Select the search type
  • Site
  • Web

Student Project

Tentacle and acontia cnidocysts of the sea anemone Exaiptasia pallida

Barbara Nuic Vidigal 2015


Cnidocysts are cells present in all cnidarians and differ through the classes. It is used for defence, prey captures, and digestion, which improves the organism survival.  The study of this cell is relevant for taxonomy and its identification has been added to species description. Not much is known about how and why it varies in different structures. This was assessed in this work by analysing acontia and tentacles cnidocysts of the sea anemone Exaiptasia pallida. Differences in type, abundance, and size of cnidocysts were observed between these structures. Based on the morphology of the cnidocysts and the function of the structures it was inferred that the distribution of cnidocysts type is related to the structure function that they are inserted. 


Cnidocysts are characteristic cells of Cnidarians that have a thick-walled capsule, which might be open at the end, and contain a coiled eversible tubule (Weill 1930, Shostak1992, Ostman 2000, Prakash et al 2009). There are three types, which are nematocysts, spirocysts and ptychocysts. The nematocysts are intracelular secretion produced by a post-Golgi vesicle located in the epithelial cell layer. It is the most diverse among cnidocysts (Fautin 2009, Anderson andBouchard 2009, Ostman 2000). In addition, they are widely distributed on the tissue and are usually poisonous and penetrating (Weill 1930, Mariscal 1984, Ostman2000, Reft 2005).

The nematocysts have many important functions such as territorial defence, predation, and locomotion (Reft 2005, Anderson and Bouchard 2009). Schlesinger et al(2004) also found that the gastrodermal tissue can extrude nematocyst, which assists the digestion of preys. In addition it can be used for phylogenetic studies due tothe range of morphological variety (Reft 2005). Its description is considered essential when describing a new species (Ardelean & Fautin 2004, Ostman2000), and it is relevant for taxonomy (Shick 1991, England 1991, Acuna 2003,Reft 2005, Fautin 2009, Ostman 2000, 2010, Rodriguez 2012, Grajales andRodriguez 2014). Last, they are well diffused in cnidarian class, while spirocysts are exclusive of Anthozoa and it has only one type of cnidocysts (Ostman 2000).

The acontia is a defence structure that is extruded trough the body wall when theanemone is disturbed (Salleo et al 1996). Fautin & Mariscal (1991) also attributea digestive function. This structure is covered of cnidocysts and even though chemical and mechanical stimulation are involved in the process of discharge,it is not very selective, which means that once it is mechanically disturbes it will discharge in everything that it touches. The control of cnidocysts discharge is important because it can only discharge once and it is not possible to replace it (Anderson and Bouchard 2009). There is a consistency within genus about nematocysts size in this structure (Stephenson), and according to England(1991) the rhabdoids, mastigophores and amastigophores, are common in the acontia but differ from one genus to another.

The cnidocysts present in the tentacle have a higher control of discharge, and are usually specific for prey capture, which is the function of the estruture (Anderson and Bouchard 2009). In anthozoans the cnidocysts of the tentacles are usually uniformly distributed, and there is a predominance of spirocysts (Shick1991, Rifkin 1991, Anderson and Bouchard 2009, Fautin 2009). Spirocysts has a function to adhere the anemone to the substrate or prey (Mariscal 1984, Simon& Scappaticci 2002).

In this present work it will be observed the predominant type of cnidocysts in different structures such as tentacle and acontia in a sea anemone and how they differ. It had been inferred by Simon and Scappaticci (2002) that there is a relation between type of cnidocysts and the structure that it is associated, although it is not known if that is a result of different development evolution or basic function. The sea anemone used was Exaiptasiapallida. The genus Exaiptasia was previously included in Aiptasia, but genetic analyses has shown that this genera was not monophyletic (Rodríguez et al.2012, 2014). In addition, there were differences in the morphology, distribution, and symbionts associated, so they were separated and redescribed by Grajales and Rodriguez (2014). The species Exaiptasia pallida was defines to substitute the followings: Aiptasia pallida, A. californica, A. diaphana, A.inula, A. leiodactyla, A. mimosa, A. pulchella and A. tagetes. One of its characteristics is the well-developed acontia. It is spread worldwide in tropical and subtropical regions, at shallow, calm water, usually between 0 to 5 meters (Grajales and Rodriguez2014).

According to Acuna et al (2003), nematocysts size is not an element that should be used to define a taxon by it self, it is also necessary to consider the qualitative variation. Based on type, size and distribution, taxonomic studies have been made (Fautin 1988, Carlgren 1949). Based on that the cnidocysts were identified and measured. They were classified based on the guideline to nematocyst nomenclature by Ostman 2000. It is based on Weill (1930, 1034a) nomenclatura with modifications made by Carlgren (1940), Cutress (1955), and Mariscal(1974). This is the most widely used nomenclature system, and it focus on the variation of the morphology of the tubule, which is possible to observe in the light microscope (LM) (Fautin 2009, Ostman 2000, 2010). But tiny spines are notusually visible in LM, so it might get unnoticed which can led to misclassification.

