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J. P h J d . 25, 206-2 12 (1 989)

ADDITIONAL EVIDENCE FOR ECOLOGICAL DIFFERENCES AMONG ISOMORPHIC REPRO D U C T I V E PHASES 0 F IR IDL4 E A LAMZNARIOID ES

(RHODOPHYTA: GIGARTINALES)'

Caroliiin Luxoro a n d Beriiabe Santelices2 Departamento de Biologia Ambiental de Poblaciones, Facultad de Ciencias Biologicas, P. Universidad Catolica de Chile

Casilla 114-D, Santiago, Chile

ABSTRACT

Gccinetoph!tes a r e more abiindaiit thou sporoph!tes i n m i x mpo.vtl rock! in ter t ida l populatiows of Iridaea lam- iriarioides Bory i i i Crutral Chile. IH this s tud j UIP exper- i t n m t o I 1 ~ trstpci thr c0Jere)i tia I eferts o f selected ecologira 1

foctor3 du k o ~ y l o g i c t i l l u d f e r e n t l f e histor! phases. In t h e j e l d . gcimrtoph~tes dominated a t higher eleilatiom a n d

I ReceiLed 20 June 1988 Accepted 15 \o\ember 1988. * Addres for reprint requats

dzi ritig sumuier; tetrasporophjtes ulere most abundant lou1 i n the ititertidal a n d dur ing the fall. Laboratory responses co rrela tecl icith these patterns. Ga m etophjtes exhibited greater desiccation tolerance t h a n tetrasporophjtes. Op- t i m u m growth of ganetophj tes occurred a t higher temper- otures ( 2 0 ° C ) and longer photoperiods (16:8 h LD) t h a n sporopliytes ( 1 5 ° C a n d 12:12 h LD). Graz ingpre frences chnngei u i t h the deilelopmentcll stage of the alga, but al l herbii*ores tested had increased pr$erence for diploid tis- sues a s coinpared to haploid. Number o f spores produced u i t h respect to total p l a n t s u f i c e , or total rocky surface,

LIFE PHASES O F IRIDAEA LMINARIOIDES 207

or settlement of spores and their germination rate did not shou! signijicant differences between phases but showed great uariabilitj in space and time. Spontaneous spore release, howetler, was alwajs higher i n cystocarpic than in tetrasporangial thalli. Such a combination of results sug- gests that some real ecological dzferences exist between the two l$e historj phases of I. laminarioides. Such ecological differences permit a prediction o f vertical and temporal patterns of distribution fo r both phases. Horizontal pat- terns of distribution cannot be explained because the sev- pral splection factors probably interact dzferentlj in var- ious habitats.

Kej index words: desiccation tolerance, grazing prefer- ences, Iridaea laminarioides, isomorphic algae, lye his- tory phases, spore production.

The understanding of heteromorphic life histo- ries as adaptations to temporally or spatially fluc- tuating environments (Lubchenco and Cubit 1980, Slocum 1980, Stebbins and Hill 1980, Dethier 198 1, Littler and Littler 1983) has raised the question of the adaptive significance of isomorphic life histories (Hannach and Santelices 1985, May 1986, Littler et al. 1987). Unequal field distribution of karyologi- cally different phases of isomorphic algae has been frequently reported (see Hannach and Santelices 1985 for a review; also Bhattacharya 1985, Dyck et al. 1985, May 1986, Hannach and Waaland 1986) and has been explained by two alternative hypoth- eses: 1) The unequal distribution observed may re- sult from demographic or physiological differences between phases (mechanistic model sensu May 1986). 2) Such distribution may reflect chance events in the settlement of phases that are equally able to survive and reproduce (stochastic model sensu May 1986). Recent studies have provided support for each hy- pothesis. Interphase differences in growth rates of species of Iridaea under laboratory (Hannach and Santelices 1985) and field conditions (May 1986) tend to support the mechanistic idea. Differences between reproductive phases of Chondrus crispus Stackhouse in resistance to forces generated by rake harvesting (Craigie and Pringle 1978) or waves (Dyck et al. 1985) further support the mechanistic hy- pothesis. Lack of interphase differences to some physiological and ecological responses of Polycaver- nosa debilis (Littler et al. 1987) as well as nearly 1:l ratios between gametophytes and sporophytes of Chondrus crispus on disturbed bottoms (Lazo et al. 1989) support the stochastic model, suggesting a lack of physiological or demographic differences be- tween the phases and an equal capacity for them to recruit and grow.

