Reef Science: Development Highlights
Vestheim, Hege, and Stein Kaartvedt. Plasticity in coloration
as an antipredator strategy among zooplankton. Limnol. Oceanogr.,
51(4), 2006, 1931-1934.
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.
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
Steinke, Michael, Jacqueline Stefels, and Eize Stamhuis.
Dimethyl sulfide triggers search behavior in copepods. Limnol.
Oceanogr., 51(4), 2006, 1925-1930.
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.
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.