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Diodora sp.

Jason Hauser 2015


Diodora sp. are benthic dwelling gastropods from the family Fissurellidae (Fleming, 1822), known generally as the keyhole limpets. They have an oblong body that is generally hidden from view under a single dome-shaped shell with a “keyhole” at the apex. They are commonly associated with intertidal marine habitats as part of the algal grazing community, and are quite small, less than a centimetre in size, making them easy to overlook. 

There are only handful of members belonging to the genus Diodora (Gray, 1821) that have been studied previously and therefore many aspects of their ecology and biology remains to be explored. It was discovered that this particular species of Diodora is a brooder with a direct lecithotrophic mode of larval development. The developmental aspect of the organism’s biology has been investigated in more detail further down on the webpage. 

Aspects relating to the specific ecology of this species were difficult to comment on in detail as the species was not fully resolved in this study. As a result much of the information relating to its ecology and conservation were generalised back to the level of genus or family.

Physical Description

Diodora sp. is a limpet-like gastropod with an oblong body that remains virtually hidden under a single dorsally located shell. The shell is dome-shaped and has an anteriorly displaced oval hole at the apex. The shell surface is rough with a radial arrangement of tightly packed spoke-like ridges running from the apex down to the base of the shell connected by multiple narrowly spaced concentric rings. Adult individuals vary in size from 6.5 to 8mm long and width of approximately 75% of the length. The shell colouration varies from pale creamy brown to deep red brown often decorated with patches of encrusting coralline algae. See figure 1 below for samples of colour variations. 

Figure 1


Reports on ecology of Diodora in the literature are brief. They have been referred to as abundant in the habits from which they were collected (Hadfield & Strathmann, 1996). Intraspecific competition is known to affect the density of some keyhole limpets within their habitat (Ortega, 1985). On the other hand, interspecific competition is thought to have only a minor influence on their ecology (Ortega, 1985). There are likely many other factors affecting their ecology that deserve greater attention due to the important functions they serve both to human society and within their ecological community.

Keyhole limpets fulfil and important role clearing rock surfaces of intertidal habitats from algae and other encrusting organisms such as bryozoans, tunicates and sponges. This helps create spaces for other sessile organisms to settle and become attached. In Peru and Chile keyhole limpets are utilised as a food source. In the last few decades a number of medical applications have been developed from a protein discovered in the hemolymph of the keyhole limpet Megathura Crenulata.  This protein, hemocyanin, is an immune stimulant used in immunotherapy treatments of certain types of cancer. Other applications in cancer vaccine research have gained attention in recent times. Many more examples of their commercial and ecological importance can likely be found.

Life History and Behaviour

Diet and Reproduction

In the laboratory setting Diodora sp. can be seen grazing across the surface of encrusting coralline algae using is rasp-like radula (follow link to video 1 below). It is not certain whether they feed on the encrusting alga itself or from the bio-films that form across its surface. Further investigations would be needed to confirm this, both in the laboratory and in the species natural habitat. It is reported in the literature that members of the genus Diodora feed on microalgae (Fretter & Graham, 1962).  Diet varies among species and they may also feed on other encrusting organisms such as bryozoans, tunicates and sponges.

Video 1 (please copy and paste the link below into your web browser)

Members within the genus Diodora are characterised as gonochoristic, i.e. having separate sexes, and without a planktonic larval phase during their development (Wilbur & Yonge, 1964) (Fretter & Graham, 1962). No reports have been found referring to sexual dimorphism within the genus, although protandry is known to exist. Protandry is the process whereby individuals start out as males and change to female as they age, e.g. Diodora gibberula (Lamarck, 1822), resulting in a short-lived stage where an individual occurs as morphologically hermaphroditic (Bacci, 1947). Histological sections of an adult Diodora sp. were viewed to determine whether the species was gonochoristic or hermaphroditic (see figure 2 or refer to Internal anatomy section). A single female gonad only was discovered therefore the species is most likely gonochoristic.

Observations of Diodora sp. yielded an interesting discovery regarding its reproductive strategy. The species displays a brooding behaviour, with retention of eggs containing embryos within the mantle cavity (see figure 3 A&B in Development section) . The retained embryos had a distinctly yolky-yellow colouration and were located within the mantle cavity above the mantle skirt. The number of eggs were estimated from images of two overturned females and approximated to between 150 and 250 per individual. This brooding behaviour was surprising given that well studied members within the genus, such as Diodora aspera (Rathke, 1833), were found to release eggs into the surrounding environment both over the head and via the apical “keyhole” during broadcast spawning events (Hadfield & Strathmann, 1996). A further question remains about how fertilisation is achieved. This was not investigated in this study; however spermcast would seem the most parsimonious explanation. Gastropods such as Diodora sp. fall under the more primitive Archeaogastropod order, which typically lack copulatory organs and rely on external fertilisation modes (Wilbur & Yonge, 1964). 

