CHAPTER 39

The Role of Sponges in the Terra Nova Bay Ecosystem

R. Cattaneo-Vietti', G. Bavestrello', C. Cerrano', E. Gaino', L. Mazzella3, M. Pansini', and M. Sara'

ABSTRACT One hundred and twenty five demosponge species are known from the Ross Sea,49 of which have been recorded in Terra Nova Bay within the framework of the PNRA ecological project. The most common species are Tedania charcoti, Axociel/a nidificata, Calyx arcuarius, Isodictya erinacea, I. cac­toides, I. conulosa, Gellius rudis, Gel/ius spp., Myxil/a elongata and Phorbas glaberrima. Two of the 49 species we found are new for Antarctica: Esperiopsis informis and Isodictya conulosa. Most of the sponges were collected at 70-120 m depth, where a great substrate heterogeneity allows the coexistence of different biocoenoses.

The success of sponges in habitats characterized by fluctuating food supply during the year is difficult to understand. In fact, in winter, oligotrophic conditions in the water column could represent a metabolic constraint for a filter feeder. A possible explanation could be the direct uptake of diatoms that manage to live for a long time within the sponge tissues. Sponges, therefore, by transferring energy from the water column to the benthos, playa key role in the Antarctic environment. An unusual physical phenomenon has been detected, by studying the sponge/diatom association, in the spicules of the hexactinellid Rossel/a racovitzae: spicules conduct light as natural optical fibres.

Sponges, and particularly hexactinellids, greatly affect sediment quality by forming mats, up to 1.5-m thick, because their siliceous spicules take a very long time to dissolve. At Terra Nova Bay the occurrence of spicules in the sediments shows three different arrangements: (1) free spicules in the sediments, (2) sphaeroid balls (aegagropila) of densely packed spicules, (3) spicule mats.

I ntrod uction

Sponges represent a major component of the Antarctic zoobenthos, remarkably contributing to the species richness of this continent, where 350 species of demo sponges are known (Sara et al. 1992). Some of these are characterizing elements of several assemblages (Dayton et al. 1974; Barthel et al. 1990) where they can reach biomass values of about 2-4kgm-2 wet wt (Beliaev and Ushakov 1957), comparable with the highest values found for sponges in tropical seas (Wilkinson 1987).

In the Ross Sea, sponge communities, between 70 and 180m depth, have been described by Bulli­vant (1967) as the McMurdo Sound Mixed and the Glass Sponge assemblages. At McMurdo, Dayton et al. (1974) described a littoral belt, at 30-60m

depth, in which sponges are the keystone species. In addition, sponge spicules constitute conspicu­ous mats (Koltun 1968; White et al. 1985; Barthel 1992), thereby affecting both sediment texture and soft-bottom community structure.

The aim of this chapter is to focus on the results obtained within the Italian Programme for Antarctic Research (PNRA) about the Porifera living at Terra Nova Bay (Ross Sea).

Taxonomic and Ecological Aspects

Forty-nine species, belonging to 7 orders and 15 families, have been recorded at Terra Nova Bay in waters 300-350m depth (Table 1), out of the 125 demosponge species known from the Ross Sea

I Dipartimento per 10 Studio del Territorio e delle sue Risorse dell' Universita di Genova, Viale Benedetto XV 5, 1-16132 Genova, Italy 1 Dipartimento di Biologia Animale ed Ecologia dell' Universita di Perugia, Via Eke di Sotto, 1-06123 Perugia, Italy J Lab. Ecologia del Benthos - Stazione Zoologica Anton Dohrn, Ischia, Naples, Italy

F. M. Faranda et al. (eds.), Ross Sea Ecology© Springer-Verlag Berlin Heidelberg 2000

540 R. Cattaneo-Vietti et al.

Table I. l.ist of demo sponge species from Terra Nova Bay so far identified

1. Plakina monolopha Schulze 1880 2. Plakina trilopha Schulze 1880 3. Cinachyra barbata Sallas 1886 4. Tetilla leptoderma Sallas 1886 5. Pseudosuberites antarcticus (Carter 1876) 6. Pseudosuberites nudus Koltun 1964 7. Sphaerotylus borealis antarcticus Kirkpatrick 1908 8. Suberites montiniger Carter 1880 9. Suberites caminatus Ridley & Dendy 1887