Iridaea laminarioides Bory is a carrageenan-pro- ducing, economically important isomorphic red alga. In central and southern Chile the intertidal popu- lations are dominated by gametophytes (Hannach and Santelices 1985, Santelices and Norambuena 1987, Westermeier et al. 1987). As also observed in

Chondrus crispus on the Atlantic coast of Canada (Bhattacharya 1985) the dominance of cystocarpic thalli of I. laminarioides in Central Chile is greater near its upper limits of vertical distribution, whereas tetrasporangial thalli become more abundant to- ward the lower part of its vertical range (Hannach and Santelices 1985). Laboratory experiments re- vealed no significant differences in the optimum growth of gametophytes and tetrasporophytes of either species under different regimes of tempera- ture, light intensity, water movement or salinity al- though some interphase differences in growth rates were observed (Hannach and Santelices 1985). In addition, laboratory experiments disclosed a signif- icantly higher consumption of gametophytic rather than sporophytic specimens by the limpet Collisella ceciliana (Hannach and Santelices 1985).

In search of interphase demographic or physio- logical differences in Iridaea laminarioides, we fo- cused this study on three specific aspects of its bi- ology. We studied, under laboratory conditions, the effects of desiccation, day-length and temperature, apparently the abiotic factors most likely to influ- ence the field distribution pattern of each phase. Secondly, we further explored the preferences of several grazers on some critical developmental stages and parts of the plant. Thirdly, we studied inter- phase differences in fertility and recruitment.

MATERIALS AND METHODS

Eftcts fifdesiccation, ttrnprraturc and daj-length. Fertile tetraspo- rangial and cystocarpic fronds of Iridaea laininarioidts were col- lected at Pelancura, 5 km north of San Antonio Port in Central Chile (33"29' S; 71'38' W), brought to the laboratory, thoroughly washed in sterile seawater and left in 25 mL of SWM-3 modified culture medium (McLachlan 1973) in darkness until spores were liberated (1-2 h). Temperature effects on growth were studied by incubating the sporelings of each phase at 10, 15 and 20" C, with a day-length regime of 12:12 h LD and 40-45 PE.rn-*.s-' of photon flux density. For the day-length experiment, sporelings were incubated under light: dark cycles of 16:8, 12:12 and 8:16 h, at 15" C and photon flux densities of 40-45 wE.m-2.s-1 pro- vided by cool-white fluorescent light tubes (40W). The culture medium was changed every 5-7 days. At the beginning and end of each experiment the diameter of the basal area of 35 randomly chosen germlings in each treatment were measured in each of two replicate dishes using an inverted microscope. Experimental data were subjected to simple ANOVA (nested plot design) (Sokal and Rohlf 1969). Desiccation effects were studied using a simple, artificial "tidal system" in the laboratory. Two 20 x 50 x 40 cm glass tanks were connected by PVC pipes. Using two water pumps (Eheim) and a clock (Grassling), filtered seawater was regulated to circulate from one tidal tank to the other at a constant flow of 12.5 mL.min-' for a 6 h cycle. A total of 20 reticulated glass slides sown with equal densities of 12 day-old gametophytes or sporophytes was suspended at a similar level in one of the tanks. T h e vertical level reached by the water along the slides while emptying or filling the tank was recorded every hour and used as "tidal marks." The system was maintained for 35 days in a walk-in growth chamber under constant temperature (1 OOC), day- length(12:12 h)andphoton fluxdensity(l0-15 ~ E . m - ~ . s - ' ) con- ditions. Relative humidity within the tank was 88 k 3%. All germlings found within two parallel transects extending along

