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Epibionts on Dromiopsis Rugosa (Decapoda: Brachyura)

Posted on: Thursday, 9 September 2004, 06:00 CDT

ABSTRACT-

Application of new preparation techniques for cleaning and study of fossil crabs to Dromiopsis rugosa (Schlotheim, 1820), from the late middle Danian limestone in the Fakse quarry, Denmark, has revealed remarkable detail of the carapace surface and epibionts infesting inner and outer surfaces of the carapace. Epibionts, identified as clionid sponges, scleractinian corals, cheilostome and ctenostome bryozoans, serpulid worms, and brachiopods, are interpreted as having attached to molted carapaces after the molted carapace had been released.

INTRODUCTION

FAKSE LIMESTONE quarry is situated east of the village of Fakse in Zealand, Denmark. It constitutes, together with the nearby Stevns Klint, the type locality of the Danian Stage, the lowermost stage of the Paleogene. The Fakse quarry exposes a section through a complex of bryozoan and coral mounds of late middle Danian age (Willumsen, 1995; Surlyk and Hkansson, 1999). The azooxanthellate coral mounds are developed in the narrow epicontinental seaway that lay within the Tornquist-Sorgenfrei lineament (Berthelsen, 1992; Willumsen, 1995). Extensive bryozoan mounds that developed in this seaway during the early and middle Danian were associated with azooxanthellate coral mounds at Fakse and at Limhamn, Sweden (Bernecker and Weidlich, 1990; Willumsen, 1995).

The coral-rich carbonates are highly fossiliferous. The fauna and lithofacies have been described by numerous authors, among them Rosenkrantz (1938), Rosenkrantz and Rasmussen (1960), Asgaard (1968), Floris (1979, 1980), Bernecker and Weidlich (1990), Willumsen (1995), and Surlyk and Hkansson (1999). This coral-rich limestone is also one of the most abundant sources for anomuran and brachyuran decapods from this interval. The decapods have been described or mentioned many times (Schlotheim, 1820; Reuss, 1859; von Fischer-Benzon, 1866; Segerberg, 1900; Woodward, 1901; Frster, 1975; Rasmussen, 1972; Jagt et al., 1993; Collins and Jakobsen, 1994). The total known crab fauna of Fakse consists of 20 species (Jakobsen and Collins, 1997).

In the course of studying the crabs, it was noted that many of the specimens were infested with epibionts. Some of these epibionts were situated on the outer surface of the carapaces whereas others were on the inner carapace surface, clearly a result of postmortem encrustation. In order to study properly the interaction of the crabs and the epibionts, special preparation techniques were developed to better expose the crab surfaces and the encrusting organisms. The purposes of this work are to describe these novel preparation techniques in order to enhance exposed surfaces of decapods and other organisms; to catalogue the epibionts found on a single species of crab, Dromiopsis rugosa (Schlotheim, 1820); and to discuss the nature of living versus postmortem encrustations. The study is based upon collections in the Geological Museum, Copenhagen, Denmark. A total of 1,096 specimens were studied, consisting of molds of the interior of the carapace and molds of the exterior of the carapace with partially or entirely preserved surface granulation.

PRESERVATION

The Fakse Limestone consists of two main facies, bryozoan and coral limestone, with a wide range of subfacies due to depositional differences and local variation in diagenesis (see Bernecker and Weidlich, 1990; Willumsen, 1995). The crab material consists almost entirely of disintegrated molted parts. The original cuticle has been converted to a structureless, chalky mass, leaving the molds of the exterior and interior with imprints of the cuticle. Only when found in matrix-poor cavities in the limestone are the crab remains exposed with a uniformly thin, fibrous calcite cement covering the original surface. It is remarkable that no evidence of corpses, or specimens with at least some associated legs, has been recognized within the crab assemblage at Fakse, apart from two specimens of the brachyuran Raniliformis baltica (Scgerberg, 1900) and two undescribed macrurans. The raninids and the macrurans, both of which tend to be burrowing organisms, lack epibionts. The burrowing behavior may account for their having been preserved with appendages as opposed to being disarticulated. The majority of specimens exhibit pre-burial fragmentation as a result of the high-energy paleoenvironment. High-energy conditions are also indicated by breakage and collapse of the scleractinian corals, none of which has ever been observed in growth position (Floris, 1980). Breakage and collapse of the corals are caused by two factors. Bioerosional activity primarily by boring sponges (?Entobia Bronn, 1837) within the coral stems weakens the skeletons. In combination with bioerosion, the turbulence on the seafloor breaks up the coral branches, subsequently leading to total collapse. Indication of very early lithificalion, as suggested by Willumscn (1995), is demonstrated in the crabs by partial sediment infilling of the carapace and absence of compaction.