Limited information is available regarding the timing of spawning events for Diodora. Anecdotal reports from a study by Hadfield & Strathmann (1996) suggested that Diodora completed spawning by late spring. Fetter and Graham (1962) found spawning occurred during the winter months. In this study spawning was observed in late autumn in the laboratory aquarium system. Both of these studies were conducted at mid to high lattidudes where seasonal variations in temperature may have a larger influence over reproductive cycles. No information around this was cited for lower latitudes. It may be the case that spawning is possible all year round, however further investigation would be needed.

Figure 2


Understanding larval development characteristics of a marine species can be valuable in understanding its distribution and abundance throughout the environment. For many species within the genus Diodora the mode of larval development remains largely unknown. In the few species that have been well documented, such as D. gibberula, D. aspera and Diodora graeca (Linnaeus, 1758), the mode of development is described as direct lecithotrophic development, ie lacks a planktonic larval stage (Bacci, 1947)(Hadfield & Strathmann, 1996)(Ward,1966). To gain a better understanding of the larval development characteristics of Diodora sp. a simple observational experiment was developed. The experiment was undertaken in a lab based setting over two weeks with the aim of following pre-hatched embryos through until commencement of metamorphosis into juveniles.

Methods: Two brooding female Diodora sp. of similar size (figure 3A&B) were selected and approximately 30 encapsulated embryos were extracted from each individual (figure 3C). Samples from each group were checked under a microscope to ensure this was the case. The embryos were then spread across six separate 10mL containers half filled with sea water from the laboratory aquarium. Care was taken to ensure embryos extracted from one individual were not mixed with embryos from the other. It should be noted that egg capsules were extremely sticky and difficult to transfer. A small fragment of rock encrusted with pink coralline algae was added to each container. The containers were covered to minimise evaporation over the course of the experiment.

One week later the containers were examined under a light dissecting microscope to document their development. Additionally, one individual from each container was removed and placed on a separate cavity slide with a drop of sea water. No cover slip was applied. The slides were then viewed under a compound microscope at high magnification. A 50% water change was applied to each container and then covered again to reduce evaporation. 

At two weeks the procedure for the previous week was repeated.

Results and Discussion: At day one microscopic examination revealed that both groups of brooded embryos were at distinctly different stages of development whilst still encapsulated within shell membrane egg. One group appeared to be in a pre-veliger development stage (figure 4A). The other group displayed typical early veliger characteristics (see figure 4B), including eye spot, protoconch, and a velum (Hadfield & Strathmann, 1996). The velum appeared to be single lobed at this stage. The shell capsule was quite obvious in this group even at low magnification as the veligers could be seen rotating around within their egg capsules (follow video two link below). The shell membrane for both groups can been seen in figure 4A&B below, indicated by the arrows. Shell size of the encapsulated veligers was approximately 220µm (see figure 5).

Video 2 link (please copy and paste the link below into your web browser)

At one week most embryos had hatched and were observed as pediveligers, a veliger that can crawl around on the substrate using its muscular foot. Hatching is known to begin as early as day seven in Diodora species, and a post-torsional shelled veliger (figure 6A&B) can be present by day six (Hadfield & Strathmann, 1996). As the time of fertilisation was not known the time until hatching was estimated to be somewhere between 7 and 13 days given findings from Hadfield & Strathmann (1996). An exception was discovered when one individual was seen freely swimming around in the container. Further invesigation would be needed to explain this occurrence; one might hypothesize that as the pediveliger still possess a bi-lobed ciliated velum soon after hatching (refer to lower left corner of figure 6A) it may retain some ability to swim. The remaining encapsulated embryos (four) were not seen moving, and were presumed to be deceased, possibly damaged during the transferring process.

On the 2nd week the pediveligers were observed rocking to and fro from the surface of the rocks fragments provided, this was presumed to be a feeding motion.  Microscopic examination of the pediveligers showed most had formed a thin strip of new shell anterior to the protoconch (refer to figure 7A&B). There was a clear demarcation line between the new shell section and the existing protoconch with obvious differences in the shell pattern. This is consistent with teleconch formation and is a good indicator that metamorphosis from veliger to juvenile stage has begun.  

