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Student Project

Diversity of sclerites in the soft coral Cladiella sp.

Julia Machado Quintaes Calvet 2015


The morphology of sclerites has always been a subject of study. Although some works came across different types and sizes of sclerites within the same colony, not many have deeply considered reasons why. The present study investigated these situations and considered reasons why they might happen. To do that, pieces of tissue from a soft coral Cladiella sp. were divided into different sections. The results found distinct shapes of sclerites for stalk and polyp regions, and also different averages of size for each one. These findings indicate there are different patterns of calcification for each region of the colony, and also considers the structural characteristics of the tissue itself as having a main role for that process.


Octocorallia is a subclass of Anthozoa, with one of its distinct orders being Alcyonacea, also known as soft corals. Soft corals are known to not produce a calcareous skeleton, like members of the reef building corals, from the Subclass Scleractinia. Instead, they secrete small individual calcium carbonate structures, the sclerites, and deposit them within the colony (Rahman and Oomori, 2008).

Sclerites are responsible for strengthening soft corals (Koehl, 1982; Lewis and Wallis, 1991) and perhaps for antipredation effect (Clavico et al, 2007; Koehl, 1982, 1996; West 1998). These structures also play a major role on species identification (Fabricius and Alderslade, 2001), considering soft corals have very similar morphological features.

Although the importance of sclerite is well-known, there aren’t many studies exploring their diversity within the same colony (Tentori and Ofwegen, 2011). The main hypothesis behind this work is there are different types of sclerites varying in the colony. And the secondary hypothesis is they also vary in size. If so, what are the morphological types and the averages of size? What are the possible impacts for the colony?

The present study was conducted with Cladiella sp. Gray, 1869. The genus is reasonably common in Indo-Pacific regions (Fabricius and Alderslade, 2001), as most coral from the family Alcyonacea. They also present association with Symbiodinium Freudenthal, 1962, which reside in the endodermal layer of the animal (Tonk et al, 2013 and Ruppert et al, 2004).  The symbiosis is also thought to be responsible for enhancing the calcification process of the coral (Kaniewska et al, 2011; Pearse and Muscatin, 1971).

Materials and Methods


The soft coral was collected in an aquarium located at The University of Queensland, Australia (Figure 1). The aquarium had a population of species sampled from the Southern Great Barrier Reef, as was the coral studied.

Figure 1

Genus identification

Soft corals have a unique calcification process therefore, the specific type of sclerites is key to species identification (Bayer, 1981; Fabricius and Alderslade, 2001). The identification of the genus of the animal was done by extracting a small section of tissue from the base stalk. The sample was then placed on a microscopic slide with 0.5 ml of 5% sodium hypochlorite in it. The sclerites found were dumbbells (Figure 4), characteristic of the genus Cladiella (Figure 7) (Fabricius and Alderslade, 2001). 

Sclerites extraction

Sections of coral tissue were removed from different parts of the colony: polyp, top stalk and base stalk (Figure 2). The sections were dissected into surface and inner areas. The samples were then placed on microscopic slides with 0.5 ml of 5% sodium hypochlorite each.

Figure 2


The sections (Figures 3-6) were observed using differential interference contrast (DIC) microscopy. The determination of sclerites size were done using the software ImageJ with the pictures of each section of the colony taken with a camera attached to the microscope. Averages of the sclerites size for each section were then calculated and are shown on Figure 8.


The images of each section of tissue with its sclerites are shown on Figures 3-6. The inner base stalk (3-A) and the surface base stalk (3-B) had the same types of sclerites, variations of dumbbells (see Figure 4). The inner section also showed a sclerite shaped like a disk (see Figure 6). Sclerites from the inner base stalk are slightly larger than the ones from the surface (Figure 8).

Images 3-C and 3-D show inner and surface top stalk, respectively. The two sections also present variations of dumbbell shapes. Adding to that, they also have figure-eight sclerites (see Figure 5). Differently from the base stalk, the surface top stalk had an average of sclerites size bigger than the ones from its inner section (Figure 8).

Lastly, the two polyp sections (3-E and 3-F) had disks and figure-eight sclerites. Sclerites from the inner section of the polyp had an average size slightly bigger than the ones from its surface section (Figure 8).

