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The worm endosymbionts in tabulate corals from the Silurian of Podolia, Ukraine/ Siluri endosumbiootilised ussid Podoolia (Ukraina) tabulaatsetes korallides.

INTRODUCTION

Macroscopic worm endosymbionts are frequently found in recent corals (Ross & Newman 1973; Smith 1984; Hunte et al. 1990a, 1990b; Marsden & Meeuwig 1990; Nishi & Nishihira 1996, 1999), but were common also in the Palaeozoic (Richards & Dyson-Cobb 1976; Tapanila 2002, 2004, 2005; Tapanila & Copper 2002; Tapanila & Holmer 2006). The earliest endosymbiotic worm fossils are known from Late Ordovician rugose (Elias 1986) and tabulate corals (Tapanila 2004) and are preserved as bioclaustrations. Bioclaustrations are produced by the embedding of an endosymbiont within the growing skeleton of a living, host organism. As a result of this interaction, a cavity is produced within the host skeleton in which the endosymbiont lives (Tapanila 2005). Three species of endosymbiotic worms, Chaetosalpinx ferganensis Sokolov, 1948, Chaetosalpinx sibiriensis Sokolov, 1948 (= Camptosalpinx estonicus Klaamann, 1958), and Coralloconchus bragensis Vinn & Motus, 2008, are known in the Silurian tabulate corals of Baltica (Tapanila 2005; Vinn & M6tus 2008). Chaetosalpinx sibiriensis has hitherto been reported from Parafavosites germana (Wenlock, NE Russia, Sokolov 1948) and Paleofavosites balticus (Llandovery, Estonia, Klaamann 1958). The specimens of Favosites pseudoforbesi muratsiensis containing C. sibiriensis, from an Ordovician-Silurian erratic of the Netherlands (Stel 1976), could also be of Baltic origin.

Similarly to modern endosymbionts of scleractinian corals (Tapanila 2005), Palaeozoic bioclaustrations are common in particular host taxa, but entirely absent from the others. Like recent endosymbionts, Palaeozoic endosymbionts appear to have preferred colonial corals with a massive, cerioid (e.g. favositids) to coenenchymal (e.g. heliolitids and sarcinulids) structure, and are more rarely found in solitary rugosans (Tapanila 2005). Palaeozoic bioclaustrations are hitherto unknown in cateniform, fasciculate, and auloporoid tabulate corals (Tapanila 2005).

The Silurian rocks (Wenlock-Pridoli) of Podolia (Ukraine) are exposed in an approximately 80 km wide area along the Dniester River and its tributaries (Fig. 1). The Silurian deposits were formed in variable conditions from the normal-marine to lagoon facies (Tsegelnjuk et al. 1983). The massive coral-stromatoporoid-algal bioherms (Grytsenko 2007) from Lower to Upper Ludlow of Podolia are characteristic of a shallow shelf environment (Fig. 2).

The aim of this paper is to test the following hypotheses: (1) the endosymbiotic worms occurred only in certain host species; (2) the infestation rates are host-specific; (3) endosymbionts preferred a certain type of tabulate morphology (heliolitid versus favositid); (4) the number of infested tabulate species in the coral reef and reef-related community changes over time; (5) the infestation rate of coral species changes with time.

[FIGURE 1 OMITTED]

MATERIAL AND METHODS

A total of 182 tabulate specimens were collected from 20 Silurian outcrops of Podolia (Fig. 1). The following species were represented: Cladopora sp. (Homerian to Gorstian), Cystihalysites sp. (Ludfordian), Heliolites sp. A (Gorstian to Ludfordian), Heliolites sp. B (Gorstian to Ludfordian), Heliolites sp. C (Gorstian to Ludfordian), Heliolites sp. D (Ludfordian), Heliolites sp. E (Ludfordian), Favosites gothlandicus Lamarck, 1816 (Gorstian to Ludfordian), Favosites sp. A (Gorstian), Favosites sp. B (Ludfordian), Favosites sp. C (Gorstian), Favosites sp. D (Gorstian), Favosites sp. E (Gorstian to Ludfordian), Paleofavosites cf. collatatus (Homerian to Gorstian), Stelliporella sp. (Ludfordian), Syringopora sp. A (Gorstian), Syringopora sp. B (Gorstian), Syringopora sp. C (Sheinwoodian), and Thecia sp. (Sheinwoodian). A number of widely distributed species are found in this collection of the Silurian of Podolia. We followed the stratigraphy by Kaljo et al. (2007) (Fig. 2). The material was collected from the carbonate rocks of reef and reef-related shelf facies. Several longitudinal and transverse sections were made from each tabulate corallum. Coral endosyrnbionts were searched under a binocular light microscope. Digital calipers (accurate to 0.01 mm) were used to measure the diameter of endosymbiont shafts in thin sections.

