We have decided to begin a short column that reviews scientific articles of both recent developments and those that have been written that may be of interest to aquarists. We feel that one of the shortcomings of our hobby is the continued presence of anecdote for which information might be available to support one idea or another. Furthermore, for many well-meaning aquarists, access or even the ability to read such information may be very limited and may pose a distinct disadvantage to those desiring such information. We would greatly appreciate suggestions for topics any of you might want to know more about, and the submission of potentially interesting article citations that any of you may have read and would like to see mentioned in a future column. Please use the relevant reviewer's author forum to present such topics.

Coral Reef Science:  Development Highlights

Eric Borneman

This month I cover a few articles on herbivory and algae.

Miller MW, Hay ME, Miller SL, Malone D, Sotka EE, Szmant AM. 1999. Effects of nutrients versus herbivores on reef algae: a new method for manipulating nutrients on coral reefs. Limnology and Oceanography 44(8): 1847-1861.

Summary:

This article set a new standard for investigating the roles of nutrients and herbivores on algae growth on coral reefs. In the method, cinder blocks were used to create diffusible compartments filled with fertilizer. A carbonate substrate was placed on top of the cinder blocks and then the blocks were either totally or partly enclosed in chicken wire to allow large and small herbivores, or only small herbivores, access to the blocks which were either nutrient enriched (by addition of fertilizer) or not enriched (control).

Water drawn from the surface of the enriched blocks showed elevated levels of dissolved inorganic nitrogen and soluble reactive phosphorus compared to controls (1.1-3.1 v. 0.3-0.9 TIN, and 0.11-0.14 v. 0.01-0.03 SRP, in mmoles/liter). Thus, it effectively produced levels comparable to naturally eutrophied water.

Algal response to elevated nutrients with exclusion of large herbivores showed that only one type of cyanobacteria increased in response to enriched nutrients. However, herbivore exclusion treatments had large effects. Turf algae and frondose macroalgae both increased when herbivores were excluded, and coralline algae decreased. It was found that increasing nutrients did not cause more algae to grow, but resulted in more nutrient rich algae that were, incidentally, grazed at a faster rate (thought to be a result of them being more nutritious to herbivores preferentially seeking and requiring nitrogen rich material). It appeared, from this article, that algae are not nutrient limited on coral reefs and the primary control of algae on reefs is herbivory, not nutrients.

Note: the results of this study have been supported many times since it was published, and it is becoming widely accepted that the primary control of algae is the rate of herbivory and not nutrient enrichment (termed "top-down" control rather than "bottom-up").

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Belliveau Stephanie A., Paul Valerie J. 2002. Effects of herbivory and nutrients on the early colonization of crustose coralline algae and fleshy algae. Marine Ecology Progress Series 232: 105-114.

Summary:

This study is one of many that support the findings of the preceding findings by Miller et al. (1999). Once again, fertilizer was used to enrich tiles either exposed to or excluded from herbivores. The tiles were examined after a period of time for percentage cover of coralline algae, fleshy algae, sediment load and coral recruitment. Results showed that fleshy algae biomass and sediments (correlated with the presence of sediment trapping turf algae) were highest on tiles excluded from herbivores. Nutrients did not have a significant effect on macroalgae, and had a slight positive effect on coralline algae. Coral settlement rates were too low to measure during the experimental study (although other studies have shown coral recruitment to be correlated to the amount of coralline algae covered substrate available). Herbivory was found to exert primary control on algal cover, even in the presence of increased nutrients.

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Taylor Richard B., Sotka Erik, Hay Mark E. 2002. Tissue-specific induction of herbivore resistance: seaweed response to amphipod grazing. Oecologia 132: 68-76.

Summary:

This paper examines the role of defensive metabolite composition in algae. Plant tissues vary greatly in the amount of permanent and inducible defenses, with it being thought that defenses occur mainly in areas of the plant most at risk of being grazed. This study used the brown algae, Sargassum filipendula and the amphipod herbivore, Ampithoe longimana to examine plant-induced defenses that occur from grazing. The most important parts of the plant, the bases that attach the plant to the seafloor, were defended from grazing primarily by their toughness and permanent resistance to grazing. However, the areas of new growth that are the preferred tissues of amphipod grazing, were stimulated by grazing to produce defensive chemicals that deterred grazing by making the tissues more unpalatable. This study confirmed that some algae are able to locally ramp up production of feeding deterrent chemical metabolites in response to grazing and this has also been found for Dictyota and Padina species in the tropics. It has also been suggested that primary chemical defenses, at least in algae, are a response to smaller grazers (mesograzers) and not macrograzers (fish, urchins).

