Tethya (Porifera, Demosponglae) Species Coexisting in a Maldivian Coral Reef Lagoon: Taxonomical, Genetic and Ecological Data

P.S.Z.N. I: Marine Ecology, 14 (4): 341-355 (1993) 8 1993 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0173-9565

Accepted: May 25,1993

Tethya (Porifera, Demospongiae) Species Coexisting in a Maldivian Coral Reef Lagoon: Taxonomical, Genetic and Ecological Data MICHELE SARA,GIUSEPPE CORRIERO & GIORGIO BAVESTRELLO Istituto di Zoologia dell’Universit8 di Genova, Via Balbi 5, 1-16126 Genova, Italia. With 7 figures and 4 tables Key words: Porifera, coral reef lagoon, spicular morphometry, genetic distance, niche differentiation, speciation.

Abstract. Tethya microstella, T. orphei, T. robusta, and T. seychellensis live in sympatry in a Maldivian coral reef lagoon (Gangehi Island, A n Atoll). A morphological, morphometric, and electrophoretical study of these species show that their Maldivian populations are well separated morphologically and genetically. The genetic distances and the other data are more consistent with an immigration than with a local speciation hypothesis. Their coexistence may be explained by a differentiation in ecological requirements. The four species inhabit the shallow water biotope of the lagoon among the rubble of dead corals. Our data show that T. orphei and T. seychellensb occupy the upper rubble layer, T. microsfella and T. robusta the lower one. This distinction agrees with the different morpho-functional features of the two pairs of species.

Problem The coexistence of different species of the same sponge genus in the same restricted area raises interesting ecological and evolutionary questions in relation to the competition and niche theories (HUTCHINSON, 1959; MILLER,1964; BROWN,1981; JACKSON, 1981) and to the debate on the allopatric versus sympatric mechanism of speciation in sponges (SARA, 1990a). The two species T. aurantium and T. citrina are known to coexist in the Mediterranean Sea (SARA & MELONE,1965); the ecological and evolutionary aspects of this sympatry has been analyzed by CORRIERO et al. (1989) at the Stagnone of Marsala lagoon. The coexistence of Tethya species is also known from an Australian coral reef lagoons (SARA, 1990b) and, in different habitats, from a Bermudian creek (CROZIER, 1918). The present study deals with four species of Tethya (T. microstella, T. orphei, T. robusta, and T. seychellensis), living together in the coral reef lagoon of Gangehi Island (Maldive Islands). The integration of the taxonomic survey with U.S. Copyright Clearance Center Code Statement: 0173-9565/93/1404-0341$02.50/0

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the electrophoretic and ecological data is designed to shed new light on the problem of sympatric coexistence in sponges. In addition this paper provides new data on the geographical variability of morphological characters of a systematically difficult and confused genus and on its poorly known field ecology, especially in the coral reef environment.

Material and Methods The study area is located around Gangehi Island, in the NW of the A n Atoll (Maldive Islands) (Fig. 1). A total of six sampling stations was established in the study area: three in the mouth of the channels connecting the lagoon with the open sea (A, B, C), one in the lagoon (D), and two in the platform reef (E, F) at about 1.5m depth (Fig. 1). Since the four Tefhya species live in a sub-rubble community (MEESTERSet al., 1991) exclusively on dead branches of Porires sp., sampling was restricted to this substrate. Dead Porites branches 3 4 c m in diameter were characteristic for all sampling stations; they formed pluristratified beds 80-120 cm thick. Quantitative sampling of the Tethyu species was conducted by removing, at each station, all specimens present on the Porites branches in 5mZareas. For each Tethya specimen the distribution on different layers of Porites branches - upper, middle and lower, each 3040cm thick - was recorded.

..:... I

..... ,., .... . .,' ;i

0

Fig. 1. Sampling sites in the Gangehi lagoon.