208 CAROLISA LLXORO A S D BERSABE S.4STELICES

Type of feed offered: ju\enile holdfasts Duration: 3 days Date: June 87 Litlor~izn p e r u m n o (Lamarcki 12 0.7-1.5 3 2/3 Co//isel/n sp. 12 0.8-2.2 3 2/3 Siphonorx i I p s c n i i i (Blainville) I ? 0.7-1.4 3 213 Fixsurdlo r r n w (Lamarck) 12 1.6-2.5 4 2/3 T ~ g u / n n t r n (Lesson) 7 0.9-1.5 3 2/3

.I'>pe of feed offered: adult holdfasts Duration: 6 da! s Date: Jul\ 87 L. pprui, iniin 9 1.0-1.7 5 2/9 C o / / i d / o sp. 6 0.7-2.7 5 2/9 s / P 5 5 o l l i 12 0.9-1.6 5 219 F . r r n m 4 2.3-4.9 7 219 T. a t f m 5 0.8-1.8 6 2/9

'l'jpe o f feed offered: reproductive blade5 Duration: 5 to 7 day5 Date: \fa\ 8'7 L. p P r l l i ~ l f 7 ~ l ~ l 25 0.8-2.0 C O / / l d i n sp. 50 0.8-3.0 s. / ? J m l l l 100 0.6-1.8 F . r r m w 7 1.7-4.0 7. nt rn 12 1.2-3.2 H w / r i n d i n (Dana) 3 0.8-1.5 TPtrnpjgu i n i p (Mol i II a ) 19 3.3-4.2

Chitoil p n i i o s u J (Fremblv) ti 2.7-4.2 Lo w h i 11 21 (I I h u ~ ( \ I oli na) 20 2.8-4.1

5 2/9 5 2/9 5 2/9 6 2 j 9 5 2/9 3 10/3 9 3/9 Y 3/9 5 3/9

each of.ten replicate slides \ \ere counted at the beginning and at the end of the esperiment.

Grrixiig p r p , f p r m c ? Ehperimentz were carried ( jut to compare the consumption rates of germlings, adult holdfasts and fertile blade, of the t x v o phases of l r i d o ~ o /(i ivii iciriuid?A b! different types of herbiLores common in the habitats occupied b\ the alga. Germ- lings and adult holdfasts of I . / o i j i i r i o r i o i d r ) \\-ere offered to mol- lu\cs, which are the important consumer^ of these two tissues (Santelicec et al. 1986, Santelices and \Iartinez 1988). In addition to the molluscs. fertile blades of both phases were offered to tu'o common species of sea urchins ( T e t m p p r riiger (lfolina) and Louechiiiur n/buA (Molina)) and to one amphipod species (H?o/r i , ipc/ in (Dana)). Data on experimental conditions are summarired in Table 1. Animals were collected in Pelancura, taken imme- diately to the laborator) and placed in 20 L plastic tanks with circulating seawater. 'The amphipods were carried in plastic bags with I . /niniunrioide\ blades without seawater.

For experiment\ on consumption of Zridnrti /c i i i i t i inr/o/dpj j u - \-eniles, carpospores and tetraspores \\ere cultured separately on 25 x 75 mm glass slides at 15' C and 45 z 5 pE.m-2.s-l for 20 da)s. The slides !\.ere then transferred to four transparent plastic tank, of 1000 cm3 ivith holes drilled 5 cm above the bottom. I7aseline \\-as used to stick two slides with each reproductive stage to the !\ails of each container. TKO containers \\.ere kept as con- trols. and betu een 7- 12 individuals of each invertebrate species uere placed in the tico replicate containers. T h e esperiment was I-rpeated \<ith rach o f the five species of molluscs (Table 1 ) . Con- tainers \\ere maintained Ivith circulating seaM'ater at 10" C and 12: 12 h of. dail! light for 3 da!s. Plant densit! was measured on each \lid? at the beginning and at the end of the experiment

using ti\.o parallel permanent transects running along one face of the slide. Consumption ivas calculated as the difference in densitt at the beginning and at the end of the experiment. Rel- ati \e consumption was calculated as consumption on one slide in relation to the total consumption in that container. All relative contumption data lvere subjected to arcsine transformation and the consumption of sporophytic and gametophytic juveniles by an! invertebrate species was compared with the Student's t-test.