Materials and Methods

Approximately 30 Exaiptasia pallida were collected in a jetty at Cleveland, QLD, Australia (27.5333° S,153.2667° E) around 10 to 30 cm deep. The anemones were kept live in the aquarium of the Universityof Queensland so that fresh material could be examined.

To analyse the morphology and extract the nematocysts, one specimen was removed of the substrate and put in small container. First, the anemone was disturbed to extrude the acontia, and than it was relaxed with MgCl2, so the tentacles could stay expanded to facilitate the extraction. The tentacles and acontia were cut with needles, and separated in different slides with the minimum amount of seawater as possible,and than squashed with the coverslip. The slides were observed in light microscope and photos were taken. Measurements and abundance were made analysing the photos with the 40x and 100x objectives.

Figure 1



This present work will use the nomenclature described by Ostman (2000), which corresponds to Weill (1934) but with several modifications by Carlgren (1940),Cutress (1955), Mariscal (1974), and England (1991). The cnidocysts nomenclatura defined by Weill (1934) had in total 16 categories, but with the improvement of microscopy equipment and reviews it was modified by Carlgren (1940), Cutress(1955), Schmidt (1969), Mariscal (1974), Ostman (2000) among others, totalizing over 30 categories and subcategories at present. Carlgren (1940) contributed by adding two subcategories to Mastigophores, which were named by “b” and “p”. The second refers to the presence of o v-shaped notch in the end of the tubule that is visible in undischarged nematocyst (England 1991, Ostman 2000).

Schmidt(1969) tried to revise and change in many aspects the nomenclatura creating more categories and subcategories, which makes it complicated and limiting, since it is based on material discharged and well preserved, which is not always possible (England 1991, Acuna et al 2003 Ostman 2000). The concept of mesobasic was introduced by England (1991) and is an intermediary term between microbasic and macrobasic, first introduced by Weill (1934). These terms refers to the length of the discharged shaft (Ostman 2000). Below there is a hierarchy picture, which resumes the types and nomenclature considered (Fig 2).

Figure 2


Acontia presented a variety of cnidocysts but mesobasic p-amastigophore was the predominant, followed by b-mastigophore (Fig 4). Macrobasic b-mastigophore was also identified (Fig 5).There was a diverse morphology of p-amastigophores, but most presented a medium to large size capsule, with undulating shaft, with two visible regions, the proximal with less dense spines, and the main with denser and bigger spines.  In addition, most shafts ended with a cone shape but some presented a tiny thin tubule or a rod shape (Fig. 3, 4, 6-9,11). In a few was possible to see the broken tubule inside the capsule (Fig 3).

The p-amastigophore presented an average capsule length of 52.5μm, width 10μm and shaft length 87.5μm. In relation to b-mastigophore, the average capsule length was 20.93μm, width 2.73μm, shaft length 22.75μm, and tubule length 81.9μm.

Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10


A variety of cnidocysts were identified but there was a dominance of spirocysts followed by mesobasic p-amastigophore and b-mastigophore (Fig 15, 16). Only one variety of p-amastigophore was identify and had a shaft without spines with a cone shaped end. No tubule was visible even in the undischarged capsule, which had a clear narrow proximal shaft and V-shape notch end (Fig 13). The spirocysts presented a capsule average length of 19.82 μm. The p-amastigophore average capsule length was 39.63 μm, the width was 5.17 μm ,  and shaft average length was 84.47 μm.

Figure 11
Figure 12
Figure 13
Figure 14


The cnidocysts found in the structures analysed differ in quantity and type. The tentacle presented more variety of cnidocysts than acontia. It had spirocysts; p-amastigophoreand b-mastigophore, while the acontia had p-amastigophore and b-mastigophore. But p-amastigophores were fairly bigger in the acontia than in the tentacles, it presented a mean capsule length of 52.5μm, while in the tentacle it had an average length of 39.63 μm. The results are very similar with what was found by Grajales and Rodriguez (2014) for the same specie. They found p-amastigophore, basitrichs, and spirocysts in the tentacles, with the last being the dominant, followed by p-amastigophore. In the acontia they also found p-amastigophore, which presented in two very distinguished sizes and basitrichs. Largep-amastigophore and basitrichs were very common. Cutress (1955), England (1991), and Ostman (2000) considered that basitrichs and b-mastigophores were overlapping nomenclatures, so it was considered that the basitrichs reported by Grajales and Rodriguez (2014) correspond to b-mastigophore. The cnidocysts identify in this study, also coincides with what was found by Reft (2012) for two species from the family Aiptasiidae, which is the same of the specie of this work.

