Mazzaella flaccida (was Iridaea) is Isomorphic
A Terrific Triphasic You Can Even Tell Apart
The genus Iridaea belongs
to the Division Rhodophyta and has all of the characteristics of a good
red alga. Just like the other members of this phylum, Iridaea uses
Floridaean Starch as its carbohydrate storage product. Its major photosynthetic
pigments include chlorophylls a and d, cartenoids, zeaxanthins, and the
water soluble phycobiliproteins, R phycocyanin, allophycocyanin, and phycoerythrin.
These later pigments serve as light absorbing antennae which aid in electron
transport to the chlorophyll a.1
Within the Division Rhodophyta, Iridaea stands in the Subclass Florideophycidae. Accordingly, Iridaean cells are multinucleate (except apical or reproductive cells), and each have several to many small, discoid chloroplasts. Division is apical and the plant is multicellular. Pit connections commonly connect cells. A pit connection allows a cytoplasmic strand to connect two adjoining cells through a "pit" in their cell walls (See Glossary). Iridaea has a triphasic life history with no flagellated stages and oogamous sexual reproduction.1
Iridaea belongs to the Family Gigartinaceae because of its cruciate tetraspores and
three-celled, procarpial carpogonial branch, and multiaxial growth (See
Figure 3). An alga is considered procarpial when its auxiliary cell and
carpogonium lie in close proximity on the same branch system. The types
of carrageenan in several genera of this family are specific to certain
life history phases. In both Chondrus and Iridaea, kappa-carrageenan
appears in the gametophytes while
all diploid stages (carposporophyte and tetrasporophyte tissue) possess
lambda-carrageenan.1 Kappa-carrageenan and lambda-carrageenan
can be chemically identified using the resorcinol method; this permits
identification of a plant's life history phase
before it becomes reproductively mature.2
Post-fertilization development places Iridaea in the Order Gigartinales (See Life History for diagrams). Among the Gigartinales, the auxiliary cell is an ordinary vegetative filament present before zygote formation. Fertilization proceeds as expected. After the spherical, colorless, nonmotile spermatium arrives at the carpogonium's hairlike trichogyne and fertilization occurs, the carpogonium's zygote migrates to the auxiliary cell. After transfer, gonimoblast fibers develop from the auxiliary cell and bear red carpospores. Iridaea also has the Gigartinalean features of a filamentous medulla, definitely nonparenchymatous in appearance. The isomorphic Gigartinales grow from perennial crusts and have diplobionic life histories in which both the gametophyte and the tetrasporophyte are free living phases.
Two Former Gigartinales:
research has resulted in rearrangements within Iridaea's family
and many people now refer to this alga as Mazzaella. New understanding
of development of the carpogonium from
the gonimoblast fibers has apparently forced the association of I. flaccida (but
not I. cordata) with the genus Mazzaella.3 According
to Hommersand et al., the inward direction of gonimoblast fiber formation
from the auxiliary cell separates Mazzaella from all other genera
except Chondrus (including the remaining, non-reclassified species
of Iridaea). Chondrus differs from the Mazzaella in
that its gonimoblast fibers have secondary pit connections (see Figure
4) to medullary cells. Mazzaella links its gonimoblast fibers
to medullary filaments through "terminal tubular cells". Moreover, Chondrus forms
its tetraspores from medullary
filaments while Mazzaella transforms cells in its cortex into
Phycologists disagree on which species of Iridaea should be reclassified as Mazzaella. Most recently, a phycologist with the Monterey Bay Aquarium Research Institute included both I. flaccida and I. cordata in the Mazzaellas.5 Until the phycological community comes to an agreement, the most descriptive name, Iridaea, remains appropriate. The term Iridaea stems from the Latin word iris, meaning rainbow. In Latin the noun declines most mellifluously:
Abbott and Hollenberg differentiate the species of Iridaea according
to appearance rather than cellular developmental structure.4 They
use size, shape, and color of the blades as well as distribution of tetraspores
on the thallus to identify species. In addition, the presence or absence
of an apophysis is
a useful taxonomic characteristic. An apophysis is a gradual blending
of the blade into the stipe, rather than a sudden transition. However,
these characteristics are highly variable and make identification difficult.
Please refer to the later discussion of the morphological
variation of Iridaea.
It would seem quite difficult to distinguish I. flaccida from the other species of Iridaea using the characteristics laid out by Abbott and Hollenberg. No need to worry--I. flaccida subtly communicates its presence. Unlike all other species of Iridaea, the tetrasporangial blades of I. flaccida have smooth margins lacking tetrasporangial bumps.4 This feature is most easily distinguishable in the top two-thirds of the blade. Note well, however, that I. flaccida lacks sori on the edges of its blade only in the tetrasporangial, not carposporangial stage. (Although technically the carposporangial thallus is still the female gametophyte, for brevity I refer to this blade as carposporangial after cystocarps have appeared on its surface.) All Iridaean carposporangial blades have sori extending to their margins, but when mature these blades become easily distinguishable from tetrasporangial thalli. The carposporangial sori appear coarse and wartlike, extending out of both sides of the thallus, while the tetrasporangial sori are not much more than raised speckles on the blade.