For experiments on consumption of adult tissues, plants were \ectioned above the holdfasts and sorted by reproductive struc- ture t!pe. Holdfasts of each phase as well as whole cystocarpic or tetrasporangial fronds were blotted, weighed and placed in the corresponding experimental tank. The different tissues were left for 6-7 days in the tank, then recovered, blotted and weighed. Consumption {\.as calculated as the difference in algal biomass between the beginning and end of the experiment, and relative consumption as the proportion of each stage ingested. A similar algal biomass of each reproductive stage, but without grazers, icas used as control. Data Fvere subjected to arcsine transforma- tion and compared using a simple analysis of variance.

S p r r p w d u r t i o i i ni id r e l ~ n s ~ . hleasurements of spore production were made by counting the number of spores in the cystocarpic and tetrasporangial sori as well as the total number of sori per unit surface of fertile frond. Then the total number of spores per plant and per unit of rocky surface were estimated.

A similar methodology \\-as used to count carpospores and tet- I-aspores. Using a dissecting microscope, the entire spore mass in rach sows w s escised and placed for 30 min in 0.25 mL of a 25% solution of SaHCI. The spore mass was transferred to a huffer solution (pH 7.2-7.4: pK 8) and homogenized using glass rods. The solution xvas then centrifuged at 10,000 rpm for 15 min. the spores resuspended in 0.005 mL o f a 0.25% methyl blue solution. and separated into three drops where the number of rpores was counted separately using a hemocytometer. Each sea- \on a total of 10 tetrasporangial and 10 cystocarpic sori randomly \elected among fertile blades was studied. Sample size was pre- \ iously determined using the running mean method.

T o measure the number of sori per unit of blade area and to estimate the number of spores per unit of plant and rocky sur- faces, seasonal field samplings were carried out at two localities in Central Chile. T h e first site was a rocky platform at Pelancura. 'The second site \\'as a rocky outcrop surrounded by sand at Ma- tamas, some 70 km south of San .4ntonio (35"56' s). Both local- ities are described by Hannach and Santelices (1985) and San- telices and Sorambuena (1987). In October 1986 and January, .April and .-\ugust 1987, two transects running perpendicular to the coast and extending from the uppermost to the lowermost tertical limit of the Iridrirci belt were established in each study \ite. The transects \ \ere approximately 50 cm wide and 1 m apart. .-\I1 plants enclosed in a 50 x 50 cm quadrat were removed at the base irith a spatula. Quadrats were set contiguously along the transect. Samples Icere placed in numbered plastic bags, trans- ported to the laboratory and kept in a freezer for 1-2 days before being washed in seawater, sorted by life history stage (female gametoph: tes, tetrasporophytes, and male and sterile plants to- gether), blotted and weighed. T o evaluate temporal variation in biomass. means rvere obtained from the samples in a given month. To determine fertile biomass, portions of blades with sori were escised and Ifeighed separately. T h e fertile biomass was calcu- lated as a percent of total biomass in any given locality and season. Through 20 \<eight determinations of known areas of fertile blades of each phase, together with counts of sori in such areas, the total number of sori per plant and per unit rocky surface could be determined for each season.

To study spontaneous spore release, 75 g of freshly collected fertile fronds of each phase \vere \cashed and brushed for 2 min in fresh water to remove epiphytes, then passed through filtered rea\\ater (0.45 pm pore size), blotted and exposed to air for 20 min at room temperature (20" C; 70% relative humidity). T h e \pore solution !\.as then filtered to remove pieces of fertile fronds,

LIFE PHASES OF IRIDAEA LAiMINARIOIDES 209

i o c i ( h i

FIG. 1. Effects of abiotic factors on survival and growth of phases of I . Inrninnrzozdps. (A) Survival of plants under different emersion times. (B) Growth of plants at different temperatures. (C) Growth of plants under different photoperiods.

and spore concentration was measured using a Coulter Counter (M2A).