Tethya from a Maldivian coral reef lagoon

343

A morphometric analysis on the megasters of five specimens each of the four Maldivian Terhya species and of a population of T. robusta from Massaua (Ethiopia, Red Sea) was carried out. Two spicule slides were prepared from each specimen and the size of 50 asters from each slide was measured under the light microscope using a camera lucida and “Graphtec” digitizer. The average diameter of the asters (D) and the ratio between ray length and the diameter of the center (R/C) (SARA& MELONE,1965; PULITZER-FINAL], 1983) were calculated. The differences among the size frequency distributions were tested using the non-parametric KOLMOGOROV-SMIRNOV test. Five specimens of T. orphei, T. robusta, and T. seychellensis from the Maldives, together with ten specimens of a T. robusta population from Massaua (Red Sea) were also analyzed by cellulose acetate electrophoresis. The samples were stored in liquid nitrogen before crushing in a homogenizing solution (10mg NADP, 1 0 0 ~ 1mercaptoethanol per lOOml distilled water, p H 7) in a proportion of 1rnl per gram of tissue. Only the choanosome of sponges was utilized to avoid the presence of autotrophic symbionts. Electrophoresis was carried out on CELLOGEL (Chemetron) et al. (1986) sheets. Procedure and staining solution were similar to those described by RICHARDSON & SARA(1992). and were previously used for sponges by SARAet al. (1989, 1992) and BAVESTRELLO The loci scored were 6-phosphogluconate dehydrogenase (6Pgd), mannose phosphate isomerase (Mpi), hexokinase (Hk), malate dehydrogenase (Mdh), glucose-phosphate isomerase (Pgi), esterases (Es), isocitrate dehydrogenase (Idh), and glutamate-oxalacetate transaminase (Got). Allelic frequencies and NEI’S(1972) D indices were calculated.

Results 1. Species description Tethya orphei SARA(1990b, p. 155)

Spherical, 1 to 1.5 cm in diameter. Colour in life, externally black or dark violet, internally dark gray. Consistency soft, very compressible. Surface with tubercles, 1 mm wide and 2 mm high. Fine sediment occurs in the surface of many specimens. Cortex thin, with large subdermal lacunes. The outer layer shelters a dense population of filamentous Cyanobacteria. Styles (strongiloxeas) in the two categories: main (600-1080 X 5-14 pm) and accessory (250-350 x 2-5 pm) with intermediates. Megasters, infrequent in the whole cortex, placed mainly under the subdermal lacunes. D = 15-70pm, exceptionally 80pm, W C = 0.7. Small megasters occur also in the outer choanosome. Cortical tylasters often cruciform in projection, with slender, cylindrical, roughened rays. Generally with a small swelling at the tip of the rays. With irregularities such as bendings and forks of the rays. D = 10-12 pm. Choanosomal micrasters (Fig. 2) include larger chiasterdoxyasters with roughened rays. The rays may be irregularly bent or bifurcated. Intermediates exist with the cortical type. D = 15-30 pm, generally 20-25 pm. Tethya orphei has been previously found only in Australia (Orpheus Island, Great Barrier Reef). The Maldivian specimens of T. orphei are consistent with, particular in skeletal features, the Australian type specimen, suggesting a lower variability in the spiculation of this species. However, differences with the Australian specimens exist in the greater size of the Maldivian sponges and in their cortex colour, which is black due to the presence of symbionts.

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Fig. 2. SEM micrographs of micrasters of T. orphei. Bar = 1 pm.

Tethya microstella SARA (1 990b, p. 153)

Spherical, from 1 to 2cm in diameter. Colour in life, externally pale grayish, internally yellowish. Consistency hard, poorly compressible. Surface with indistinct tubercles, sometimes covered by a thin layer of sediment. Cortex thin, without distinct lacunes. One specimen contained a central nucleus measuring 600pm in diameter; it was formed of tangential short styles (200-250pm) transversal to the bases-of the bundles of styles.

Tethya from a Maldivian coral reef lagoon

345

Fig. 3. SEM micrographs of micrasters of T. microsfella (a-c) and T. robusfa (d-e). Bar = 1 pm.

Styles (strongiloxeas) variable in size and shape, subdivided into a main category 700-1050 x 10-15 pm, and a small accessory category: 250-500 x 4-10 pm, with intermediates. The slender styles often with a subtylote head. Larger megasters occur as a main layer in the middle cortex. Smaller ones also in the inner cortex and superficial choanosome. D = 10-50 pm, FUC = 0.7-0.8, sometimes 0.5. Cortical tylasters in a dense layer on the surface of the sponge; with slender cylindrical and roughened rays which bear a swelling, generally small, at their tip; cruciform in projection. D =

346

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4-6 pm, sometimes 8 pm. The rays may be irregularly developed with forks and bendings. Choanosomal tylasters, similar to the cortical ones, yet generally larger: 6-11 pm. The Maldivian specimens are attributed to T. microsrellu chiefly on the basis of the shape and size of their very small cortical micrasters (Fig. 3 a-c). On the other hand these specimens differ from the Australian type specimen (SARA, 1990b) and the New-Guinean collection (SARA,1993) in the smaller size of main styles and megasters. Moreover, as opposed to the Australian and NewGuinean specimens, those from the Maldives show no clear cortical lacunes and are more compact and less compressible. They represent an extension of variability in cortical structure previously observed in the Australian and NewGuinean specimens.