Among the cnidocysts in the acontia, the biggest was the p-amastigophore, which had more than double size capsule length than b-mastigophore. But thep-amastigophore shaft and b-mastigophore tubule had almost the same length. Another difference between p-amastigophore in these structures is the pattern of spines in the shaft. While in the acontia the shaft had spines, it was spineless in the tentacles. Spirocysts were the most abundant cnidocysts in the tentacles, and was absent in the acontia, which had p-amastigophore as the most abundant. Some nematocysts that were present in the acontia were not present inthe tentacles, such as p-amastigophore with a rod shape end. Although, rod-shaped p-amastigophore found in the tentacles of M. farcimen by Ostman (2010) were interpreted as an immature amastigophore instead of a variety. Although both structures presented b-mastigophore, they were less abundant in the acontia than in the tentacle. Most of them were undischarged and none were seeing discharged in the tentacles. In contrast, there was a great number of p-amastigophores discharged in the acontia. Accordingto England (1991) rhabdoids (mastigophores and amastigophores) are frequent in the acontia but differ betweenfamilies and genere.

The high abundance of spirocysts in the tentacles was expected according to previous studies (Mariscal 1984, Simon & Scappaticci 2002, Fautin 2009, Ostman 2000, 2010, Grajales and Rodriguez 2014) and can be related to the structure function. Spirocysts are usually used by the anemone to adhere to the substrate and prey (Mariscal 1984, Simon & Scappaticci 2002). So its abundance is important and associated with the success of prey capture. On the other hand, cnidocysts of the acontia, p-amastigophore and b-mastigophore have shafts usually with spines that indicate their penetrating function (Ostman 2000, 2010). They are also capable of injecting toxins, and are mostly used for defence. That is evidence by the anemone behaviour of extruding only when disturbed and by the nematocysts type.

Comparing these results with previous ones, it is possible to observe the consistency of nematocysts types with structures, and taxonomic groups. That emphasize the idea that this is a reliable data to use for taxonomy, as pointed out before by Fautin 1988, Carlgren 1949, England 1991, Ostman 2000, Reft 2012. Even though there was a significant difference in the capsule sizes found in this study comparing to previous ones, the qualitative data was compatible. Also, as Acunaet al (2003) pointed out, nematocysts size should not be used to define a taxon by itself, it is also necessary to consider the qualitative variation. On the other hand, Ostman (2010) emphasized the relevance in cnidocysts size to differentiate at species level, and that it can differ according to the size of the animal, so it is a data that is important to be described.

Based on type, size and distribution, taxonomic studies have been made (Fautin 1988,Carlgren 1949, Ostman 2010, Grajales and Rodriguez 2015). But there is not enough studies about cnidocysts distribution on the body and how the type and function is associated with the structure that they are inserted. It was already proved that they are associated taxonomically and that there is a distintics distribution among the different tissues (Ostman 2000, Reft 2012), but not a clear explanation of why and how that happened. In this study was observed greater abundance of cnidocysts related to prey capture in the tentacles and to defence in the acontia. So there is a relation between cnidocysts type and abundance to the function of the structure that they are insert. It was not considered here which part of the structure the cnidocysts observed were located, and it can vary in size and abundance from the base to the tip of the tentacles, so that should be observedin future studies. Also it is necessary to analyze more data from different classes and see how another groups differ in morphology and how the nematocysts associated also differ.


Acuña FH, Excoffon AC, Zamponi MO, Ricci L. Importance ofNematocysts in Taxonomy of Acontiarian Sea Anemones (Cnidaria, Actiniaria): AStatistical Comparative Study. Zoologischer Anzeiger - A Journal of ComparativeZoology. 2003;242(1):75-81.

Anderson PAV, Bouchard C. The regulation of cnidocytedischarge. Toxicon. 2009;54(8):1046-53.

Ardelean A, Fautin DG. Variability in nematocysts from asingle individual of the sea anemone Actinodendron arboreum (Cnidaria:Anthozoa: Actiniaria). Hydrobiologia (incorporating JAQU). 2004;530(1):189-97.

Carlgren, O. A contribution to the knowledge of thestructure and distribution of the cnidae in the Anthozoa. Lunds Univ.Årsskrift. 1940;36: 1–62.

Carlgren, O.  Asurvey of Ptychodactiaria, Coral- limorpharia and Actiniaria. Kungl. Svens.Vetens. Handl. 1949; 1: 1–121.