10. Latrunculia biformis Kirkpatrick 1908 11. Stylocordyla borealis Loven 1868 12. Eurypon miniaceum Thiele 1905 13. Homaxinella balfourensis (Ridley & Dendy 1887) 14. Homaxinella flagelliformis (Ridley & Dendy 1886) IS. Asbestopluma be/gicae Topsent 1902 16. Mycale acerata Kirkpatrick 1907 17. Mycale fibrosa Boury-Esnault & Van Beveren 1982 18. Mycale tridens Hentschel 1914 19. Esperiopsis inform is Stephens 1915 20. Isodictya cactoides (Kirkpatrick 1908) 21. Isodictya conulosa (Ridley & Dendy 1886) 22. Isodictya erinacea (Topsent 1916) 23. Inflatella belli (Kirkpatrick 1908) 24. Ectyodoryx antarctica (Hentschel 1914) 25. Ectyodoryx nobilis (Ridley & Dendy 1886) 26. Ectyodoryx ramilobosa (Topsent 1916) 27. Ectyomyxilla sp. 28. Iophon radiatus Topsent 1902 29. Iophon unicornis Topsent 1907 30. Iophon sp. 31. Lyssodendoryx flabellata Burton 1929 32. Myxilla asigmata (Topsent 1902) 33. Myxilla e/ongata Topsent 1917 34. Myxodoryx hanitschi (Kirkpatrick 1907) 3S. Tedania charcoti Topsent 1907 36. Tedania tantula (Kirkpatrick 1907) 37. Phorbas glaberrima (Topsent 1917) 38. Axociella nidificata (Kirkpatrick 1907) 39. Artemisina tubulosa Koltun 1964 40. Clathria toxipraedita Topsent 1913 41. Calyx arcuarius (Topsent 1913) 42. Haliclona dancoi (Topsent 1902) 43. Haliclona penicillata (Topsent 1908) 44. Microxina benedeni (Topsent 1901) 4S. Gellius pilosus Kirkpatrick 1907 46. Gellius rudis Topsent 1901 47. Gellius sp. 1 48. Gellius sp. 2 49. Dendrilla membranosa Pallas 1776

(Sara et al.1992).A species belonging to the genus Iophon is probably new for science and will be described in a separate chapter. Among the most common demo sponges, Tedania charcoti, Gellius rudis and Axociella nidificata, are present in more than half of the total samples, Calyx arcuarius,

Isodictya erinacea and Gellius spp., in more than 40%, Isodictya cactoides, 1. conulosa, Myxilla elongata and Phorbas glaberrima, in more than 30%.

A large part of these species was collected, using a triangular dredge, between 80 and 120m depth, inside a wide belt characterized by a great substrate heterogeneity allowing the coexistence of different communities.

Only three species (Inflatella belli, Calyx arcuarius and Axociella nidificata) have been recorded deeper, below 300 m depth. On sparse rocky outcrops (Fig. 1), the community is dominated by a diversified filter-feeder assem­blage characterized by sponges, anthozoans and holothuroids (Di Geronimo et al. 1992; Cattaneo­Vietti et al., this Vol.). This community develops within the Encrusting Algal Zone, charac­terized by the coralline alga Clathromorphum lemoineanum and shows a high richness in species and biomass, which can be only partially com­pared to that described for McMurdo Sound, between 30 and 60 m depth (III zone of Dayton et al.1974).According to our data, the Terra Nova Bay community seems to differ from that of McMurdo in composition and structure because it is domi­nated by demosponges, whereas hexactinellids and calcareous sponges are represented by only a few species, with a low density. Among the volcano species (Dayton et al. 1974), Rossella racovitzae is uncommon and R. nuda and Scolymastra joubinii are occasionally found (Fig. 1).