Tethya robusta BOWERBANK (1873, p. 10); SARA(1990, p. 186)

Spherical, hemispherical, or cushion-like. Size from 0.5 to 3cm in diameter. Colour in life, externally pale yellow, grayish or ochraceous; internally pale brown. Consistency hard, poorly compressible. Surface with flattened or scarcely prominent, polygonal, 2 4 mm wide tubercles; without sediment or epibiontic organisms. Cortex well developed. Subdermal lacunes, when present, small. Styles (strongiloxeas): main category: 700-1450 X 12-22 pm; small accessory category: 350 X 5-9 pm; with intermediates. Megasters: very abundant, densely packed into the whole cortex and less abundant in the choanosome, not confined to its outer region as in the Great Barrier Reef specimens (SARA, 1990b). D = 25-120 pm with R/C = 0.6. In the choanosome smaller megasters, 20-30pm with R/C = 1. Micrasters (Fig. 3 d-e): cortical tylasters and chiasters which form a dense layer on the surface, with or without a small center and with a more or less pronounced swelling at the tip of the rays. D = 10-14pm. Medullar chiasters and oxyasters with or without a small center. D = 14 pm. Terhya robustu is a widely distributed species in the Indo-Pacific region. Its occurrence has been hitherto documented from Australia (Great Barrier Reef and Abrolhos Islands), Salomon Islands, Papua-New Guinea, New Zealand, and the Red Sea. Recent observations of the senior author (SARA)indicate the presence of this species in the Levant Sea (Eastern Mediterranean). The Maldivian specimens of this species differ from the Australian type 1873) in the larger size of megasters. specimen (BOWERBANK, Tethya seychellensis WRIGHT(1881, p. 13: Alemo seychellensis)

Spherical to hemispherical. Diameter from 0.5 to 4cm. Colour in life, externally carmine red, rarely orange; internally orange. Consistency soft and compressible. Surface covered by flattened, generally well distinguished tubercles. External budding occurs in many specimens. Buds are developed on 5-20mm high stalks. Epibiontic algae sometimes cover the external cortex surface. Fine sediment often covers the depressions between the tubercles. Cortex relatively thin, with large subdermal lacunes and smaller lacunes in the outer part of the

Tethyo from a Maldivian coral reef lagoon

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choanosome. A skeletal nucleus, 1mm in diameter and formed by the intersection of transversal small styles, has been found in the center of some specimens. Styles (strongiloxeas) main category: 1100-1250 x 18-22 pm; small accessory category: 240-470 x 2-9 pm. Small styles, 300-400 x 10 pm, bent at the proximal end, most likely participating in the formation of the skeletal nucleus. Megasters: distributed in the whole cortex and the outer choanosome, with a main layer under the subdermal lacunes. D = 30-80pm with R/C = 0.7. More rarely, chiefly in the choanosome, smaller megasters, D = 1&20pm, with R/C = 0.5. Micrasters (Fig. 4): cortical tylasters densely packed on the external surface of the cortex and coating the cortical canals and lacunes. With roughened rays which show a small swelling at the tip of the rays and many irregularities such as bendings and forks. Often cruciform or tridentate in projection. D = 8-14 pm. Choanosomal oxyasters triradiate, tetraradiate

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(cruciform), or pentaradiate in projection with a small center or without a center. Rays very often forked, bent, or with other irregularities. D = 30-50 pm. Tethyu seychellensis has a wide intertropical, Indo-Pacific, and Atlantic diffusion. "he Maldivian specimens show skeletal features very similar to the type specimen from the Seychelles.

2. Morphometric analyses Morphometric data of the Maldivian species and the Massaua population of T. robustu regarding megaster diameter and shape, evaluated by the R/C ratio, are summarized in Table 1. The size ,frequency distributions (Fig. 5) show that the range of variation of the T. robustu asters is well separated from those of the other species which

Tablel. Morphometric features of megasters. For each species 5 specimens were selected and 100 spicules measured for each specimen. diameter

T. seycheffensb T. robustu T. robustu (Massaua) T. orphei T. microstella 0.6

raylcenter

mean + S D

max. (Pm)

min. (crm)

mean k SD

(Pm)

54.7+ 8.5 92.3 t 15.8 59.6 t 7.6 4 3 . 4 t 9.3 31.0 5 10.7

77.4 118.4 80.3 69.2 53.4

32.4 25.9 31.0 14.3 5.8

0.66 0.21 0.56 t 0.18 0.5850.11 0.67+ 0.19 0.76 5 0.67

+

I

h

-0

0.4

-

0.3

-

0.2

-

0

5

e

c I

0

t!

g

L

0

10

20

30

40

50 60 70 80 size classes @m)

90

1 0 0 1 1 0 1 2 0 ' I0

Fig. 5. Size frequency distributions of the diameter of the megasters of the four species.