PREPARATION TECHNIQUES

Selection of technique.-The preparation of the Fakse material employs two different kinds of treatment: a mechanical cleaning process by which the surrounding matrix and any remaining cuticle of the specimen is removed, exposing the mold of the interior of the carapace; and the removal of the mold of the interior leaving a mold of the exterior of the carapace in which a cast can be made, the so- called negative preparation. The first step in preparation is to consider the best choice of method.

Partly exposed molds of the interior, selected for the staining method described below, can be cleaned by mechanical tools such as pneumatic air scribes to remove any remaining obscuring matrix. To expose roughly the mold of the interior of a carapace, the Desoutter Power Pen is recommended, as it quickly removes the thicker portion of surrounding matrix. The Murray Engineering Micro-Jacks and the Air scribe W224 are suited for more precise removal of the matrix within intricate portions of the fossils, such as the rostrum and orbits. Subsequently, the molds of the interior can be cleaned using sandblasting or the waterblasting technique described below.

FIGURE 1-1, Acid-prepared silicone rubber cast of a male brachyuran decapod, Panopeus bessmani Collins and Jakobsen, 2003, from the middle Eocene of Denmark showing dorsal aspect. 2, Same specimen, showing ventral aspect of a male specimen. Scale bar equals 10 mm.

FIGURE 2-1, Highly abraded sideritic nodule with partly exposed carapace of a brachyuran decapod, Glyphithyreus bituberculatus Collins and Jakobsen, 2003, from the middle Eocene of Denmark. 2, Acid-prepared silicone rubber cast from the same nodule showing ventral aspect of a male specimen. Scale bar equals 10 mm.

In other cases where the limestone contains numerous crab remains exposed either by hammer and cold chisel or by using hydraulic pressure, a negative preparation has proven to be the fastest, most convenient, and most informative treatment. This is a destructive process in which a mold of the exterior of the carapace is exposed, a cast of silicone rubber or resin (polyester, epoxy) is prepared, and the limestone is removed by acid digestion (Fig. 1). Silicon rubber is preferred, as small individual specimens on a larger cast easily can be cut out using a scalpel for scanning electron microscopic (SEM) examination without any loss of material.

Waterblasting and examination of molds of the exterior of the carapace for epibionts.-Molds of the exterior of decapod carapaces potentially carry information about the epibionts that attached themselves to the crab during its life, those that may have attached to the animal following death, and the surface ornamentation of the cuticle. Epibionts on the interior of the carapaces are clearly postmortem infestations (Waugh et al., 2004). It is not possible to check every specimen from Fakse for epibionts on both sides of the carapace due to preservational condition. All the investigated, stained specimens represent interiors of the carapaces and are, therefore, postmortem encrustations. Examination of the true mold of the exterior is frequently impaired by the presence of a thin layer of exocuticle or epicuticle so that what appears to be the mold of the exterior is, in fact, an intracuticular surface that is a plane of weakness (Waugh et al., 2004). Removal of this material is essential for proper examination of epibionts.

By using an air scribe, most of the enclosed crab remains can be removed and then the surface can be cleaned by high pressurized water from a watergun, such as the Wagner Model W 400 SE. The gun generates a fine, highly powered jet stream of water, adjustable up to 180 bars, that immediately blows off most of the particles in the imprint without harming the surface details.

