Anatomy & Physiology
(Research Project)
Dorid nudibranchs
possess large rhinophores and branchial plumes surrounding their anus on the dorsal
surface. One may think that this could be disadvantageous for a nudibranch when
trying to avoid predation as these structures “stick out” and provide extra
surface area for capture. As a cryptobranch, Dendrodoris nigra has the ability to retract these structures into
pockets but does it utilise any other types of defences to avoid predation? A research project was carried out to
investigate the defence mechanisms associated with Dendrodoris nigra through the examination of its internal anatomy
and physiology. Other systems investigated include the sensory, feeding and
digestive and reproductive system.
Two Dendrodoris nigra individuals were
collected from Moreton Bay in April 2014 and kept in an aquarium system at The
University of Queensland. Once ready for
fixation, the specimens were placed in a petri dish with sea water and held on
ice until relaxed and no longer moving. The specimens were then transferred
using tweezers into a labelled tube with 4% Paraformaldehyde buffer and stored
for sectioning. The fixed specimens were sectioned, mounted and stained with
Hematoxylin and Eosin by UQ Lab Technicians.
The prepared slides were then examined using a dissecting microscope and
an Olympus Differential Interference Contrast (DIC; Normarskl) Microscope.
Sensory System
The rhinophores of D.nigra are perfoliate, also known as lamellate, and appear as a series of flat
plates or leaves (Figure 1). In adults, the clavus (the distal portion of the rhinophore above the stalk) tilts
backwards and bears approximately 17, sloping lamellae (Brodie et al., 1997). At the base of these lamellae are what thought to be
glomeruli which are associated with the detection and processing of olfactory
information (Figure 1)(Wertz et al., 2006).
Feeding and Digestive System
As expected, there was no
cuticualrization present within the buccal cavity of D.nigra due to its lack of radula present in most Mollusca (Figure
2A)(Wägele et al., 1999). The buccal cavity is displayed as a large
circular cavity with a pore opening. This large area within the cavity is likely
to house the suctorial tube in lieu of the radula. Musculature can also be
observed surrounding the mouth and may aid in the movement of the mouth while
feeding. The digestive enzymes associated with feeding are secreted by the
salivary gland located to the right of the buccal cavity (Figure2B)(Wägele et al., 1999). Sponge tissue is externally
digested into a “stew” which can then be consumed and the nutrients extracted
by the large digestive gland which fills a considerable amount of the body
cavity (Figure 2C) (Young, 1966).
Defence
As mentioned
earlier D.nigra has the ability to
retract its highly sensitive chemoreceptive rhinophores and respiratory gill
plume. The rhinophore retractile muscles can be seen as extending from each
rhinophore into a joint U-shaped concentration of muscle which runs ventrally down
the body of the nudibranch (Figure 1)(Wägele et al., 1999). The same pattern can be seen
in the retractile muscles of the brachial plum which extend from each plume
into a joined U-shaped muscle running up the animal (Figure 3A). It was
observed that D.nigra has a soft
gelatinous body upon touch and it has been reported that some species of
nudibranch contain spicules in their mantle as a mechanical defence (Behrens et al., 2005). No such spicules were
observed in the mantle of D.nigra.
Other papers have also reported that D.nigra
employs chemical defences by secreting acidic mucopolysaccharides from glandular
cells in its mantle (Wägele et al., 1999). Glandular cells were
observed in the epidermis of the specimens below the main pigment layer (Figure
3D). The cryptic colouration of D.nigra may also act as a means of
defence through camouflage and this can be seen by the dispersed black pigment
granules throughout the epithelium, gill and rhinophores (Figure 3B and C)(Wägele et al., 1999).
From the research project carried out it would appear that D.nigra’s primary mode of defence is its
acidic mucous secretions and cryptic colouring which it uses to camouflage with
its benthic surroundings. Extensive research could help us gain a deeper
understanding of the defence mechanisms employed by D.nigra.
Reproductive System
The reproductive
organs of nudibranchs are located on their right hand side and include a
genital opening from which the copulatory organ emerges (Brodie et al., 1997). Figure 4 shows an irregular shaped, tube like
organ presumed to be the penis of the nudibranch due to its shape and location
on the right hand side mid-way between the rhinophores and start of the
digestive gland. Many papers also
mention the presence of penial hooks in nudibranchs to avoid separation while mating;
however penial hooks could not be identified in the sectioned specimen(Valdés and Gosliner, 1999). One could assume that the
oval organ below is related to reproduction and could resemble that of a
seminal vesicle. To the left of both of
these structures is a highly muscularised ellipse with muscles running both longitudinally
and latitudinally. While no papers could
be located to explain this muscle it may resemble the columnar muscle used in the
retraction of gastropods into their shells. As there is no longer a need for
this in nudibranchs, the structure may be vestigial. Other components of the reproductive system
were unable to be identified in the sections but include the ampulla, prostate,
bursa copulatrix, deferent duct, vaginal duct and vestibular gland (Valdés and Gosliner, 1999).
References
Behrens, D. W., Petrinos, C. & Schrurs, C. 2005. Nudibranch behavior, New World
Publications.
Brodie, G. D., Willan, R. C. & Collins, J. D.
1997. TAXONOMY AND OCCURRENCE OF DENDRODORIS NIGRA AND DENDRODORIS FUMATA
(NUDIBRANCHIA: DENDRODORIDIDAE) IN THE INDO-WEST PACIFIC REGION. Journal of Molluscan Studies, 63, 407-423.
Valdés, À. & Gosliner, T. M. 1999. Phylogeny of
the radula-less dorids (Mollusca, Nudibranchia), with the description of a new
genus and a new family. Zoologica
Scripta, 28, 315-360.
Wägele, H., Brodie, G. D. & Klussmann-Kolb, A.
1999. Histological investigations on Dendrodoris nigra (Stimpson, 1855)
(Gastropoda, Nudibranchia, Dendrodorididae). Molluscan Research, 20,
79-94.
Wertz, A., Rössler, W., Obermayer, M. &
Bickmeyer, U. 2006. Functional neuroanatomy of the rhinophore of Aplysia
punctata. Frontiers in Zoology, 3, 1-11.
Young, D. K. 1966. Systematics, Food and Functional Morphology
of the Feeding Apparatus of Some Dorid Nudibranchs, University of Hawaii.
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