HUNTING
WITH A MICROSCOPE, PART II
by ALF
JACOB NILSEN
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A
long time ago, back in 1978 at my friend’s room I saw my very first
marine aquarium.I did not realise it just then, but my friend’s aquarium was
very different from what was marine set ups those days.Instead, of the then, commonly used graveyard-white coral
skeletons, calcareous rocks were used as decoration. Instead of changing
the water regularly and cleaning the dead coral of all sorts of algae,
green thread algae were allowed to grow and flourish.The tank appeared green and exciting as the long green threads of
algae swung back and forth.But, most importantly, the fishes recovered completely from a
parasite infection that they had when bought in the local pet shop.
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In
this aquarium the green thread algae represented something new,
biologically as well as aesthetically.For the first time we discovered that some sort of
“biological balance” could be achieved in a closed aquarium
system, and that it was indeed possible to cure disease and
prevent the fishes from being further attacked by parasites.The solution was, however, not copper sulphate but the growth
of algae!
To-day
the “green aquarium,” overgrown by thread algae, is not loved
by many aquarists.Although
the algae have a positive influence on the well being of many
fishes, it tends to overgrow corals and other sessile
invertebrates.Let us
for a moment forget about this, and observe the general beauty and
the interesting reproductive biology of one of the algae that
appear as long green threads in the marine aquarium.We are referring to members of the genus Derbesia.
The
many species of Derbesia are members of green algae (division
Chlorophyta), where they belong to order Caulerpales, in the family
Derbeciaceae. The genus Derbesia seems to be cosmopolitic with
species found in cold, temperate and tropical waters.The most commonly mentioned species is Derbesia marina,
which is reported from Polynesia (Payri & al., 2000), from the
Caribbean (Littler & al, 1989), from the Mediterranean (Riedl, 1983)
and from cold waters, such as from the Atlantic, North-Sea, and along
the Norwegian coast (Nicholls, 1976).I have observed the algae myself in the Maldives and along the
coast of Norway, and as mentioned above, is found it to be very common
and often abundant in marine aquariums.
Figure
1 + 2:My first marine aquarium from 1979 was covered with green
thread algae.The
invertebrates, which at that time were rarely seen in the local
pet shops, were soon overgrown by the algae, but the fishes
remained healthy and recovered from parasite attacks and
infection they had when bought.
Figure
3: Growth of Derbesia in the shallow water, close to the
beach of a coral cay in the Maldives, at about two metres in
depth.
Figure
4: Growth of Derbesia from live rock in a reef aquarium.
Before
we continue, it is necessary to address the reproductive life history of
algae in general:
The
life history of all plants follows a single basic pattern where there is
an alteration between a haploid, gamete-producing phase (the gametophyte)
and a diploid, spore-producing phase (the sporophyte).In most plants one of the two is non-photosynthetic and cannot
grow independently from the other.In some groups - such as the ferns and many algae - the two
phases are so distinct, and independent of each other that the
connection between them would not be suspected if the fate of the
reproductive bodies that they each produce was not known.Very simplified the life history among algae can be separated
into three groups:
1)Those with a single morphological phase producing haploid or
diploid cells that develop into new plants.
2)Those with two morphological phases where a haploid gametophyte
produces gametes that fuses and grows to a sporophyte, which in turn
produces diploid spores.
3)Several red algae with three morphological phases involving a
haploid gametophyte, a diploid carposporophyte developing on female
gametophyte, and an independent diploid tetrasporophyte.
Derbesia
spp fall within group 2, while the red algae of the genus Ceramium
– commonly found in marine aquariums and which we deal with later on
in this article – belong to group 3.(Table 1 explains the terminology involved, while table 2
gives some details about which genera of algae that belong in the three
different groups).
Table
1 – Algae reproduction terminology
Word
Explanation
Haploid
A
cell, nucleus, or organism, having a single set of unpaired
chromosomes.
Diploid
A
cell, nucleus, or organism having two haploid sets of chromosomes;
two copies of each chromosomes are present, except (in some cases)
for the sex chromosomes.