High pressure water, known as waterblasting or wetblasting, is a common industrial procedure for removing stubborn deposits such as paint, rust, etc., from various surface materials. With this procedure, the water creates an abrasive spray as effective, but not as damaging, as sandblasting. Waterblasting has been utilized in preparation of Fakse crabs as wellas crab nodules (Fig. 2) in order to clean the molds of the exterior of the carapace in search of epibionts. The high pressure water (180 bars at 1 cm from the orifice) will penetrate the residual exocuticular and epicuticular layers, if voids representing molds of epibionts are present.

Waterblasting has other applications as well. By using the watergun or an ordinary pressure washer with adjustable water pressure up to 150 bars, bulk samples of cirriped-bearing bryozoan limestone from Fakse have been cleaned with outstanding results (Fig. 3). The cleaning process of the illustrated sample containing numerous cirriped plates was accomplished within 10 seconds! Waterblasting technology can also be used to extract microfossils from weakly cemented rocks of all kinds. Despite the apparently tough preparation treatment, the microfossils in the rock are left unharmed without any traces of abrasion. Prolonged cleaning treatment will even result in complete disintegration of this bryozoan limestone.

Following waterblasting, in the cases where pieces of the mold of the interior or some cuticle remains in the imprint, they must be painstakingly removed by an Ultra Sonic Sealer, used in the dental profession to remove hard calculus and stains from the teeth. Because the water rinse used in the dental office to clean the gumline of loosened debris is damaging to some fossils, the water supply can simply be disconnected. The cleaning should be done under a stereomicroscope to permit controllable cleaning of the details and to avoid damage to the surface of the mold of the exterior.

FIGURE 3-1, Unprepared slab of bryozoan limestone from the Fakse quarry containing plates of pedunculate cirripeds. 2, Same slab cleaned with waterblasting (approx. 10 seconds) has revealed numerous plates of cirripeds, Pycnolepas sp. Scale bar for 1, 2 equals 50 mm. 3, Close-up image showing the extraordinary abundance of cirriped plates, all presumably belonging to Pycnolepas sp. Scale bar equals 10 mm. 4, Detailed image of a single scutum of Pycnolepas sp., displaying the outstanding results of the cleaning technique. Scale bar equals 5 mm.

When cleaned, the prepared limestone mold of the exterior is cast with cither latex rubber, with black toner incorporated to enhance the photographic image, or RTV silicone rubber (Wacker silicone 4,400 or Silastic 9,161). For rapid examination, a fast-curing vinyl polyxilosane impression material such as Exaflex Putty can be prepared. Exaflex Putty is a dental product composed of a base and a catalyst component which, when thoroughly mixed, applied to the imprints, and allowed to set for from two to five minutes, can be removed from the mold. The casting result can be as good as that resulting from other casting media, even with rather complex negative imprints. In the Fakse Limestone, however, with imprints of a very complex nature, a vacuum-impregnated silicone rubber cast is recommended. After polymerization (24 hours), the surrounding matrix is removed completely by soaking in a bath of 10 percent hydrochloric acid until the matrix is completely dissolved. The acid- prepared casts reproduce extraordinary detail (Fig. 4) and often reveal unexpected encrustations which are impossible to "capture" in latex casts or Exaflex impression compound. When choosing a negative preparation procedure, this method can reduce the preparation time to a fraction of that accomplished by other methods-and with better results.

Staining and examination of molds of the interior of carapaces in search of epibionts.-Staining methods have been used widely to facilitate rapid identification of certain minerals in carbonate rock. To our knowledge, the methods have not previously been utilized in identification of encrusting fossil organisms in limestone. Use of the staining technique in combination with the new cleaning techniques described herein to examine encrusting epibionts inside carapaces of the brachyuran decapod Dromiopsis rugosa has revealed epibionts not easily visible otherwise (Fig. 5).

Treatment of the molds of the interior of carapaces with methylene blue as a staining agent provides useful information about obscured, encrusting epibionts. Methylene blue is a water-soluble, nontoxic substance available in powder form from chemical suppliers. Although the use of the dye is primarily biomedical (Sambrook et al., 1989), methylene blue has also been used as a stain in some clay-rich rocks (Miller, 1988). Miller (1988) suggested diluting 0.5 g of methylene blue in 250 ml of water for a working solution.