RESULTS

Two endosymbiotic worms were present in the Silurian tabulate corals of Podolia. The common ichnofossil Chaetosalpinx sibiriensis Sokolov, 1948 (Table 1, Fig. 3) was found in 45 tabulate coralla and a rare cornulitid Coralloconchus bragensis Vinn & Motus, 2008 (Fig. 4) in two coralla of Heliolites sp. (Vinn & Motus 2008).

The infestation pattern of tabulate corals by C. sibiriensis is not random. The worms responsible for C. sibiriensis bioclaustrations were capable of infesting certain tabulate species: Paleofavosites cf. collatatus, Heliolites sp. A, Heliolites sp. B, Heliolites sp. C, Favosites gothlandicus, Favosites sp. A (6 of the 19 species studied). One of these species was infested both in the Late Homerian and Gorstian, one in the Gorstian as well as in the Ludfordian, one in the Ludfordian, and three infestations occurred in the Gorstian (Tables I and 2). The infestation rates of P. cf. collatatus changed only moderately over time (Table 3).

Chaetosalpinx sibiriensis preferred the tabulates P. cf. collatatus (71% of the specimens infested) over Heliolites sp. B (25% infested) and Favosites gothlandicus (I I % infested) in the Gorstian, however, no preference was observed concerning the favositid (infestation rates from 11% to 71%) or heliolitid (infestation rates from 25% to 42%) type of tabulate morphology. The multiple peaks on the histogram of the C. sibiriensis diameter (Fig. 5) in most infested P. cf. collatatus indicate that worms responsible for the bioclaustration could have had more than one distinct growth type.

DISCUSSION

The number of infested tabulate species in the Silurian of Podolia increases from one in the Late Homerian to five in the Gorstian, and decreases to two in the Ludfordian (Table 2). However, these numbers are based on relatively small samples, while different results can be obtained from larger samples.

It is not possible to detect on the fossil material whether variations in the infestation rates in the Gorstian (Table 3) result from active larval selectivity by the worms or simply indicate their success of colonization. The environmental differences in worm infestation rates could be an alternative way to explain the species-specific percentage of infested specimens, if the infested corals lived in different environments. All studied tabulates were collected from shallow shelf facies and we believe that all our C. sibiriensis-infested corals were living in a similar environment.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Chaetosalpinx has been interpreted as a tabulate parasite, considering its position between the corallites, perforation of the host's skeleton and soft tissue, modification of its phenotype and possible inhibition of its growth (Zapalski 2007). Thus, some of the uninfested tabulate species could have evolved active means of protection against infestation by C. sibiriensis. On the other hand, as we did not find any adverse effects of Chaetosalpinx on the neighbouring coral skeleton (Fig. 3), we suggest that the relationship was commensal. In Palaeozoic endosymbiosis it is possible that the particular tabulate coral taxa which tend to contain bioclaustrations may have been among the community's least aggressive or toxic corals (Tapanila 2005). Three peaks on the histogram of C. sibiriensis diameter in P. cf. collatatus (Fig. 5) could be explained by the presence of three distinct morphotypes. Thus, there may have been more than one species of endosymbiotic worms responsible for C. sibiriensis in P. cf. collatatus. We did not find any correlation between the growth form of the corals and the occurrence of Chaetosalpinx bioclaustrations.

In Palaeozoic tabulate corals bioclaustrations are preferentially found in common host taxa that span millions of years of geologic time, e.g. Favosites (Tapanila 2005). Our observations on the Silurian tabulates from Podolia support this opinion. We found the highest infestation rates in P. cf. collatatus, which is a common tabulate species in the Muksha Subformation (Motus & Grytsenko 2007) and ranges from the Late Homerian to the Gorstian (Table 1). Common and longranged species had sufficient abundance in space and a long exposure time for their potential symbionts, increasing so the probability of the development of an endosymbiotic relationship as compared to short-ranged and less common coral species.

In Baltica Chaetosalpinx has hitherto been reported from Favosites vicinalis (Stel 1976), Heliolites sp. (Stel 1976), Parafavosites germanica (Sokolov 1948), and Paleofavosites balticus (Klaamann 1958), and probably also from Favosites pseudoforbesi (Stel 1976) and Thecia swindereniana (Stel 1976). Our new data from Podona show that Chaetosalpinx was infesting many more coral species in the Silurian of Baltica, and besides favositids, also heliolitids were common hosts. Chaetosalpinx is hitherto known from eight species of favositids (Tapanila 2005; our data), three species of heliolitids (our data), and one species of Thecia (Stel 1976). Thus, in the Silurian favositid corals could have had the highest number of Chaetosalpinx-tolerant species as compared to the rest of tabulates. However, among the Chaetosalpinx-tolerant corals no preference of favositids over heliolitids is observed as both the highest and lowest infestation rates characterize favositids (Table 3). The scanty data on the occurrences of Chaetosalpinx outside Baltica do not allow analysing the palaeobiogeographic distribution patterns of the genus in the Silurian.