Biogeographic Aspects

Appropriate oceanographic conditions are re­quired to allow a long-range dispersal of sponge species, because these sessile organisms have also short living larval stages. The main biogeographic characteristics of the Antarctic sponge fauna can be defined by the following characteristics (Burton 1932; Koltun 1970): (1) a high level of species-endemism coupled with negligible genus endemism; (2) a circumpolar distribution for many species; (3) a closer faunistic relationship with southern South America and Falkland Islands, than with South Africa, Australia and New Zealand. More recently, Sara et al. (1992) high­lighted typical Antarctic demo sponge fauna indi­cated as the Antarctic Faunistic Complex (AFe), which is homogeneous all along the continent. The distinction made by several authors (Ekman

The Role of Sponges in the Terra Nova Bay Ecosystem 541

Fig. 1. Underwater photograph showing the sponge assemblage at Terra Nova Bay (95 m depth). Scale bar = 1 m

1935; Andriashev 1965; Kussakin 1967; Knox 1970) into western and eastern biogeographic subunits seems to be artificial. The ordination pattern from the correspondence analysis of sponge species from Antarctica and neighbouring areas (Sara et al. 1992) shows that Magellan, Falklands and South Georgia tend to be close to the continental sector west of the Antarctic Peninsula. This analy­sis confirms the existence of faunal exchanges between Antarctica and South America via the Scotia Arc, proposed by Knox and Lowry (1977) as one of the mechanisms for the origin and disper­sal of the Antarctic marine fauna.

The biogeographic analysis of the demo­sponges from Terra Nova Bay concerns 45 spe­cies, because four taxa were identified only to the generic level. Most of these (36 species) were already known from the Western Ross Sea, while only 3 species (Pseudosuberites antarcticus, Suberites caminatus and Isodictya erinacea) were reported from the Eastern Ross Sea sector. Of the remaining 6 species, Homaxinella flagelliformis was known from Kerguelen and Magellan area, and has been recently recorded from Terra Nova Bay (Pansini et al. 1994) and from the Weddel Sea (Gutt and Koltun 1995). Mycale tridens and Artemisina tubulosa were already known from other Antarctic areas (Sara at al. 1992) and Mycale fibrosa from the western side of the Antarctic Peninsula and from Kerguelen Islands. Esperiopsis informis was known from South Africa and Isod­ictya conulosa from South Africa, South America and Australia. Both are new for Antarctica.

The available data support the circumpolar

distribution of many Antarctic sponges. In addi­tion, the new record for Antarctica of two species known from South Africa is remarkable, because the biogeographic affinity of the two areas is thought to be low.

Relationship Between Sponges and Autotrophic Organisms

It is difficult to explain the success of sponges in Antarctic habitats which are characterized by a quantitatively fluctuating food supply during the year. Oligotrophic conditions in the water column in winter (Matsuda et al. 1987) could represent, in fact, a metabolic constraint for filter-feeding organisms. Antarctic sponges take up and store planktonic and benthic diatoms by means of the so-called pinacoderm entrapping activity (Gaino et al. 1994). This strategy, if confirmed, would imply that these organisms can rely on an alterna­tive energetic source during oligotrophic months, considering also the large amount of polysaccha­rids produced by diatoms (Edgar 1983). In addi­tion, this entrapping (Fig. 2) supports the existence of a functional adaptation of Antarctic sponges which, in summer months, rely on a food uptake mechanism that bypasses the aquiferous system, preventing a possible clogging of the choanocyte chambers caused by large amounts of organic matter that settle on their surface (Fabiano et al. 1997).

Relationships between sponges and auto­trophic organisms (cyanobacteria, zooxanthellae

542 R. Cattaneo-Vietti et al.

Fig.2a-d.

and zoochlorellae) are well known in tropical (Wilkinson 1980) and temperate (Sara 1971; Sara and Vacelet 1973; Gaino et al. 1976; Arillo et al. 1993) waters where they represent a food source, in addition to organic particles coming from the water column (Wilkinson 1979, 1983). However, few data are available on possible feeding rela­tionships between sponges and diatoms (Sara 1966; Cox and Larkum 1983).

A few species of pennate and centric diatoms (Fig. 3), typical of Antarctic waters (Krebs 1983) and well adapted at very low irradiance levels (Palmisano et al. 1985), were found in Suberites montiniger, Tedania charcoti, Phorbas glaberrima and Isodictya conulosa. Benthic diatoms were rep­resented by Achnanthes sp. (Fig. 3a,c) and Pseudo­gomphonema sp., whereas Fragilariopsis curta (Fig. 2) and Fragilariopsis sp. can be considered sympa­gic species, which represent the typical compo­nents ofthe sea-ice diatom flora (Krebs et aI.1987). Centric diatoms belong to the genera Porosira (Fig. 3a,b), Coscinodiscus and Rhizosolenia.