FIGURE 4-1, A slab of coral limestone from the Fakse quarry with numerous remains of decapods preserved as imprints in the limestone. Scale bar equals 50 mm. 2, Acid-digested silicone rubber cast of the same specimen as shown in 1. The limestone has been cleaned by waterblasting prior to casting. 3, SEM image of a well-preserved chela of a juvenile brachyuran, Dromiopsis elegans Reuss, 1859. Scale bar equals 1 mm. 4, SEM image of the same specimen showing details of the posterior portion of the palm. Scale bar equals 200 . 5, Close-up showing frontal region of a juvenile brachyuran, D. elegans, a carapace (left below) of a galatheid squat lobster, and a chela of a galatheid decapod. Scale bar equals 5 mm. 6, SEM image of a carpus presumably referable to Galathea sp. Scale bar equals 1 mm. 7, Left portion of a carapace of Dromiopsis rugosa (Schlotheim, 1820). Scale bar equals 2 mm. 8, Abdominal segment of an anomuran decapod, possibly a galatheid (right) and a merus of an unidentified anomuran (left). Scale bar equals 2 mm. 9, Left cheliped of a brachyuran, Caloxanthus ornatus (von Fischer-Benzon, 1866). Scale bar equals 2 mm.

Prior to staining, the mold of the interior of the carapace and any remaining structureless cuticular material must be removed by using the mechanical techniques previously described. Gently sandblasting the surface, preferably by using acrylic powder as a blasting medium, prepares the specimen and leaves the surface dry so that the mold can be stained immediately. Alternatively, waterblasting may be employed; however, this technique requires careful drying before proceeding to the staining process. The step- by-step procedure with use of methylene blue is as follows:

FIGURE 5-1, Mold of the interior (steinkern) of a carapace of an adult specimen of Dromiopsis rugosa. Scale bar for/, 2 equals IO mm. 2, Same specimen stained with methylene blue clearly demonstrates the presence of epibionts (calcitic serpulids, cheilostome bryozoans, and brachiopods) attached to the inside of carapace. 3, Close-up of the anterior portion of the carapace in 2, showing details of the serpulid attachment, cheilostome bryozoans, and a single thecidean brachiopod. Scale bar equals 4 mm.

1. The specimen (steinkern) must be completely dry prior to staining for improved penetration of the color.

2. Prepare a solution of methylene blue by dissolving 0.5 g of dry chemical in 250 ml water. Stock solution may be diluted in one to three parts of distilled water.

3. The dried specimen is placed in the solution for approximately 10 seconds. The penetration of the solution depends largely upon the texture and porosity of the specimens and rate of diffusion of dye solution. The staining clearly reveals obscure encrusting bryozoans, serpulids, and brachiopods that were attached to the interior of the carapace.

4. The specimen is then removed from the dye solution, briefly rinsed with tapwater and placed in a drying oven (100C) for one hour.

5. To enhance intensity of the stain, apply a few drops of light machine oil to saturate the stained surface.

6. After the oil has soaked into the rock, the specimen is placed in a waterbath containing a photographic wetting agent, such as Kodak Photo-Flo, in order to avoid development of air bubbles on the surface of the specimen prior to photography. Best results have been achieved by photographing the stained specimens under water.

PHOTOGRAPHY

To create a uniform appearance for photography, the material was blackened either with black impermanent ink airbrushed onto the actual fossils, or by using lampblack or other dry colored powder dusted onto the silicone rubber cast. Excessive powder may be blown away by compressed air. The powder creates a fine, uniform black coating. The ink applied to the fossil specimens can be removed subsequently by soaking the specimens in a bath of an ordinary household bleaching agent for five minutes and then washing in water. The pre-treatment makes a superb background color prior to application of a light coating of sublimated ammonium chloride which further enhances the contrast. The final images have the appearance of an SEM photo (Fig. 6). Digital images of stained material can be easily processed in various image-editing programs. Converting the colors to gray tones often creates an even clearer final image.