In order to compare settlement and germination of tetraspores and carpospores, a spore solution was prepared as above with 5.5 g of fertile frond of each phase incubated separately in 200 mL of SWM-3 modified culture medium. To avoid spore sedimen- tation and settlement, this stock spore solution was stirred at 100 rpm on the platform of a Junior Orbit Shaker at 15" C and a 12: 12 h LD cycle. Three petri dishes, with three sterile cover slips stuck to the bottom with solid vaseline, were used per treatment and filled with the spore solution 1, 5 , 10, 15, 22, 28, 34, 40, 46 and 52 h after spore release. T h e dishes were incubated under constant conditions of temperature (12" C), photon flux density (40-50 pE.m-2.s-') and day-length (12:12 h) for 120 h. T h e cov- er-slips were removed, fixed in a 3% formaldehyde-seawater so- lution, stained with a 0.25% methyl-blue solution and counted under an inverted microscope. The number of spores which set- tled, germinated and elongated was counted in two microscopic fields ( 2 0 0 ~ ) on the three cover slips from each of three petri dishes. The two field measurements were used as pseudorepli- cates, the others as true replicates and results were compared using a nested analysis of variance. To evaluate seasonal changes in settlement and germination capacity of the spores, the exper- iment was repeated in late summer (March) and winter (August).

RESULTS

Sporophytes of Iridaea larninaroides responded to desiccation stress differently than gametophytes. Under laboratory conditions used, short emersion periods slightly stimulated survival of sporophytes (Fig. 1A). The percent of plants surviving 1 h of emersion in a cycle of 12 h was greater than when plants grew continuously immersed. However, ga- metophytes survived better than sporophytes during long emersion periods. After 3 h desiccation, den- sities of surviving sporophytes were reduced to near-

FIG. 2. Relative consumption of different herbivores on the two phases of I . Initzznnrzoid~s. (A) Juvenile holdfasts. (B) Adult holdfasts. (C) Reproductive blades.

ly l % , whereas 8-10% of the gametophytes were still alive and growing. In these experiments desic- cation tolerance of gametophytes was at least 2 h longer than that of sporophytes.

Although both phases were able to grow between 10 and 20" C, the response curves differed (Fig. 1B). Optimum growth temperature for sporophytes was 15" C, and growth decreased at higher (20" C) and lower (10" C) temperatures. Optimum growth tem- perature for gametophytes was 20" C, showing no significant differences in growth rates when grown at 10 and 15" C.

The phases exhibit a different response to day- length regimes. Tetrasporophytes showed signifi- cantly higher growth rates under 12 h of daily light than under 8 h; longer photoperiods (16:8 h) did not seem to increase growth (Fig. 1 C). On the other hand, growth of gametophytes under long days (16 h) was significantly higher than under the other two day-length conditions.

The herbivores used clearly discriminated be- tween the two algal life phases, and their degree of discrimination varied with plant age. Young tetra- sporophytes were consistently preferred over young gametophytes (Fig. 2A) by the five species of mol- luscs tested, with significant interphase differences (P < 0.05). Differential consumption of adult hold- fasts was shown only by Sifhonaria lessonii and Tegula atra (Fig. 2B). The amount of algae consumed by Littorina perutliana and Collisella sp. was not signifi- cantly different from zero and Fissurella crassa ex- hibited an equally low consumption of both phases.

210 CAROLISA LUXORO A S D BERNABE SANTELICES

Tetraspores Carpspores in) P n )

FIG. 4. Spore amount released spontaneously through 20 h.

FIG. 3. Euprersions of phases fertilit\ of I . / ~ t m I n o r - m d e ~ in different seamiis. (.4) Spore amount per wrus . (B) Sumber of wri per area of reproductiir blade. (C) Biomass of reproductive blades per area of rock\ surface. (D) Spore amount per area of rock) wrface. Sp: spring: S: summer: .4: autumn: i V : winter.

Differences in consumption of fertile fronds of either phase r\.ere not statistically significant. HOW- ever, the gastropods L. p r r w i n v n , Collisalln sp., S. l r n o u i i and T. ntro and the sea urchin Lo.wc1ziuzi.i nlbus all exhibited a higher consumption rate of fertile tetrasporangial over fertile cystocarpic blades (Fig. 2C). T h e reverse was true for the sea urchin, T ~ t m - Iijgzts uigar and the amphipod Hjnlr mpdio. However, the consumption by the amphipod tvas highly spe- cific and restricted to cystocarps, thereby affecting diploid rather than haploid spores.

