Coral Reef Science:  Development Highlights

Habib Sekha

Vestheim, Hege, and Stein Kaartvedt. Plasticity in coloration as an antipredator strategy among zooplankton. Limnol. Oceanogr., 51(4), 2006, 1931-1934.

Abstract:

We show that marine zooplankton change their level of coloration both with depth and time of the day. The carnivorous copepod Pareuchaeta norvegica caught near the bottom in 200-400m deep-water columns were darker than specimens caught higher in the water column. A diel rhythm in coloration occurred even at several hundred meters' depth, with individuals caught during night time being more pigmented than the ones caught during the day. We hypothesize that individuals actively adjust their degree of coloration to achieve optimal camouflage at the prevailing light regime.

Comments:

Some hobbyists are suffering from "red bugs," Tegastes acroporanus, a copepod parasite that consumes Acropora coral tissue. So far there are no confirmed reports of predators of this red bug. I have often wondered why they are red. Is it because of their diet? Do they really need the red-colored substances or their precursors? The above abstract immediately reminded me of the red bugs. Perhaps they are red due to their diet and because many marine fishes' eyes are not sensitive to red colors, they might appear invisible or as grey spots? Perhaps this is a form of camouflage from predation by fish?

Sure, this is just speculation but nevertheless, in my opinion, an interesting one. At least judging from the above abstract, some copepods do seem to adjust their pigmentation as a camouflage technique. If the lack of predators is due to their camouflage, then perhaps with some luck and, depending on the spectrum of their red coloration and other substances in the red bug, light of a certain wavelength might make the copepods visible to fish.

Steinke, Michael, Jacqueline Stefels, and Eize Stamhuis. Dimethyl sulfide triggers search behavior in copepods. Limnol. Oceanogr., 51(4), 2006, 1925-1930.

Abstract:

The oceans are nutritionally dilute, and finding food is a major challenge for many zooplanktonic predators. Chemodetection is necessary for successful prey capture, but little is known about the infochemicals involved in the interaction between herbivorous copepods and their phytoplankton prey. We used females of Temora longicornis to investigate chemodetection of dimethyl sulfide (DMS) in this calanoid copepod and quantified its behavioral response to plumes of DMS using video-microscopy in combination with laser-sheet particle image velocimetry (PIV). Slow injection of a 1-µmol L-1 DMS plume into the feeding current resulted in a characteristic behavioral pattern ("tailflapping"), a redirection of flow equivalent to 30% of the average current velocity, and changes in the location of flow-induced vortices. In free-swimming individuals, this likely results in somersault-type movements that are associated with search behavior in copepods. In comparison to seawater controls, DMS injections significantly increased the average number of tail-flaps per copepod during the first 2 seconds after exposure to DMS gradients. Our results demonstrate that copepods can detect and react to plumes of DMS and suggest that this biogenic trace gas can influence the structure and function of pelagic foodwebs.

Comments:

Plumes of chemicals released by organisms can attract predators but may also deter them depending on the chemical and the organism involved. The above abstract shows that copepods sense dimethyl sulfide (DMS), which is a chemical released under certain circumstances by phytoplankton. They sense the DMS, which directs them to their food source.

Phytoplankton produce DMSP which, when degraded, forms DMS. Some phytoplankton can degrade their DMSP into DMS by the use of an enzyme when their cell becomes damaged or by other physical stress such as water turbulence. It is, therefore, believed that DMS production is a mechanism to deter herbivores. However, copepods seem to use that as a chemical cue to find the phytoplankton which is a part of their diet.

Supplements primarily containing phytoplankton as a food source can be an indirect food source for corals by serving as food for copepods which, in turn, can be captured and consumed by corals. However, if copepods need DMS to sense and find phytoplankton, and if DMS is the only chemical that would allow them to do so, then it makes me wonder to what extent the bottled phytoplankton products can produce DMS. Perhaps most, if not all, of the DMSP is already converted to DMS and would no longer cause a plume to direct the copepods to it, or the enzyme could have become inactive so no direct production of DMS could take place. That is, copepods might not be able to find such quality phytoplankton and hence be unable to consume it. Despite that, some bacteria excrete the same enzyme, but the phytoplankton might already be the victim of such bacteria.

DMS in high concentration has a garlic-like odor, but when found in low concentrations it can smell fruit-like and can contribute to the smell of the sea. Similar odors can be found in some supplements such as bottled phytoplankton, in which DMS was set free.