RESULTS

A total of 1,096 specimens of Dromiopsis rugosa were available for study in the collections of the Geological Museum. The nature of preservation of the Fakse material is such that all specimens preserve molds of the interior of the carapace and fewer than 10 percent of those examined preserve molds of the exterior of the carapace. Of the 1,096 total specimens, 100 specimens exposed partial or complete molds of the exterior of the carapace. These 100 specimens were carefully examined for epibionts under a binocular microscope. They could not be further prepared because they were preserved as thin, fragile calcitic layers in the coral limestone.

From the remaining 996 specimens, a sample of 100 specimens was randomly selected for staining to determine the extent to which epibionts had encrusted the interior of carapace material. Any cuticular material that remained on the mold of the interior of the carapace was removed by sandblasting using acrylic powder. The structureless, chalky cuticular material could have been removed easily by waterblasting; however, that process requires careful drying before proceeding with the staining process.

Six groups of organisms were identified as epibionts (Fig. 7) on Dromiopsis rugosa. Encrusting cheilostome bryozoans were the most common epibionts with aragonitic serpulid worm tubes as the second most common forms. In addition, encrusting bryozoans interpreted to be ctenostomes, encrusting thecidcan brachiopods, scleractinian corals, and clionid sponges, referable to Entobia Bronn, 1832, were also identified. With regard to the identification of the bryozoans, Paul Taylor suggested (personal commun., 2003) that what are interpreted to be ctenostomes might, in fact, be the cheilostome Allantopora Lang, 1914. Certainly, the bryozoan fauna of the Fakse limestones is rich enough to warrant study by a specialist; that is well beyond our capabilities.

Considering the 100 specimens on which part or all of the outer surface of the cuticle could be examined, 44, or 44 percent, bore some epibionts. This number represents 4 percent of the entire population of 1,096. Within the 44 specimens with epibionts, 15, or 34 percent, were partial carapaces. All but eight of the specimens, 82 percent, had a single type of epibiont, although there may have been one or more individuals or colonies on one specimen. The remaining eight specimens exhibited no more than two different types of epibionts. Numerically, encrusting cheilostome bryozoans were the most abundant epibionts, with more carapaces exhibiting cheilostomes than any other taxon and with as many as nine colonies on a single carapace. Only one specimen exhibited a thecidean brachiopod on the exterior and only two showed evidence of clionid borings.

FIOURK 6-1, Invisible epibionts (bryo/,oans and brachiopod) on a carapace of Dromiopsis rugosa with preserved surface ornamentation. Scale bar for/, 2 equals 10 mm. 2, Same specimen blackened with ink (airbrushed) prior to application with sublimated ammonium chloride revealed bryozoan encrustation on the posterior portion of carapace and a firmly attached small thecidean brachiopod in the branchiocardic region. 3, Close-up of the thecidean brachiopod. Scale bar equals 1 mm.

The 100 specimens selected for examination of the inner surface of the cuticle were also heavily infested by epibionts. Although 28 of the specimens showed taphonomic breakage, 74 percent-52 complete and 22 broken specimens-had epibionts. Of the 74 encrusted specimens, 26, or 35 percent, had only one type of organism represented and 48, or 65 percent, had more than one. Four, or 5 percent, of the specimens bore four epibiont types. As in the case of the external surfaces, encrusting cheilostome bryozoans were numerically the most abundant, both in terms of numbers of carapaces encrusted and in terms of numbers of colonies present on a single carapace, with as many as 11 colonies on one carapace. Serpulid worms were the next most common epibiont.

DISCUSSION AND CONCLUSIONS

Application of the watcrblasting technique to clean the surface of molds of the interior and exterior of crab carapaces in the Fakse Limestone is a simple procedure for removing extraneous material from surfaces to maximize exposure of epibionts. Although the technique has only been tested on the present type of rock (coral and bryozoan limestone) and two crab-bearing phosphoritic/sideritic concretions from the Lillebaelt clay Formation in Denmark (Figs. 1, 2) (Heilmann-Clausen et al., 1985), it clearly has the potential for broader application. Likewise, the staining of carbonate rocks with methylene blue to reveal epibionts also has the potential for providing valuable new information on biotic associates.