Diplohaplont
An
organism characterized by alternating diploid and haploid
generations.
Sporophyte
Plants
with alternation of generations, the diploid plant that produces
haploid spores.
Gametophyte
An
individual plant, or a haploid generation of a plant exhibiting
alternating generations, that produces gametes.
Tetrasporophyte
A
diploid sporophyte (see above) producing spores in groups of four,
which are released from a tetrasporangium.
Carposporophyte
The
diploid sporophyte developing on the female gametophyte, producing
haploid or diploid spores.
Gametangia
Any
cell or organ that produces gametes.
Isomorphic
Identical
or similar in shape, structure, or appearance.
Heteromorphic
An
organism that has different forms during different seasons, during
different stages of its life cycle, or from generation to
generation.
Spores
A
single-celled or multi-cellular, asexual, reproductive or resting
body that is resistant to unfavorable environmental conditions and
that is capable of developing into an adult without fusion with
another cell when the environment is favorable.
Zoospores
Motile
spores that can move for instant by the means of flagella.
Gametes
A
mature haploid reproductive cell that unites with another such
cell of the opposite sex to form a diploid zygote. Typically sperm
and egg.
Antheriduim
Structure
on red algae gametophytes that produces male gametes
Oogonium
Structure
on red algae gametophytes that produces female gametes.
Table
2 – Life histories of marine algae.Modified from Dring, 1986.
Groups
1)Single morphological phase: somatic
cells haploid or diploid
Haploid
Diploid
Chlorophyta
Dunaliella
Codium,
Caulerpa, Udotea, Halimeda
Phaeophyta
None
Fucus,
Sargassum
Others
Dinoflagellates
Diatoms,
some dinoflagellates
2)
Two morphological phases:
haploid gametophyte and diploid
sporophyte
Isomorphic
Heteromorphic
Gametophyte
Sporophyte
Chlorophyta
Ulva,
Cladophora, Enteromorpha
Monostroma,
Acrosiphonia, Halicystis, Bryopsis
Codiolum,
Derbesia
Phaeophyta
Ectocarpus,
Dictyota
Scytosiphon,
Petalonia,
Laminarian gametophytes
Laminaria
Rhodophyta
-
Porphyra,
Bangia
Conchocelis
3)
Three morphological phases: haploid
gametophyte, diploid carposporophyte eveloping on female
gametophyte, and independent diploid tetrasporophyte
The
exciting thing is that although the filamentous growth of Derbesia
might look ugly and unpleasant, the life history of the slimy threads is
indeed interesting and highly observable in the marine aquarium if you
look at the algae threads through a microscope.
Figure
5: The thalli of Derbesia at 100X magnification on the
film plane.The
chloroplasts are clearly visible, but note that the green colour
is lacking from the tip of the individual thallus where the
thread grows.
Figure
6: The
growth tip of a Derbesia thread at high magnification
(1000X on the film plane).The cell wall, the individual chloroplasts and the water
filled vesicles are clearly visible.
Put
some Derbesia threads under your microscope and use 40 or 100X
magnification and you will easily see the thalli (the body of plants
that cannot be divided in root, stem or leaves) being composed of
delicate and rather beautiful threads that can grow to 10 cm in length.The green colour reflects the chlorophyll pigments that are
stored in chloroplasts, and even at this low magnification you can see
that the chloroplasts are scattered inside the threads with
liquid-filled vesicles (vacuoles) in between.Increase the magnification to 1000X and you will be able to see
the individual plastids and the cell wall, which is composted of mannan.Perhaps you can also spot some of the many nuclei?
Drawing-1:
THE REPRODUCTIVE CYCLE OF Derbesia(From
Hoek & al (1978) in Fosså & Nilsen (1996).