Fragilariopsis species were frequently found associated with all the examined sponges and reached a remarkable concentration in Phorbas glaberrima (Fig. 2) and Tedania charcoti. The assemblage living in Suberites montiniger (Fig. 3a-c) was characterized by Achnanthes sp. and Porosira sp., whereas Pseudogomphonema sp. was rarer and observed only in Isodictya conulosa. Navicula and Nitzschia were occasionally observed.

The occurrence of diatoms in sponge tissue was confirmed by chlorophyll values rec­orded from the sponge body. However, remark­able variations were measured among the con­sidered species: the sole calcareous sponge exam­ined (Leuconia sp.) (Table 2) showed low values of chlorophyll whereas the highest concentrations were found in the demo sponges Isodictya eri­nacea, I. conulosa, Inflatella belli, Phorbas glaber­rima and Myxodoryx hanitschi. The comparison of three specimens of Myxodorix hanitschi showed significant differences in ChI a content with values ranging from 5.5 to 16.1 ~g g-l Ww. In addition, chlorophyll concentrations were not uniform within the sponge body. The outer ectosome was richer in ChI a than the inner choanosome, char-

..

The Role of Sponges in the Terra Nova Bay Ecosystem 543

acterized by a greater amount of ChI c in Phorbas glaberrima. By contrast, in Iophon sp. and partic­ularly in Inflatella belli, the highest concentrations of ChI a and ChI c were found in the choanosome.

The amount of ChI a was about 2 to 4 times higher than ChI c; in Phorbas glaberrima the ratio ChI a/ChI c decreased from 4.7 to 1.3 from the ectosome to the choanosome. The very high values of ChI c, double those of ChI a (Table 2) recorded in Suberites montiniger, could be related to pres­ence of Achnanthes sp., the dominant associated diatom in this sponge.

By studying these sponge-diatom associa­tions, an unusual physical phenomenon was detected: the long spicules of the hexactinellid Rossella racovitzae were able to channel light as natural optical fibres (Cattaneo-Vietti et al. 1996). Presumably, this light-conducting system plays a metabolic role, affecting the occurrence, growth and distribution of associated diatoms inside the sponge body. Indeed, SEM observations showed numerous diatoms adherent to the spicules, thereby making such a physical phenomenon es­sential for diatom survival in rather deep waters at very low levels of light intensity.

Spicule Mats

Particular attention has been paid to elucidate the role of sponge spicules in the composition of the littoral sediments of Terra Nova Bay. Even though siliceous sponges are known to be involved in the formation of important silica deposits (Bullivant 1967; Dearborn 1967; Koltun 1968; Dayton 1979; Barthel 1992; Frignani et al. 1992), their role has not been deeply investigated. The sponge siliceous spicules, especially those of the hexactinellids, take a long time to dissolve, can form mats, up to 1.5 m thick, contributing to a significant enrichment of biogenic silica (BSi) in the sediment and causing consequently a loss of available silica in the water column.

At Cape Armitage (Ross Island) these mats are mainly composed of Cinachyra antarctica spicules (Battershill 1989), while the thick spicule mats described from deeper waters by Dayton et al. (1974) are built up by an accumulation of spicules

Fig.2a-d. SEM view of settled diatoms (Fragilariopsis curta) on the ectosome of Phorbas glaberrima (a), and different steps of the their uptake (b-c) and final incorporation (d). Bars lOO~m

544 R. Cattaneo-Vietti et al.

Fig.3a-e.

