimeda species reveal that the chemicals have diverse biological activities. One of these is antibacterial, specifically, against Staphlococcus aureus and Vibrio spp., suggesting a possible disease-resisting role. Another activity relates to toxicity to damselfishes of 1h photograph of green alga Halimeda growing on sand in Bonaireexposure to physiological levels of terpenoid extracts. Damselfishes are herbivores and eat algae cultivated in their gardens. Some terpenoid extracts are known to be toxic to marine and terrestrial fungi, and fungal-related diseases are fairly common in coral-reef ecosystems. Another finding with survival implications for Halimeda is that 13 species of herbivorous fishes are deterred from eating test pellets containing natural concentrations of terpenoid extracts . Finally, larvae of sea urchins are killed after 1h exposure to physiological levels of the terpenoid extracts, a finding that also extends to survival benefits for Halimeda. Paul 1985 Proc 5th Int Coral Reef Symp 5: 39.

NOTE tests done outside of the living system as, for example, in dishes in the laboratory. In contrast, in vivo tests are done within the organism

NOTE concentrations occurring naturally in the seaweed

 

Halimeda sp. growing on sand in Bonaire

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs photograph of Caribbean reef

"There's mostly herbivorous fishes in this area of the reef...parrotfishes and blue tangs. There may seem to be a lot of algae for them but, in fact, this species of brown alga contains chemicals which are thought to ward off feeding by fishes." - Little Cayman 2002

NOTE Dictyopteris delicatula

 
 

photograph of brown alga Dictyopteris delicatula courtesy Mark & Diane Littler, Smithsonian Institution
We've seen the broad-spectrum chemical potency of the green alga Halimeda, but many brown and especially red tropical algae also have considerable protective chemistry. For example, studies on the brown alga Dictyopteris delicatula in the Grenadines reveal that special chemicals known as dictyopterenes partially deter feeding by parrotfishes and surgeonfishes, but do not seem to affect amphipod crustaceans that commonly inhabit and consume the seaweed. Hay et al. Mar Ecol Progr Ser 48: 185. Photograph of Dictyopteris courtesy Mark & Diane Littler, Smithsonian Institution, Washington.

NOTE these are also of interest because of potential insecticidal properties

 

Brown alga Dictyopteris delicatula (1X) with an amphipod
of the type that eats brown seaweeds superimposed (5X)

 

photograph of brown alga Padina sp.Another observation relating to these and related brown seaweeds is that otherwise palatable algae may survive better if they grow near to species that are chemically defended. Hay 1991 p. 96 In, The ecology of fishes on coral reefs (Sale, ed.) Academic Press, San Diego.

photograph of brown alga Stypopodium sp.

Filamentous red and green algae grow in abundance near the chemically defended brown alga Padina sp. 0.5X

 

 





The brown alga Stypopodium zonale is known to be
rich in herbivore-deterrent secondary metabolites,
and plants growing nearby may benefit 1X

 
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