349

Tethya from a Maldivian coral reef lagoon

Table 2. Allelic frequencies and heterozygosity. Hn: number of heterozygotes; Ho: observed heterozygosity; He: expected heterozygosity. loci

alleles

T. seychellensis Maldives

T. robusta Maldives

T. orphei Maldives

Hk

a b

1 0 0

1 0 0

0.1 0.1 0.8 Hn = 2 Ho = 0.4 He = 0.34

1 0 0

0 0 1

1

C

T. robusta Massaua 0 1 0

~~

Got

a b C

0 1 0

0 0 ~~

Es

a b

0 1 0 0 0

C

d e

0.4 0 0.6 0 0 Hn = 2 Ho = 0.4 He = 0.48 ~

Idh

a b

0 1 0 0

C

d Mdh

a b C

0.4 0.6 0

1 0 0

1 0 0 0 0 0 1

1 0 0

Hn

=2 Ho = 0.4 He = 0.48 ~

Pgi

Mpi

6Pgd

a b

0 0

C

1

0 0.4 0.6 Hn = 2 Ho = 0.4 He = 0.48

1 0 0

0 1 0

a b

0

0

0.2

C

0.8 Hn = 2 HO = 0.4 He = 0.48

1 0

1 0 0

0 1 0

0 1 0

0 1 0

0 0 1

a b C

1 0 0

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overlap to a certain degree. Nevertheless all distributions are significantly different using the non-parametric KOLMOGOROV-SMIRNOV test (0.05 > P > 0.01). Regarding megaster shape, the R/C ratio increases from T. robusta to T. microstella, while T. seychellensis and T. orphei, with a very similar ratio, occupy an intermediate position. A comparison between the Gangehi population of T. robusta and the Massaua one (Table 1) shows that the megasters are very similar in shape yet significantly different (P < 0.01) in size.

3. Electrophoresis Enzymes where all the tested specimens gave reliable results are listed in Table 2. Each enzyme coded for only one gene locus. The data show that all populations are monomorphic for the same allele at three loci (Got, Zdh, 6Pgd), while the other loci show varying amounts of polymorphism. All the examined populations are fixed for alternative alleles at several loci; in particular, the T. robusta population from Massaua shares only two loci with Maldivian T. robusta, and one locus with T. seychellensis and T. orphei.

2

1.5

1

0.5

0

I

I

I

I

I A

B

C

D

Fig. 6. NEI’Sdistances among the examined populations. A, T. seychellensb; B, T. robustu (Maldives); C, T. orphei; D, T. robustu (Massaua).

The number of polymorphic loci is low (one or two in each species) and similar to that observed by SARAet al. (1989) in Mediterranean T. citrina. In contrast with this species, however, the Maldivian species do not show significant differences between the observed and expected heterozygosity. The genetic distances (D) (NEI, 1972) between the Maldivian species range from 1to 1.5 (Fig. 6) and are comparable to those between the European Tethya species, while the genetic distance between T. robusta from the Red Sea and the Maldivian species (including T . robusta) are considerably higher (1.74.3).

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Tethya from a Maldivian coral reef lagoon

4. Distribution The four Tethya species are present only in stations A, B, and C. They are absent in the most internal lagoon station (D) and outside the lagoon (E, F) (Table 3). In the three stations where Tethya occurs, no appreciable differences were found in the frequency values distribution of specimens for each species. Fifty specimens of T. seychellensis, 31 of T. robusta, 21 of T. orphei, and 4 of T. microstella were found in a total area of 30m2 (Table 3). Density values (per square meter) in the three stations with Tethya are: T.seychellensis, 3.33; T . robusta, 2.06; T. orphei, 1.4; T. microstella, 0.27 (Table 3 ) . The distribution of the four species in different dead coral branch layers (Table 4) shows a significantly (P < 0.01 in the Chi-square test) higher frequency of T. seychellensis and T. orphei in the upper layer than in the middle and lower ones; a significantly (P