The Role of Sponges in the Terra Nova Bay Ecosystem 545

Table 2. Chlorophyll values in Terra Nova Bay sponges collected from 70 to 100m depth

Species No. of specimens Chi a (!lglg) Chi c (!lglg)

Calcispongiae Leuconia sp. 0.67 ± 0.1 0.27 ± 0.1

Demospongiae MicTOxina benedeni 2.00 ± 0.3 1.53 ± 0.8 Artemisina tubulosa 2.67 ± 0.5 0.99 ± 0.1 Haliclona dancoi 3 2.86 ± 0.3 1.09 ± 0.3 Pseudosuberites nudus 3 2.95 ± 0.5 2.62 ± 0.6 Axociella nidificata 3.41 ± 0.2 0.99 ± 0.1 Gellius pilosus 6 3.43 ± 0.3 0.80 ± 0.1 Gellius rudis 3 5.28 ± 1.6 0.85 ± 0.1 Gellius sp. 1 3 2.41 ± 1.0 0.90 ± 0.3 Dendrilla membranosa 3 5.68 ± 1.4 2.77 ± 1.2 Suberites montiniger 6 6.01 ± 0.9 11.12 ± 2.2 Tedania charcoti 6 6.11 ± 1.6 0.30 ± 0.7 Myxilla elongata 9 6.11 ± 2.0 1.67 ± 0.4 Isodictya erinacea 3 7.42 ± 0.9 3.52 ± 0.5 Myxodoryx hanitschi 9 9.80 ± 2.2 4.34 ± 0.8 Isodictya conulosa 3 10.73 ± 2.9 3.59 ± 0.8 Iophon sp. (ectosome) 3 1.56± 0.4 0.88 ± 0.4 Iophon sp. (choanosome) 3 3.68 ± 1.2 1.13 ± 0.5 Inflatella belli (ectosome) 0.69 ± 0.3 0.44 ± 0.1 Inflatella belli 9.00 ± 2.4 5.96 ± 1.7

(choanosome) Phorbas glaberrima 6 9.79 ± 3.2 2.10 ± 0.6

(ectosome) Phorbas glaberrima 6 5.44 ± 0.6 4.35 ± 1.3

(choanosome)

coming from many sponge species. At Terra Nova Bay, spicule mats are formed by hexactinellid spicules particularly from Rossella sp. (Fig. 4a) while in the sediments from Cape Confusion, tetractinellid spicules, especially from Cinachyra sp., prevail (Fig. 4b).

free spicules were found practically in all the considered stations with densities varying from less than 5000 up to 500000 spiculeg-1 DW of sediment (Fig. 5). In addition, spicule mats were also detected in isolated sampling areas along the Campbell glacier tongue and in front of the Cape Confusion glacier at depths between 200 and 250 m. The amount of settled spicules gradually increases from the coast towards the open sea, showing a significant relationship with the filter-feeder assemblages of the hard bottom belt dominated by sponges between 80 and 120m depth.

The spicules found in Terra Nova Bay sedi­ments show various aggregation types: (1) loose spicules, often broken, mixed with the sediment; (2) densely packed spicules forming sphaeroid balls (aegagropila) very variable in diameter, 1O-90mm in diameter (Fig. 4c), (3) stratified spi­cule mats, often formed by large amounts of aega­gropila (Fig. 4d) with a silica ponderal value ranging between 50 and 80% of the total sediment weight. Aegagropila and mats were found along the coast from Tethys Bay to Adelie Cove, within a bathymetric range from 90 to 120m depth, while

SEM analyses confirm the difficulty to dissolve opal at low temperatures. No signs of etching were observed on recent spicules (Fig. 4b), as well as on subfossil ones, collected in a deposit of the Hell Gate Glacier tongue.

Fig.3a-e. SEM and TEM micrographs of diatoms Ijving inside Antarctic sponges. a SEM general view of assemblages of Achnan­thes sp. (e) and POTOsira sp. (b) in Suberites montiniger. d, e TEM micrographs of incorporated diatoms. Note the sequence of the frustules (d) and the detail of intact photosynthetic apparatus (e). Bars a 20 !lID; b-e 10 !lID

546 R. Cattaneo-Vietti et aJ.

Fig.4a-e. Spicule mats at Terra Nova Bay. SEM micrographs of a spicule mat constituted by hexactineBid (a) and telractinel­lid (b) spicules. Free aegagropila in the sediments (c) and a stratified spicule mat constituted by innumerable aegagropila (d). Note diatoms growing on a burrowed spicule (e). Bars a,e 1 mm; b IOO!lm; c-d 1 cm

The Role of Sponges in the Terra Nova Bay Ecosystem 547

> 100000 spicules/g spicules mats 2Km

Fig. 5. Sponge spicule distribution in the sediments along the coast from Tethys Bay to Adelie Cove. The amount of spicules gradually increases from the coast towards the hard-bottom belt characterized by the sponge community, between 80 and 120m depth