Invertebrate Tidbits

Ronald L. Shimek, Ph. D.

It's A Snap,

This month, I will discuss some interesting articles on snapping shrimp that have appeared over the last couple of years.

Versluis-Michel; Schmitz-Barbara; von-der-Heydt-Anna; Lohse-Detlef , 2000, How snapping shrimp snap: Through cavitating bubbles. Science: 289 (5487): 2114-2117.

Summary:

We all know that snapping shrimp snap. It has been assumed that the snap was caused by a mechanical snap of one part of the shrimp's exoskeleton against another or by a release of a mechanically loaded skeletal string. The authors found the actual reason for the snap to be quite different. Looking at the snapping shrimp, Alpheus heterochaelis, they found the loud snapping sound was caused by an extremely rapid closure of its snapper claw. During the rapid snapper claw closure, a high-velocity water jet is emitted from the claw with a speed exceeding cavitation conditions. Hydrophone measurements in conjunction with time-controlled high-speed imaging of the claw closure demonstrate that the sound is emitted at the cavitation bubble collapse and not on claw closure. One of the effects of the snapping is to produce sound loud enough to stun or kill prey animals. The claw closure itself is silent.

And that ain't all…Those cavitation bubbles flash like flashbulbs!

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Lohse, Delef, Barbara Schmitz and Michel Versluis. 2001. Snapping shrimp make flashing bubbles, Nature 413, 477 - 478

Summary:

The authors found that the cavitation bubbles created by shrimp in stunning their prey have some surprising properties. As the paper above indicated, the sound created by snapping shrimp originates from the rapid and violent collapse of a large cavitation bubble generated under the tensile forces of a high-velocity water jet formed when the shrimp's snapper-claw snaps shut. As this bubble collapses, a short, intense flash of light is emitted. This means that inside the collapsing bubble there are extremely high pressures and temperatures of at least 5,000° K (about 8,000 °F).

The authors were the first people to observe the flashing and they named it "shrimpoluminescence."' This is also the first observation of this mode of light production by any animal. It is similar to sonoluminescence, the light emission from a bubble created by ultrasound. During their investigations they positioned a snapping shrimp (Alpheus heterochaelis) in a seawater aquarium maintained at 20 °C and evoked a snap by gently stroking its snapper claw with a paintbrush. The sound pressure was recorded using a hydrophone in close proximity (1-3 cm) to the imploding bubble. The light emitted during the collapse was detected using a calibrated photodetector, and all experiments were carried out in complete darkness. The flash duration is extremely short, less than 10 billionths of a second. Only about 50,000 photons are produced by the hot bubble interior, and consequently the light is too dim to be seen with the unaided eye. Through the examination of two different shrimp, they found no correlation between sound and light intensity, so loud and soft pops both produced light flashes. There doesn't appear to be any biological significance to the flashes, rather they seem to be simple byproducts of the snapping noise. However, the light emission certainly indicates the extreme conditions inside the bubble at collapse and, therefore, demonstrates the violence of the event.

And finally, one last snap and flash…

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Duffy, J. Emmett and Kenneth S. Macdonald. 1999. Colony structure of the social snapping shrimp Synalpheus filidigitus in Belize. Journal of Crustacean Biology. 19: 283-292.

Summary:

In an investigation of some snapping shrimp living in a sponge, these researchers found evidence of a second case of a true social hierarchy, similar to that found in social insects. The social insects, such as ants and bees, have highly organized colonies based around a single reproductive female, and a "caste system" for the partitioning of labor within the nest. These snapping shrimp, Synalpheus filidigitus, live as colonies of 30 to about 120 individuals within large sponges in the Caribbean. The colonies were produced and dominated by a single large reproductive female who was over twice the size of other females in the colony. Such a colony structure had previously been demonstrated for another snapping shrimp, Synalpheus regalis.

In addition to the interesting symbiotic behavior shown by some snapping or pistol shrimps, we now have the opportunity to observe, and possibly maintain true social crustaceans. Presently, it is unlikely we could maintain these in hobbyist sized tanks; the sponges are too large and require specific conditions we can't provide. Nonetheless, such maintenance may be a possibility in the near future.