Derbesia
is a heteromorphic diplohaplont.This means that during a complete life cycle the algae
alters between a haploid and a diploid stage that appear
differently. The sporophyte is the algae threads (A), which
develop pear-shaped sporangia (B) that are attached to the algae
threads by a little stalk. Inside the sporangia, zoospores (C
& D) develop by the means of meiosis. About half of the
zoospores settle and develop to female gametophytes while the
other half settle and develop to male gametophytes (E & F).The gametophytes grow to about 5 mm height and are known as
Halicystis-stages.The
Halicystis contain chloroplasts and many nuclei and a large
vacuole in the centre. The fertile male gametophyte develops
light-green patches and male gametes are released through pores (E
& G).The female
gametophyte develops dark-green patches and releases female gametes
(F & H).The
gamete fuses to a zygote (I) that settles and grows to a new
sporophyte.The life
cycle is completed.
Figure
7: Halicystic
stages of Derbesia.Light green gametophytes grow from a live rock in
a reef aquarium.9X
magnification on the film plane.
Continue
to view the algae threads at low magnification, and it will not
be long until you discover pear-shaped outgrowths on the
threads.These are
sporangia and give you the first hint that you are approaching
the reproductive cycle of the green thread algae.Now it gets exciting!(Please
keep your eye on drawing 1 and text outlining the
reproductive cycles of this species).
The
first time I saw the sporangia myself, I did not realise what
this actually was.I
did, however, locate three of these structures that grew side by
side and placed them in the centre of the microscope’s field
of view.It was
clear that the three were in different stages of development as
the one in the middle was much lighter green that the other two
were coloured very dark green.The one to the right in the field of view had also its
cell membrane expanded more than the other two.And then, as the strong light from the microscope heated
the object viewed, this sporangium exploded releasing all its
zoospores in a few seconds.
Figure 8: The
sporophyte with sporangia as it appears through a binocular lens
at 6X magnification on the film plane.
Figure
9: The sporophyte and sporangia as seen through a microscope at
40X magnification on the film plane.Note the many epiphytic diatoms, a free living diatom,
and what are probably zoospores and small free-living
flagellates scattered all over the field of view.
Figure 10
+ 11: Three sporangia have grown from the thallus of a Derbesia
sporophyte.Note
that the middle sporangium is immature and light green while the
two others are mature and darker.The right sporangium has the cell membrane expanded.When the heat from the microscope’s lamp warms up the
object, the right sporangium explodes releasing the zoospores.100X on the film plane.
Luckily,
I managed to have it all in focus and was there to capture it all on
film.It was one of those
moments you do not forget. It can very well illustrate how much biology
a reef aquarium teach!
The
zoospores moved rapidly away with their circulating movements, and as I
looked at other samples I discovered that zoospores from Derbesia
were everywhere.
When
I later on examined the surface of some live rock pieces covered with
red calcareous algae, I discovered what I first believed to be
ball-shaped algae from the genus Valonia or Cyrtocaria,
but later on realised that what I actually found was the gametophytes of
Derbesia (known as Halicystis-stages, though resembling Valonia).
By
altering between a sexual and non-sexual stage the green thread algae
manage to spread most efficiently and at the same time secure genetic
variability.By using a
microscope you will be able to observe this interesting piece of biology
close up!
A
Red Algae
If
the life cycle of Derbesia seemed complex, those of red algae
(Rhodophyta) are even more complex. As Table 2 shows, many red algae
have life cycles that involve three different stages.Among these are members of the genus Ceramium, red
alga that typically appear in the marine aquarium.
Ceramium
is a diverse genus with cosmopolitan distribution.Most species are turf-like in colder waters most, and are
often found from the mid littoral zone and downwards.On the coral reef, some species of Ceramium are
members of the turf algae, commonly found on reef boulders and at
the base of corals.Payri & al. (2000) mention six species from French
Polynesia, while Ceramium rubrum is a common species along
the coasts of Europe and is listed in many publications (such as
Campbell, 1977).Littler & al. (1989) mention the genus from the Caribbean
and Brazil, while Abbott (1997) lists 6 species from Hawaii.