Cutress, C. E., An interpretation of the structure anddistribution of cnidae in Anthozoa. Syst. Zool. 1955; 4: 120-137.

England KW. Nematocysts of sea anemones(Actiniaria, Ceriantharia and Corallimorpharia: Cnidaria): nomenclature.Hydrobiologia. 1991;216-217(1):691-7.

Fautin, D. G. Importance of nematocyst to Actiniantaxonomy. Pp. 487–500 in: HESSINGER, D. A. & LENHOFF H. M. (eds.) Thebiology of the nematocysts. Academic Press, San Diego. 1988

Fautin DG. Structural diversity, systematics,and evolution of cnidae. Toxicon. 2009;54(8):1054-64.

Fautin, D. G. & Mariscal, R. N. Cnidaria: Anthozoa.(Chapter 6). Pp. 267–358 in: HARRISON, F. G. & J. A. WESTFALL (eds.)Microscopic Anatomy of Invertebrates.1991.Vol. 2. Wiley-Liss Inc., New York.

Grajales A, Rodriguez E. Morphological revision of the genusAiptasia and the family Aiptasiidae (Cnidaria, Actiniaria, Metridioidea).ZOOTAXA. 2014;3826(1):55-100.

Kramer, A., Framcis, L. Predation resistence andnematocysts scaling for Metridium senile andM. farcimen. Biol.Bull, 2004; 207:130-140.

Mariscal, R.N.. Nematocysts. In: L. Muscatine and H.M. Lenhoff (eds.), Coelenterate Biology, 1974; pp. 129-178.

Kramer, A., Framcis, L.Predationresistence and nematocysts scaling for Metridiumsenile and M. farcimen.Biol.Bull. 2004; 207:130-140.

Mariscal, R. N. Cnidaria:Cnidae. Pp. 57-68 in Biology of theIntegument. Vol. I. Invertebrates.J. Bereiter-Hahn,A. G. Maltolsy,and K. S. Richardseds. Springer-Verlag,Berlin. 1984.

Östman C, Institutionen för evolutionsbiologi, Uppsalauniversitet, Biologiska sektionen, Teknisk-naturvetenskapligavetenskapsområdet. A guideline to nematocyst nomenclature and classification,and some notes on the systematic value of nematocysts. Scientia Marina.2000;64(1):31-46.,

Östman C, Kultima JR, Roat C, Institutionen förorganismbiologi, Uppsala universitet, Biologiska sektionen, et al. Tentaclecnidae of the sea anemone Metridium senile (Linnaeus, 1761) (Cnidaria:Anthozoa). Scientia Marina. 2010;74(3):511-21.

Östman C, Kultima JR, Roat C, Rundblom K, Institutionen förorganismbiologi, Uppsala universitet, et al. Acontia and mesentery nematocystsof the sea anemone Metridium senile (Linnaeus, 1761) (Cnidaria: Anthozoa).Scientia Marina. 2010;74(3):483-97.

Reft AJ. Significance of nematocyst morphology to phylogenyof sea anemones and other Cnidarians [dissertation]. ProQuest, UMIDissertations Publishing; 2005.

Reft AJ, Daly M. Morphology, distribution, and evolution ofapical structure of nematocysts in hexacorallia. J Morphol. 2012;273(2):121-36.

Rodríguez E, Barbeitos M, Daly M, Gusmão LC, Häussermann V.Toward a natural classification: phylogeny of acontiate sea anemones (Cnidaria,Anthozoa, Actiniaria). Cladistics. 2012;28(4):375-92.

Salleo, A., La Spada, G. & Robson, E.A. 1990,"Discharge characteristics of nematocysts isolated from acontiaofCalliactis parasitica", Marine Biology, vol. 104, no. 3, pp. 459-464.

Scappaticci AA, Kahn F, Kass-Simon G. Nematocyst discharge inHydra vulgaris: Differential responses of desmonemes and stenoteles tomechanical and chemical stimulation. Comparative Biochemistry and Physiology,Part A. 2010;157(2):184-91.

Shick JM. Functional Biology of Sea Anemones. SpringerNetherlands; 1991.

Schmidt, H. Die Nesselkapseln der Aktinien undihredifferentialdiagnostische Bedeutung. Helgol. wiss. Meeres. 1969;19: 284–317.

Shostak S. A symbiogenetic theory for theorigins of cnidocysts in Cnidaria. BioSystems. 1993;29(1):49-58.Kass-SimonG, Scappaticci A. The behavioral and developmental physiology of nematocysts.Canadian Journal of Zoo. 2002;80(10):1772-94.

Weill,R.Contribution à l’étude des cnidaires et de leurs nématocystes. Trav. Stn.Zool. Wimereux 1934;10: 1–701