Blade size and shape provide some help in distinguishing I. flaccida. Its iridescent, lanceolot blades are 20-30 cm wide and 8-20 cm long and range in color from yellow-green to purple. "Flaccid" does not properly describe the lively, slightly puckered thallus; unfortunately, the individual who named this species worked from poorly dried specimens.6 The plant will overwinter as a mere crust if winter storms tear off all of its thalli, so it can be difficult to locate all specimens of the plant within a quadrat. This overwintering stage give birth to a profusion of small blades in the spring. At Pt. Pinos, rocks which I observed in early February to bear no Iridaean thalli produced a remarkably dense crop of inch-long, chocolate brown, shiny new leaves by early March.
I. cordata is the species most commonly confused with I. flaccida in the Monterey area. While I. flaccida may have an apophysis present or absent depending on wave exposure, I. cordata always has a prominent apophysis. Of course, this is not a very useful characteristic for differentiation because the very conditions under which flaccida would have a more pronounced apophysis (more exposed) are the conditions in which I. cordata is more common. I. flaccida generally has smooth edges on its blades while the borders of I. cordata are commonly scalloped, fringed, or irregular. Again, these characteristics change with wave exposure. The most reliable criterion for separating these two species is that the tetrasporangial sori of I. cordata extend to the margins of the blade.
I. flaccida ranges form Alaska to Northern Baja, California, and is most abundant in the midtidal to low intertidal.4 In contrast, I. cordata specializes in the low intertidal to subtidal zone of substantially wave-swept shores. I find I. flaccida most commonly on decently exposed, vertical rock faces looking into the surf. It is uncommon in sheltered coves such as the one off Agassiz Beach at Hopkins Marine Station, but plentiful at Soboranes Point and Pt. Pinos on sloping rocks facing the surf. It forms dense patches in its ideal habitats but grows solitarily in less favorable locations.
The species of algae and invertebrates which live in close proximity
to Iridaea vary with the depth and exposure of the site. Generally, I. flaccida begins
to appear a half foot below the last Pelvetia specimen,
just below the zone of maximum Mastacarpis cover. However, at
Pt. Pinos I have found it above even the last line of hardy Endocladia in
places where strong waves periodically rake the supralittoral zone. Prionitis,
Chondrocanthus canuliculata, Mastacarpis, Corallinales, and Endocladia are
its most plentiful neighbors at both Pt. Pinos and Hopkins Marine Station.
Various species of Tegula enjoy grazing on the blades of Iridaea while
chitons, colonial anenomes, the muscle Mytilus, the crab Pachygrapsus,
and the starfish Pisaster ocracious occupy the rocks beneath its
thallus. Long tendrils of Egregia mark the lower edge of ideal I. flaccida habitat.
Morphology varies with wave exposure. On more wave-swept locations the thallus is thicker; this more sturdy blade better withstands wave forces. It is also more cleft or lobed. These divisions in the thallus allow it to rearrange under flow and overlap pieces of its thallus, and this decreases the amount of area upon which flow forces can act. By better conforming its shape under high water velocities it can have a lower (better) Vogel number. In addition, the stipe of the exposed plant is longer and more definite. This also probably helps it conform to flow and press the thallus against the substrate in order to present the smallest frontal area to the dangerous drag forces. (Picture: carposporophyte with well-defined stipe)
On less exposed shores, the stipe becomes shorter, more indefinite, or even absent. This type of plant cannot reconfigure easily under high water velocities, and would suffer substantial drag forces in more exposed locations. Its thallus is thin, making it less sturdy. The blade has a more lanceolot shape on calmer shores.
Iridaea also varies its pigments according to light availability, and this makes its color a variable characteristic. The thallus appears darker red under low light levels because of increased levels of phycoerythrin, which assists photosynthesis. The thallus can range from dark purple to almost black when growing very low in the intertidal and subtidal where light is scarce. Individual plants are often more red towards the stipe where the blade is most shaded. When growing on the sides of rocks instead of their top surfaces, the thallus also darkens because it spends more hours shaded. Photobleaching contributes to color differences because plants growing in high light levels get bleached by excess light as well as producing less phycoerythrin. Color varies a great deal even among specimens growing on the same rock surface. For instance, when Iridaea grows shaded by Pelvetia, it assumes the dark red tone of a lower tide-depth specimen.