T h e only significant (P < 0.05) difference found between phases in fertility measurements was the number of sori produced per unit of frond surface (Fig. 3). Differences lvere statistically significant (P < 0.05) in autumn at Matanzas and during autumn and winter at Pelancura. However, no significant (P > 0.05) differences were detected in the number of spores produced by each type of sorus, in the total number of sori produced by plant, or in the total number of spores produced by unit of rocky surface. On a population basis, the larger number of sori per unit surface found in the sporophytic blades was balanced by the larger size of the gametophytic blades, the larger number of fertile gametophytic blades per plant and the increased abundance of gametophytes in these populations. There were ob- vious inter-season and inter-habitat differences in the number of spores produced per unit of rocky

areas. Ho\vever, there were no significant (P > 0.05) differences in the total number of spores produced over a one year period.

T h e number of carpospores spontaneously re- leased Ivithin a 20 h period after collecting was (Fig. 4) about 10,000 times higher than the number of tetraspores released. Even though this response was quantified only once, the observations were made in all the experiments involving spore manipula- tions.

Interphase differences in spore settlement and germination varied seasonally (Fig. 5A). In winter, both types of spores exhibited a gradual decrease in settlement capacity. By contrast, in late summer, settling decreased more rapidly in carpospores than in tetraspores up to 28 h after release. Thereafter the reduction in settlement capacity was more in tetraspores.

Germination rates exhibited conspicuous seasonal but only minor interphase differences (Fig. 5B). T h e only statistically significant differences were restrict- ed to germination rates in spores settled 5-1 5 h after spore release, with carpospores in winter and tet- raspores in late summer exhibiting significantly higher germination rates than the other phase. No significant ( P > 0.05) differences were found in ger- mination rates between either type of spores a t other settlement times.

DISCUSSION

T h e results provide additional information on bi- ological differences between phases in the life his- tory of ZridaPn laminnrioidps. Some of these differ- ences are highly consistent with the observed pattern of distribution of these phases in space and time. Experiments with abiotic factors characterize the gametophytes as individuals with higher desiccation tolerance than sporophytes and with growth rates

LIFE PHASES OF IRIDAEA LAMINARIOIDES 21 1

stimulated by higher temperatures and longer days than sporophytes. These data are consistent with the increased abundance of gametophytes at the upper- most limits of the species distribution observed in Central and Southern Chile (Hannach and Santel- ices 1985, Westermeier et al. 1987). Our results also provide an explanation for the spring-summer max- imum of gametophyte occurrence and the autumn maximum sporophyte occurrence of I . laminarioides. Similar differences in distribution patterns were de- scribed in Chondrus crispus, Gigartina stellata (= Mas- tocarpus stellatus) and Iridaea cordata (Prince and Kingsbury 1973, Chen et al. 1974, Mathieson and Burns 1975, Bhattacharya 1985, Dyck et al. 1985, May 1986) and perhaps reflect equivalent ecophys- iological differences between the life history phases of these isomorphic algae.

Similarly, grazing experiments illustrate a differ- ential preference for the diploid sporophyte by mol- lusc grazers that are normal inhabitants of the Iri- daea laminarioides habitat. T h e increased gametophyte occurrence found in several popula- tions of the alga (Hannach and Santelices 1985) may reflect the choices and activities of herbivores on those populations. The differential resistance to grazing often described as characteristic of hetero- morphic algal species (Lubchenco and Cubit 1980, Slocum 1980, Dethier 1981, Littler and Littler 1983) also occurs, and quite consistently, between the iso- morphic phases of I. laminarioides. In this case, how- ever, the least consumed is the haploid, gameto- phytic phase.