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A
typical life cycle of red algae is outlined in Drawing-2.In general, a typical life cycle is like this:A diploid tetrasporophyte develop sporangia and produces tetra
spores by meiosis.The tetra spores develop to male and female gametophytes,
which develop male and female gametangia that produce gametes.The male gametes are produced in structures called antheridia,
while female gametes are produced in oogonia, which often develop as a
sort of container or cavity on the gametophyte.The male gametes enter the oogonia where the fertilization
occurs.The fertilized egg
cell develops to a haploid carposporophyte, which is an almost invisible
parasitic plant living attached to the mother gametophyte. The
carpo-sporophyte does in turn produce carpospores that grows to a new
diploid tetrassporophyte.
Drawing-2:Outline
of a typical red algae life cycle.After Wallace & al. (1986)
in Fosså & Nilsen (1996).
Obviously
the reproductive cycle in red algae is very complex.Using a microscope I have been able to detect
carposporophytes of Ceramium as shown in figures 12 and 13.So far I have not been able to locate sporangia myself, but I
encourage aquarists to examine their red algae to possibly reveal more
of the red algae’s complex but interesting life cycle.
Figure
12: Close up of the carposporophyte on Ceramium.400X on the film plane. Do also note the shape of the
individual cells in the alga thallus.
Figure
13: Thalli and Carposporophytes of Ceramium. 100X on the
film plane.
Figure
14: Unknown green alga growing on the front glass of a small
reef tank.200X on
the film plane.
Figure
15: Zooxanthellae in close up.Sample from Euphyllia ancora.400X magnification on the film plane.
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In
the next part of this series we shall take a look at some of the
peculiar animals that you can find hunting with a microscope in your
reef aquarium.However, I cannot let you go just yet……
A
few years back, when I operated a small experimental aquarium (Nilsen,
1999), I spotted a strange patchy growth of green algae on the front
glass of my tank.I used the microscope and found a strange alga as shown in figure
14.Still
this alga remains unidentified, even to genus.Can anyone help identify it?
And
at the end: Do not forget that you can use the microscope to take a
closer look at the most important of all algae on the coral reef and in
the coral reef aquarium – the zooxanthellae.The symbiotic algae belong to the genus Symbiodinium.Traditionally only one single species was believed to exist (S.
microadriaticum), but to day we know that several species exist and
that the algae develop specific strains as a response to environmental
differences.
Cut
a tentacle of a broad polyp stony coral (such as Euphyllia sp.)
or cut a little piece of a soft coral branch.Squeeze the tissue gently and let the tissue fluid of the sample
spread on the microscope slide.Cover it and use your microscope.You will easily observe the tiny brown dinoflagellates that are
responsible for the growth and ecological success of the tropical giant
underwater structure called “the coral reef”.
CAMPBELL,
A.C. (1977) Planter og dyr I grunne farvann.Gyldendal Norsk Forlag, Oslo, Norway (in Norwegian). Original
edition: The
Hamlyn Guide to the Seashore and Shallow Seas of Britain and Europe.
The Hamlyn Publishing group Ltd. (1976), U.K.
DRING,
M. J.1986.The Biology of
Marine Plants. Edward Arnold, London, U.K.199 pp.
FOSSÅ,
S. A. & A. J. NILSEN (1996): The Modern Coral Reef Aquarium
vol. 1. 1. edition. Birgit Schmettkamp Verlag (BSV), Bornheim, Germany.
HOEK, C. vand den1984Algen. 2. Ausgabe. Thieme Verlag, Stuttgart, Gernany. 481
pp.
LILLTER,
D. S., M. M. LITTLER, K. E. BUCHER and J. N. NORRIS. (1989).Marine Plants of the Caribbean. Airelife Publishing
Ltd., Shrewsbury, U.K.
NILSEN,
A.J. (1999):A small
experimental Aquarium.Marine
Fish and Reef1(1):102-120.
PAYRI,
C, A de R. N’YEURT and J. OREMPULLER (2000).Algae of French Polynesia. Au Vent Des Iles, Tahiti (ISBN
2-909790-82-7).
RIEDL,
R. (ed.) (1983): Fauna und Flora des Mittelmeeres. Parey Verlag,
Hamburg-Berlin.
WALLACE,
R. A., J. L. KING & G. P. SANDERS (1986): Biology the science of
life. 2. Edition. Scott, Foresman and Company, USA.1217 pp.
