A new species of red algae from the Hawaiian Islands Laurencia molokiniensis et. nov. (Ceramiales, Rhodophta)

Jennifer Smith

Graduate Student

Botany Department

University Of Hawaii, Manoa

email: jesmith@hawaii.edu

The Hawaiian Islands represent one of the most remote island systems in the world. They are located in the middle of the Pacific ocean, some 2500 miles away from any continental land mass (Figure 1). This unique location and subsequent isolation has resulted in several speciation events. The flora and fauna of Hawaii is composed of many terrestrial and marine endemic species as a result of this isolation. In fact the marine flora of Hawaii has been virtually ignored in the past except for the few phycologists that have dedicated their lives to understanding, describing and documenting these unique organisms. Hawaii is home to over 450 species of marine algae with only 10 of these being nonnative. The islands of Hawaii represent a very opportune setting in which to study biodiversity and biogeography.

Figure 1.  Map of the coral reefs around the world. Note the complete isolation of the Hawaiian Islands, in the middle of the Pacific Ocean (yellow circle).

The main Hawaiian Islands consist of 6 large islands but are actually made up of many other atolls, craters and smaller islands. Biogeography and biodiversity has mostly been studied on the main islands: Hawaii, Maui, Lanai, Molokai, Oahu and Kauai. However, these islands are also the areas which have been subjected to large scale human disturbances. Therefore, it seems that the smaller islands might represent more pristine conditions and should provide a better environment for understanding the natural composition of the flora and fauna of this region (Figure 2). Suprisingly enough, I was invited to conduct some of the first marine surveys ever done from the island of Kaho'olawe (off of the coast of Maui) in the winter of 1998, and I later conducted the very first marine algal collections from the island of Molokini in March, 1998.

Figure 2. Map showing the main islands of Hawaii.

To the casual observer or nonbiologist,  one may think that there are only a few plants (actaully algae are protists) in the subtidal marine coral reef environment. This however, is a large misconception in that this habitat type may house the highest diversityof algae per unit area than any other habitat. Many marine algae are small or microscopic, and may not even be recognizable until a microscope is used. In my work at Molokini, I have found over 100 species of algae within just a 25 meter area. As part of this work, I discovered a new species of red algae in the genus Laurencia that has never been described before, and may even be endemic to Hawaii (or even Molokini). The island of Molokini is actually a sinking and eroding crater that is about 500 meters in diameter. The island is situated approximately 2 miles off of the southwestern coast Maui, and has very little terrestrial vegetation (Figure 3).

Figure 3. Two views of the island of MOLOKINI.

THE GENUS LAURENCIA

The genus Laurencia consists of plants in the Division Rhodophyta, Class Floridiophyceae, Order Ceramiales, Family Rhodomelaceae and subfamily Laurencieae. Several species of Laurencia have been described from the tropical, subtropical and temperate regions of the world. Many of the species in the tropics make up a large component of the algal biomass in intertidal habitats. In Hawaii, the subtidal flora has not been extensively surveyed, and therfore probably consists of many yet undiscovered species. McDermid(1988) reported that the Hawaiian Islands are home to 15 or more Laurencia species, whereas Saito (1969) reported 17 members of this genus in the Hawaiian flora. It is evident that Laurencia has many species in Hawaii and more taxonomic and systematic attention should be given to it in the future. 

DESCRIPTION

Plants in this genus are chiefly erect and bushy or prostrate and creeping with a more or less fleshy or cartilaginous consistency. The axis is either compressed and flattened or round and terete. Plants can range in size from less than a millimeter to several centimeters in height. Branch apices are blunt with a terminal pit containing a single apical cell (Figure 5). Rudimentary colorless trichoblasts often protrude from this apical pit region. Lateral growth of the main axis occurs when cells from the main central axial filament begin to divide laterally, first producing  a ring of five pericentral cells, and these later dividing to produce the outer layer of small pigmented cortical cells. In mature axes this growth pattern is obscured and actually quite unrecognizable. Adult axis structure is composed of a parencymatous matrix, with an obscure polysiphonous core, surrounded by a cortex of isodiametric cells, these sometimes with lenticular thickenings. The tetrasporangia are tetrahedrally divided and scattered in the outermost cortical layer of the branchlets. The spermatangial branchlets are cylindrical and crowded in the apical pit. The cystocarps are morphologically terminal.     

HABIT of Laurencia molokiniensis

Several specimens of the new species were found in the subtidal, coral reef habitat off of Molokini Island, Hawaiian Islands. This alga was a fairly abundant component of the epilithic turf algal community. Specimens were collected from depths of approximately 15-60 ft. The substrate upon which this and other plants were commonly attached was essentially dead or dying coral fragments, branches, or rubble. Due to the extremely high percent cover of coral in the benthic zone, there was virtually no empty limestone/bassalt subtrate for colonization by algae. Figure 4 shows the typical habitat in which this and many other turf algal species exist.

Figure 4. Epilithic turf habiat of Laurencia molokiniensis and many other algal species

VEGITATIVE CHARACTERISTICS

Thalli are prostrate and creeping with upright branches, not exceeding 4 mm. in height. From the horizontal axis the branching pattern is secund and only 1 or 2 orders of branching are present. Horizontal axes have frequent rhizoids and large cup or disk-shaped holdfasts which anchor the plant to the sustratum. Thalli are rose red to pink and when wet appear to have rings on the upright portions.

The thallus is terete with a branch diameter averaging 160 micrometers. Branch tips are truncate with an apical pit and many specimens have colorless sterile trichoblasts radiating from the apical pit. Growth occurs from an apical cell that divides in two planes to produce a central axial filament and surrounding pericentral cells (Figure 5). 