Chemical differences in carrageenan composition between gametophytes and sporophytes have been well established for several species of Gigartinaceae (Pickmere et al. 1973, Chen et al. 1974, McCandless and Craigie 1975, Waaland 1975). Kappa carra- geenan, which gels, occurs in the haploid gameto- phytes; lambda carrageenan, which is viscous, occurs in the diploid tetrasporophytes. One wonders whether the herbivory and desiccation differences between phases, so clear and consistent, could be derived form physico-chemical differences in cell walls. The idea would provide a single explanation for several observed interphase biological and chem- ical differences and is consistent with the gameto- phytic predominance, which seems more common in carrageenan-producing members of the Gigarti- naceae than in other algal families. Care should be taken, therefore, when looking for generalizations on isomorphic algal species. For example, the well known field scarcity of haploid gametophytes in the isomorphic agar-producing Gelidium species may re- flect a set of genetic and environmental constraints completely different from those regulating the car- rageenan-producing Chondrus, Gigartina or Iridaea.

In contrast to the above clear-cut interphase dif- ferences in tolerance to grazing and to some abiotic factors, the studies on spore production, release, settlement and germination exhibit only minor dif-

A

loo.

' ,

'f 10 50 10 30 50rinulupnd.dinrulru"

l h l

FIG. 5 . Settlement and germination spore rates in late summer and winter. (A) Settlement. (B) Germination.

ferences between phases. One of the most significant differences, number of sori per plant, is counter- balanced by the number and size of fertile blades per unit surface. Spontaneous spore release, the oth- er important difference observed between phases, cannot be fully understood as long as the fate of the unreleased spores is unknown. In addition, there may be field complications. Amphipod grazing has been shown to facilitate carpospore release in Iridaea laminarioides (Buschmann and Santelices 1987), and perhaps the same could occur with tetrasporophytic sori. The two populations of I . laminarioides studied here were obviously unequal in their seasonal rep- resentation of fertile sporophytes and gameto- phytes. Yet the total number of spores produced by each of the two phases at each locality and season was more or less similar and always characterized by great variability. Spore production seems to be a function of several factors such as the relative abundance of each phase, the size and number of fertile blades per unit of rocky area, the number of fertile sori per blade and number of mature spores per sorus. Furthermore, spore recruitment can be modified by abiotic and biotic factors. Therefore, one should not assume recruitment rates based only on percentage of reproductive female gametophytes and sporophytes (May 1986). It would be wrong to conclude that no demographic or physiological dif- ferences exist among supposedly competitively equivalent phases because their occurrence in the field appears to be equal (Lazo et al. 1989). Perhaps biotic or abiotic factors are compensating for phys- iological differences and favor differential spore re- cruitment of one phase over the other.

The ecophysiological differences between the iso- morphic phases of Iridaea laminarioides further sup- port the mechanistic model of May (1986). These differences permit the prediction of vertical and sea- sonal patterns of distribution of the phases in hab-

212 C4ROLIS.I L U X O R O A N D BERSABE S.4TTELICES

itats 1t.her-e these differences are expressed. HOT\-- ever, this is not the case in horizontal distribution w.here stochastic et-ents could be more important or )$.here the importance of one factor (e.g. tempera- ture) could be compensated b\. the effect of another factor (e.g. grazing).

T h e marked preference that herbii.ores shot$. for the diploid phase contrast w.ith the patterns report- ed for species ivith heteromorphic life c)-cles (Lub- chenco and Cubit 1980) 1t.here the gametophytic phase is preferred. In light o f these results, it seems doubtful that heteromorphic alternation of gener- ations could be derived from isomorphic alternation of generations (Stebbins and Hill 1980).

'This stud? is based o n the thesis submitted b! the first author as partial f~ulfillment o f the requirements for a Licenciatura degree i i i Biological Science5 at the P. C i i i \ e r d a d Catolica de Chile. I \ ? are iiiclebted to Dr. I . .I. . ibbott for re\ ieiting and improving thc final draft of the paper-. Our appreciation also to the tivo i-c\iviier\ (Dr j . .I. \fathieson and G. Sharp) for helpful sugges- t ion\ . Thi\ manuscript \\as complrtrd while B. Santelices Jvas a Guggenheim Fellor\. at the Department of Botan! of the Uni- \ en i t> of Haitaii. Hi5 gratitude extended to both of these in<titutions. Funding for the rewarch \\ark \ \as provided by In- ternational Development Research Centre. Grant 3-P-85-0069 t o the second author.

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