Application of these techniques to one species of crab, Dromiopsis rugosa, has produced conclusions that contrast markedly from those obtained by Waugh et al. (2004) and from observations elsewhere (Glaessner, 1969) that the proportion of fossil crabs bearing epibionts is far lower than on living crabs. However, taphonomic history may provide an explanation. In the studies cited by Waugh et al. (2004), most of the specimens represented corpses that arguably were buried very rapidly, with no time for postmortem encrustation. Those specimens exhibited no evidence of scavenging or dissociation of skeletal parts. Thus, they argued that the epibionts attached to a living organism, and the smaller proportion of epibionts on fossil as opposed to living crabs, were both related to taphonomic loss of the exocuticlc, the surface to which the organisms attached.

The Dromiopsis rugosa from the Fakse Limestone are invariably preserved as molted carapaces; none of the carapaces can be interpreted to be a corpse. The epibionts are found on both the exteriors and the interiors of the carapaces in large numbers, suggesting the possibility that they settled on the molted carapaces after they had been released. There is no clear evidence to differentiate encrustation prior to molting from postmortem encrustation on the external surface of carapaces. High rates of encrustation on gastropods, pelecypods, and corals associated with the crabs further reinforce the notion that epibiont attachment was largely a postmortem phenomenon. This combination of observations is consistent with accumulation of dead and dissociated remains of the benthic fauna in an area in which the epibionts could attach to a wide variety of substrates: crabs, molluscs, or corals. The essential difference between this type of occurrence and that discussed by Waugh et al. (2004) is time of exposure prior to burial: shorter exposure time in the case of burial of corpses and longer exposure time in the case of accumulation of epibionts on molts in the Fakse Limestone.

Because the specimens forming the basis for this study were taken from museum collections that were probably strongly biased in favor of complete and well-preserved specimens, the percentages of occurrence can only be taken as approximations. The results of this study do, however, clearly demonstrate that careful preparation of material and more systematic collecting of highly fossiliferous rock units will make it possible to distinguish between pre- and postmortem attachment of epibionts and to estimate more accurately the rate of pre-death epibiosis on fossil crab populations. The description of this material brings into question the conventional assumption that most crab remains are the result of rapid burial. A systematic study of decapod preservation types is needed to obtain a clearer picture of the range of crab taphonomy.

FIGURE 7-1, Internal mold of a carapace of Dromiopsis rugosa stained with methylene blue showing four colonies of encrusting chcilostome bryozoans. 2, Enlarged details of the bryozoan colony outlined in 1. 3, Carapace with preserved surface granulation exhibits attached serpulids. 4, Stained mold of the interior of Dromiopsis rugosa exhibiting a large encrusting bryozoan colony and a thecidean brachiopod. 5, Close-up image of the thecidean brachiopod. 6, Enlarged details of encrusting ctenostome bryozoan outlined in 7. 7, Mold of the interior of a carapace of Dromiopsis rugosa exhibiting encrusting ctenostome bryo/.oans. 8, Detailed image of an acid-prepared silicone rubber cast of the inner side of a carapace of Dromiopsis rugosa showing serpulids and two juvenile scleractian corals. 9, Enlarged image of the scleractian outlined in 8. 10, Mold of the interior of a carapace of Dromiopsis rugosa showing cast of clionid borings in the cuticle which has been removed by waterblasting. 11, Enlarged details of the clionid borings outlined in 10. Scale bar in 1, 3, 4, 7, 8, 10 = 10 mm; 2 = 2 mm; 5 = I mm; and 6, 9, 11 = 5 mm.

ACKNOWLEDGMENTS

We thank C. Schweitzer and D. Waugh, Kent State University, for carefully reading the manuscript, D. Waugh for assistance in the arrangement of the illustrations, and J. K. Nielsen, Geological Museum, for providing SEM images. Careful reviews by M. Key Jr., Dickinson College, and R Taylor, The Natural History Museum, London, substantially improved the paper.

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ACCEPTED 29 DECEMBER 2003

STEN LENNART JAKOBSEN AND RODNEY M. FELDMANN

ster Voldgade 5-7, DK-1350 Copenhagen K, Denmark, and , and Department of Geology, Kent State University, Kent, Ohio 44242,

Copyright Paleontological Society Sep 2004

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