Protoconch sizes again remained similar to the veligers that were observed over the previous two weeks, the only noticeable increase in size was achieved with the addition of the teleconch.  Pediveliger protoconch sizes reported in other studies of the Diodora genus were somewhat comparable. Hadfield & Strathmann (1996) in their studies on larval development of D. aspera reported a shell size of 245µm for a hatched 8-day old veliger and 255µm shell for a 28-day old pediveliger. Veliger protoconch it would seem is fixed throughout the veliger stages, with increase in shell size occuring once excretion of the teleconch initiates during metamorphosis to the juvenile form. 

In summary Diodora sp. are brooders, with the trochophore stage passed and veliger stage reached prior to hatching. Upon hatching the larvae attach to the substrate as pediveligers, most likely foregoing any planktonic larval phase. Within one to two weeks the hatched pediveligers begin to feed from the substrate and commence metamorphosis to the juvenile stage. The mode of development is generally consistent with direct lecithotrophic development. 

Figure 3
Figure 4
Figure 5
Figure 6
Figure 7

Anatomy and Physiology

External Anatomy
The shell characteristics are commonly used to differentiate between genera of Fissurellidae. Viewed dorsally the dominating feature is the shell: a single bilaterally symmetrical shell, shaped like a broad oval dome, with a rough surface patterned like a spiders web (see figure 8) . The shell has an apical hole at the peak that is displaced anteriorly. Particles suspended in the water are displaced upwards when seen moving over the hole, this is because water flows out through the apical hole from inside the cavity of the animal, facilitating respiration and waste removal (Voltzow & Collin 1995).

When viewed from underneath, refer to figure 9, many anatomical features are discernible. Prominent sensory structures include a pair of cephalic tentacles, short eye stalks with single eyes near the base of the cephalic tentacles, and a single row of stubby epipodial tentacles that circle the basal section of the visceral mass protruding (Sasaki, 1998). The tubular truncated snout sits nestled between the cephalic tentacles with mouth opening at the base. A large singular oval shaped muscular foot dominates the central area of the ventral surface. Surrounding the inner edge of the shell is a thin membranous flap known as the mantle skirt. From and slightly tiled view it is possible to see deeper into the mantle cavity, refer to figure 10. Paired ctenidia can be seen this way and appear to be approximately equal in size. Just beyond the ctenidia a bright glow can be observed from the opening of the apical hole.

Keyhole limpets such as Diodora sp. have a unique method of respiration. Ctenidial cilia beat to create a current that draws water under the shell and into the mantle cavity near to the anterior region of the animal. The “clean” water then passes through the ctenidial filaments  before collecting excreted wastes and then exiting through the “keyhole” at the apex (Voltzow & Collin 1995).

Internal Anatomy
Internal anatomy was not thoroughly investigated in this study. Histological sections were generally reflective of the typical internal anatomy at the family level, Fissurellidae, and are shown in figure 11 & 12. Fissurellidae internal anatomy includes a pair of bipectinate ctenidia with  associated osphradia, diotocardian heart  with two lateral atria and median ventricle, and two unequal sized kidneys (right kidney larger) (Haszprunar, 1989) (Sasaki, 1998). Excretory openings of kidneys are into the mantle cavity. Radula teeth typically asymmetrical and are often asymmetrically attached to radular ribbon (Aktipis et al, 2011). Through gut is present with mouth, oesophagus, stomach, intestine and anus. Digestion is extracellular, with digestive enzymes produced by gastrodermis of digestive glands and transported into the stomach (Fretter & Graham, 1962). A single gonad is present with entry into the mantle cavity via the right kidney opening (Collado & Brown, 2007).

Figure 8
Figure 9
Figure 10
Figure 11
Figure 12

Evolution and Systematics

The body plan features of the ancestral gastropod have been debated many times. Key features include torsion of the visceral mass, and a coiled shelled (now also debated) (Ruppert et al. 2004). Other hypothesised features include anterior mantle cavity, anal slit on anterior shell, a pair of bipectinate gills, hypobranchial glands, a pair of nephridiopores and a pair of osphradia. A diotocardian heart was present, one ventricle, a U-shaped gut, a single gonad, and of course a radula (Ruppert et al. 2004). Many of these features are retained in the Fissurellidae body plan, including the pair gills, diotocardian heart, and paired nephridia (kidneys), which have been lost or heavily modified in many other gastropods (Ruppert et al. 2004). Modifications to the ancestral plan include positioning the anal slit at the apex of the shell, extension of the mantle cavity posteriorly, a reduced left kidney, and secondary loss of the operculum and shell coiling in the adult (Ruppert et al. 2004) (Aktipis et al, 2011).

