Diet
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)
https://youtu.be/7-MifEFpfi0

Reproduction
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.

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)
https://youtu.be/Adbd8YxEGZI


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.