In order to verify the presence of free spicules also in off-shore sediments, additional observa­tions were carried out on samples coming from the center of the northern part of the Joides Basin (site B: 74°S-175°E) and the northern flank of the Mawson Bank (site C: n030'S-175°E), where oceanographic moorings where deployed during the 1994 ROSSMIZE cruise (Labrozzi et al. 1998). Site B (586m depth) can be considered a decantation area with very muddy sediments, while site C (456m depth) was characterized by coarse sediment in an area of strong currents. The amount of free spicules found in site B ranged around 200000 spiculeg-1 DW of sediment, but was far lower in C: 4800 spicule g-l DW. These data agree with the remarkable BSi differences found in the sediment of the same sites by Labrozzi et al. (1998).

A possible ecological role of the spicules in the sediments may be advanced. The Chi a (Fig. 6), biomass and species richness values are higher in areas rich in spicules than in sites characterized by spicule-free sediments. A possible explanation may be bound to the sponge spicule capacity of creating a 3-D substratum on which larger diatom population can develop (Table 2), supporting therefore a richer associated fauna as observed by Dearbon (1967) at McMurdo. This process could be enhanced by the opal spicule capacity of con­veying light as natural optical fibres (Cattaneo-

Vietti et al. 1996) allowing diatoms to live on the spicules (Fig. 4e).

Conclusions

In Antarctica, sponges can be considered as a key taxon, because of their high biomass and capacity to structure, by the spicules, soft-bottom sedi­ments. In Antarctic assemblages, composed of long-living species, varied biological interactions prevail over physical factors. This is particularly true for the peculiar trophic strategy adopted by many sponges, able to take up and preserve dense concentrations of diatoms within their bodies. This strategy allows them to overcome severe food fluctuations typical of the Antarctic environment. The nature of these associations needs further investigation. It could be reasonable to suppose a symbiotic relationship, likewise observed for other autotrophic organisms associated to sponges in tropical and temperate oligotrophic habitats (Wilkinson 1979), even though a true endosymbiosis causes remarkable changes in the organization of diatom frustules (Reimer and Lee 1988).

The conspicuous sponge spicule mats found at Terra Nova Bay stress the important role of sponges in determining sediment texture with strong ecological, geological and sedimentological

548 R. Cattaneo-Vietti et aJ.

o ~------------~------------------~ Fig.6. Different trends of Chl a values at 120 m depth in muddy sediments with low amounts of spicules and in spicule mats (standard deviations are represented by horizontal lines)

-C Q)

E "0 Q)

UJ 4 Q) .c -c

8~----------~------r-~~~----~ o 246

Chlorophyll- a(~g/g) B

implications (White et aI. 1985). In fact, spicules create a more rich and structured 3-D substratum which favours the development of specific assem­blages. The distribution of spicule mats is linked to some historical, biological and ecological factors. Most of these are constituted by hexactinellid spicules, a group not so frequent now in the bay to justify these large deposits. As a consequence, both spicule distribution and mat formation could reflect historical events (mass mortality of some species, iceberg scouring), in which currents have played a complex role. In fact, seafloor currents, according to their intensity, are able to disperse, aggregate and accumulate spicules along the coast, as suggested by the different shape of mat struc­tures and by the patchy distribution of spicules.

The fact that opal sponge spicules may act as optical fibres, conveying light inside the sponge body, suggests that this physical phenomenon could be strictly linked to sponge evolution. In fact, autotrophic symbiosis in ancestral sponges was established probably during the late Precambrian (Wilkinson 1983), when a great expansion of sili­ceous sponges occurred. We can speculate that symbiosis, the winning strategy of this taxon, could be supported by this peculiar property of spicules.

Acknowledgements. This research was supported by the Italian Programma Nazionale di Ricerca in Antartide (PNRA)

10

within the framework of the research project Ecology and Biogeochemistry of the Southern Ocean. The authors thank M.e. Gambi (Stazione Zoologica di Napoli), M. Ravaioli (CNR. Bologna) and N. Corradi (Universita di Genova) for their help in sampling operations. Laura Paganelli and Paola Allegretti helped in data analysis.

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