CONCLUSIONS

In Palaeozoic tabulate corals bioclaustrations are mainly found in common host taxa spanning millions of years of geologic time, e.g. Favosites (Tapanila 2005). We established the highest infestation rates in P. cf. collatatus, possibly because this species is a common tabulate in the Muksha Subformation.

Among the Chaetosalpinx-tolerant corals, favositids show no preference over heliolitids. Both the highest and lowest infestation rates are associated with favositids.

doi: 10.3176/earth.2009.3.03

Acknowledgements. V. Grytsenko is thanked for the help in collecting material in Podolia and for valuable discussions. We are grateful to Dr Steve Kershaw (Brunet University) and Dr Mikolaj K. Zapalski (Institute of Paleobiology, Polish Academy of Sciences) for reviewing this manuscript. We thank Prof. Dimitri Kaljo (Institute of Geology at Tallinn University of Technology) for useful comments on the paper. O. Vinn acknowledges the Estonian Science Foundation for grant No. 6623 'Tube formation and biomineralization in annelids, its evolutionary and ecological implications'. M.-A. Motus was supported by the Estonian target funding programme No. 014002008 and the Estonian Science Foundation grants Nos JD05-41 and 6127.

Received 17 December 2008, accepted 30 March 2009

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Mari-Ann Motus (a) and Olev Vinn (b)

(a) Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; motus@gi.ee

(b) Department of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia; olev.vinn@ut.ee
Table 1. Stratigraphic and locality information of Chaetosalpinx
sibiriensis-infested tabulate species. The first number in parentheses
shows the number of the studied samples, the second number in
parentheses--the number of infested tabulate specimens

Tabulate species   Late Homerian     Gorstian       Ludfordian

Paleofavosites     Bagovitsa area    Ustje (2; 1)
cf. collatatus     (39; 19)          Sokol (3; 3)
                   Vrublevtsy        Tsviklevtsy
                   (5; 1)            (2; 1)
                   Ustje Bagovitsy
                   (3; 3)
Heliolites sp. A                     Ustje (1; 0)   Braga (2; 1)
                                     Tsviklevtsy
                                     (2; 0)
                                     Grinchuk-27    Babshin (1; 1)
                                     (1; 1)
                                     Grinchuk       Zhvanets-39
                                     (1; 0)         (16; 6)
Heliolites sp. B                     Tsviklevtsy-   Babshin (1; 0)
                                     186 (1; 1)
                                     Tsviklevtsy    Zhvanets (8; 0)
                                     (7; 2)
                                     Grinchuk
                                     (1; 0)
                                     Grinchuk-27
                                     (1; 0)
                                     Ustje (1; 0)
                                     Sokol (1; 0)
Heliolites sp. C                     Sokol (1; 0)   Zhvanets (2; 1)
                                     Ustje (2; 0)
                                     Tsviklevtsy
                                     (1; 0)
                                     Tsviklevtsy-
                                     186 (1; 0)
Favosites                            Sokol-25       Grinchuk-27
gothlandicus                         (10; 0)        (11; 0)
                                     Tsviklevtsy-
                                     186 (6; 1)
                                     Ustje (2; 1)   Malinovetskaya
                                     Sokol (1; 0)   sloboda (1; 0)
Favosites sp. A                      Ustje (2; 1)
                                     Tsviklevtsy-
                                     186 (1; 1)

Table 2. Infestation (X) of tabulate corals by Chaetosalpinx
sibiriensis in the Silurian of Podolia

Tabulate species         Late Homerian   Gorstian   Ludfordian

Paleofavosites cf.             X             X
collatatus
Heliolites sp. A                             X           X
Heliolites sp. B                             X
Heliolites sp. C                                         X
Favosites gothlandicus                       X
Favosites sp. A                              X

Table 3. Infestation rates of the tabulate coral species by
Chaetosalpinx sibiriensis in the Silurian of Podolia

Tabulate species         Late Homerian   Gorstian       Ludfordian

Paleofavosites cf.       (n = 47) 49%    (n = 7) 71%
collatatus
Heliolites sp. A                                        (n = 19) 42%
Heliolites sp. B                         (n = 12) 25%
Favosites gothlandicus                   (n = 19) 11%

Fig. 2. Stratigraphical scheme of the study interval after Kaljo
et al. (2007)

SERIES   Stage        Formation     Subformation

LUDLOW   Ludfordian   Rykhta        Grinchuk
                      Tsviklevtsy   Bernovo
                                    Sokol
         Gorstain     Konovka       Shutnovtsy
                                    Goloskov
WENL.    Homerian     Bagovitsa     Ustje
                                    Muksha
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Author:Motus, Mari-Ann; Vinn, Olev
Publication:Estonian Journal of Earth Sciences
Article Type:Report
Geographic Code:4EXUR
Date:Sep 1, 2009
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