The average size of the sclerites showed that, no matter which section analysed from the colony, all of them ranged roughly from 0.1 mm to 0.2 mm. The sections from the polyps had the smallest average of size. The base of the stalk had the biggest size of sclerites, with the top stalk having not only an intermedium size, but also presenting sclerites encountered in the base stalk and in the polyp (except disks).

Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8


According to studies conducted by Koehl (1982) and Lewis and Wallis (1991), the sclerites of soft corals possibly have a primary function of structural support and rigidity. Considering that, it is likely that sclerites with bigger sizes would be located at the stalk of the colony, which is the situation found on the coral studied (see Figure 8).

A number of studies (Clavico et al, 2007; Koehl, 1982, 1996; West 1998) suggest that smaller sclerites in a high density would have a better role in supporting the colony, whereas bigger ones would likely have an antipredation effect (Clavico et al, 2007). This situation is supported by the hypothesis that sclerites with larger sizes would allow areas of the tissue without them to be deformed by water-motion (Koehl, 1982, 1986) and also vulnerable to predation.

Opposing to this theory, Van Alstyne et al (1994), Tentori and Allemand (2006) and Fabricius and Alderslade (2001) suggest that smaller sclerites are found in the polyps and larger ones shaped as dumbbells are found in other areas of the colony. It’s also thought that sclerites have no significant role in antipredator situations (O’Neal and Pawlik, 2002; Van Alstyne et al, 1994), leaving chemical defences of each coral to play that role (Van Alstyne et al, 1994).

Tentori and Ofwegen (2011) pointed out that, in their study, the size of the structures in lower sections increased from surface to inner regions. This was also the case with sclerites from base stalk and polyps regions. The surface top stalk was the only section of the colony which the upper layer had sclerites with bigger sizes than its inner region. This data is very likely to have been found because, as shown on Figure 3-C, undissolved tissue was on top of a vast section of sclerites, which made impossible to properly measure the length of the structures. Otherwise, the sclerites from the inner region would have possibly been bigger than from the surface.

It’s assumed that sclerites are synthesized by sclerocyte cells, forming calcitic structures within the mesoglea of the animal, developing its endoskeleton (Jeng et al, 2011; Ruppert et al, 2004). A few studies came across with morphological differences between sclerites in sections of the coral (Tentori and Ofwegen, 2011; Verseveldt, 1980, 1982), as this study has.

The present study suggested the dominance of dumbbell sclerites throughout stalk regions of the colony (Figure 4) and a different type of sclerite in the polyps sections, figure-eights (Figures 5 and 6). Although differences in size were encountered, no significant morphological difference could be found from surface and inner areas of the same region of the colony. Sclerites from the stalk have more complex crystalized forms, whereas the ones from the polyps are more compact and smaller (Noe and Dullo, 2006; Tentori and Ofwegen, 2011). Perhaps, this situation is because of higher rates of calcification in polyps when compared to stalks (Tentori et al, 2004).

A recent work has detected unique characteristics for sclerites. The study found out that soft corals have organic matrices that are protein-rich (Rahman and Isa, 2005). The relevance of this may be an explanation as to why there are distinctive morphological types of sclerites in different anatomical regions. Because of this property, sclerites may aggregate calcium differently from region to region of the colony. This might also explain disk-like structures on sections 3-A, E and F; they may be early stages of sclerite development still aggregating calcium.

The contrasts from polyp to stalk were noteworthy. The morphological type of sclerites in the polyps were very different from the stalks. All of the sections from the stalk had variations of dumbbell shaped sclerites (see representations on Figure 7). Fabricius and Alderslade (2001) say that figure-eight sclerites are only present in the polyps (Figure 7). Figures 3-C and 3-D indicate it was not the case for the Cladiella sp. studied. This might have happened because the stalk region of this animal is extremely short if compared to the rest of the colony (see Figure 2). Knowing that, it's likely there were polyps in the regions dissected from the top stalk. 

Finally, the collective data from the work showed there were indeed morphological differences with sclerites from different anatomical sites of the coral. Sizes also tended to vary from each region, either being from polyp to stalk or from inner and surface sections of the same region. The study considers that the differences encountered are likely because of the pattern of sclerite formation, which also have a significant impact on the sclerites role in the colony. Following studies should aim to correlate more physical and chemical factors to deeply understand the results found, such as the role of zooxanthellae during calcification and the production of chemical substances as predator defense.


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