The fossil history of Diodorinae (the superfamily which includes Diodora) only extends into the Cenozoic (McLean, 1984), whereas the gastropod fossil record extends as far back as the Cambrian some 480 or so million years prior to the Cenozoic (Ruppert et al. 2004).

Commonly cited phylogeny 
KINGDOM - Animalia
PHYLUM - Mollusca
CLASS – Gastropoda (Subclass – Vetigastropoda)
ORDER – Archeaogastropoda (Superfamily – Fissurelloidea)
FAMILY – Fissurellidae (Subfamily – Diodorinae)
GENUS – Diodora

Systematics for determining gastropod phylogeny has been up for debate over more recent years due to discoveries such as vetigastropods living around hydrothermal vents. Such discoveries outlined new combinations of body plan characters, challenging the interrelationships among the vetigastropod subgroups. The addition of molecular systematics induces further debate still.

Biogeographic Distribution

Members of the genus Diodora are known to commonly inhabit intertidal zones.  Biogeographical distribution of Diodora sp. is currently restricted to its assumed collection location.  The species was discovered actively reproducing in a small population within the refugium tank of the UQ laboratory marine aquarium at the St. Lucia campus in Australia. The aquarium is seeded with coral rubble collected near the reef crest area on the north side of Heron Island reef. Individuals of the species must have hitch-hiked into the aquarium attached to this coral rubble. As the species has not been fully identified it cannot be determined whether or not the species occurs elsewhere.

To view the location of Heron Island’s northern reef please copy and paste the link below into your browser on Google Maps.,+Queensland

Conservation and Threats

There are currently no known threats to Diodora sp. in its home range on the north side Heron Island reef. This reef is part of a World Heritage-listed Marine National Park and has heavy restrictions on many human activities including fishing. It should be noted that this species has not been fully identified and therefore it is not known if additional populations exist elsewhere, nor is it known how abundant they are in their current habitat.

The mode of reproduction and development for Diodora sp. is likely to significantly limit the dispersal capabilities of the species. As a direct developer with lack of a planktonic larval phase the dispersion of offspring is limited to within the immediate vicinity of the parental individual (Fretter & Graham, 1962). Additionally members of the genus are known to have very slow growth rates (Fretter & Graham, 1962). This could have implications on generation times as the age to sexual maturity may be delayed. If Diodora sp. is subject to slow generation times and has limited dispersal capabilities then a significant threat to its persistence may exist. Under such circumstances a population Diodora sp. would have a poor potential to recover from an extreme weather event or if the species was unable to adapt sufficiently to environmental impacts due to future climate change.


Aktipis, S. Boehm, E. Giribet, G. (2011) Another step towards understanding the slit‐limpets (Fissurellidae, Fissurelloidea, Vetigastropoda, Gastropoda): a combined five‐gene molecular phylogeny. Zoologica Scripta, 40:238-259

Bacci, G. (1947) Sex reversal in Patella coerulea L. and Diodora gibberula (Lam.). Nature, 160:94-95

Collado, G. Brown, D. (2007) Microscopic Anatomy of the Reproductive System in Two Sympatric Species of Fissurella Bruguiére, 1789 (Mollusca: Vetigastropoda). International Journal of Morphology, 25:315-322

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Ortega, S. (1985) Competitive interactions among tropical intertidal limpets. Journal of experimental marine biology and ecology, 90:11-25

Ruppert, E. Fox, R.  Barnes, R. (2004) Invertebrate Zoology: A Functional Evolutionary Approach. Brooks/Cole, Belmont, CA, USA p308-339

Sasaki, T. (1998) Comparative anatomy and phylogeny of the recent Archaeogastropoda (Mollusca: Gastropoda). Bulletin 38:The Univ. museum, the Univ. of Tokyo

Voltzow, J. Collin, R. (1995) Flow through Mantle Cavities Revisited: Was Sanitation the Key to Fissurellid Evolution? Invertebrate Biology 114:145-150

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Wilbur, K. Yonge, C. (1964) Physiology of mollusca. Academic Press.

Special thanks to Professor Bernard Degnan and the 2015 BIOL3211(marine invertebrates) tutors for their guidance.