Figure 5. Branch tip of L. molokiniensis showing apical pit, apical cell and axial filament. Mag=4X.

.

In cross section the central axial filament, the 5 surrounding pericentral cells and the outer pigmented cortical cells are all visible (Figure 6). In longitudinal section the pericental cells, cortical cells and the outer colorless sheath are shown (Figure 7). Each axial cell gives rise to a whorl of surrounding pericentral cells, which gives the plant a segmented appearance.

Figure 6. Cross section through a branch of Laurencia molokiniensis showing central axial filament, 5 pericentral cells and outer pigmented cortical cells (Mag = 20X).

Figure 7. Longitudinal section through a branch of Laurencia molokiniensis showing pericentral cells and outer colorless sheath. Note: segmented appearance (Mag = 10X).

One unique character that all members of the division Rhodophyta share is that cells in the vertical plane are all interconnected via pit connections. These connections may be somewhat analogous to plasmodesmata in terrestrial plants. Many Rhodophytes also have secondary pit connections, ajioning cells in the horizontal plane. The presence or absence of these secondary pit connections is a good diagnostic feature in recognizing individual species. Figure 8 shows pit connections in Laurencia molokiniensis, as well as showing the overall elongated shape of the pigmented cortical cells.

Figure 8. Surface cells of

Laurencia molokiniensis showing

pit connections and elongate

cortical cells. Note the abundance

of chloroplasts in these cells

(Mag = 40X).

REPRODUCTION

Most members of the Floridiophyceae all share the same general reproductive characteristics. Figure 9 shows the overall life history pattern of this group. There are many types of sporangia produced but the most common is the tetrasporangia, which produces 4 meiospores (tetraspores), arranged tetrahedrally, cruciately or zonately. Male gametes (spermatia) are sperical, colorless and nonmotile.  The spermatangia are formed singly or in groups and can be sessile or located on specialized branches of the thallus. The female gamete (carpogonium) has an elongate receptive structure, the trichogyne which projects away from the plant body. The primary function of the trichogyne is to increase the chances of fertilization. After fertilization, the carpogonia gives rise to a group of diploid cells (carposporangium, cystocarp, or gonimoblast) which protrude from the fertilized carpogonium and remain attached to the female gametophyte. The carposporophyte is short lived and reduced in size and gives rise to several diploid carpospores. These diploid spores are released into the water column and will germinate into the diploid tetrasporophte. The tetrasporophyte undergoes meiosis to produce meiospores which will grow into the gametophyte stage. The tetrasporic and gametophytic plants may be isomorphic or hetermorphic.

Figure 9. Generalized diagram of the life history in the Floridophyceae

REPRODUCTIVE STRUCTURES

Laurencia molokiniensis has tetrasporangia that are arranged in clusters around the apical pit of the branchlets. The tetraspores are located at right angles to the main branch axis and are tetrahedrally divided.  Mature tetraspores occur at the outer margins of the upright braches and average 40 micrometers in diameter (Figure 10).

Figure 10. Tetrasporophyte releasing tetraspores. Note the position of the tetraspores in relation to the main axis of the branch. Also note the the tetrahedral division of the spores. Mag=4X

Gametophytes and tetrasporophytes are isomorphic and male and female gametophytes exist separately. Carposporangia occur laterally in the basal regions of the upright branches on female plants (Figure 11). Cystocarps are urn shaped and sessile with dimensions of: 400-480 micrometers in width by 320-350 micrometers in height (Figure 12).

Figure 11. Female gametophyte showing carposporangia after fertilization. The carposporangia (cystocarp) is growing out of the previous carpogonium. This cystocarp is mature and about to release diploid carpospores. Mag=4X.

Figure 12. Close up of a cystocarp with mature carpospores. The carposores will be released into the water column and upon settling will germinate   into a tetrasporophyte. Mag=10X.

Male gametophytes bear spematangial branchlets which radiate out of the apical pit (Figure 13). Sterile trichoblasts bear the spermatogenous cells (Figure 14). These spermatia-bearing trichoblasts range from 96-202 micrometers in height above the vegitative branch. Spermatogenous branchlets bear naked spermatia. However, a single sterlie cell terminates each fertile branch (Figure 15). The tips of mature male plants are somewhat inflated, being about twice the diameter of vegitative branches.

Figure 13. Male plant showing spermatogenous branchlets radiating out from the apical pit. Note the inflated tips  of male branches as compared to vegitative branch tips. Mag=4X

Figure 14. Closer view of spermatogenous

branchlet bearing spermatia along the

margins of the branchlet and sterile cells at

the terminal ends of the branchlet. Mag=20X

Figure 15. A close up view

showing fertile branchlets,

individual spermatia and sterile

cells. Mag=40X

COMMENTS

Several specimens of this plant were collected on March 8, 1998 from the island of Molokini at depths that ranged from 15-60 ft. There have been no other reports of this plant or anything similar from Hawaii. Therefore, this plant should be treated as a new species. All of the photographs were taken by the author, and all work was conducted at the University of Hawaii, Manoa. Type specimens will be deposited at the Bernice P. Bishop Museum's phycological herbarium for future reference.

ACKNOWLEDGMENTS

I would like to thank several people for making this research possible. First and foremost I would not have been able to do any of this work without the help, encouragement and support of Dr. Isabella Abbott. I would also like to thank Skippy Hou and Cindy Hunter for inviting me on the research trip to Molokini. Several other people deserve recognition for their help and support along the way:

Brent Carman- for helping me collect and being a great dive buddy

Dr. Celia Smith

6th floor algae lab- for help in identifying all of my samples

The Big Island DAR- for support

UH Hilo & Manoa MOP

Carl Steppath

and all of my algae friends!!!!