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You are here:   OldClasses > 2012 > Holothuria atra |Emily Purton

 

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Holothuria atra Jaeger

Lollyfish

              

                      

 Emily Purton (2012)

   

                                                 

 

 

 

 

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Physical Description


Ecology


Life History & Behaviour


Anatomy & Physiology


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Anatomy

 

Figure 10: Dissection of H. atra showing: i) digestive tract; ii) pre-dissection animal; iii) left respiratory tree; iv) gonad (contained oocytes = female); v) ventral longitudinal muscles (LM) and ampullae (small black dots in between LM



Figure 11: Sketch of the internal anatomy of H. atra (Adapted from Ruppert et al. 2004)

PHYSIOLOGY


 
The primary characteristic that is used to identify H. atra from other sea cucumbers is the patches of black-coloured epidermis that are visible between the sand that is covering the dorsal side of the animal. Although number, size and location can vary (Phys. Des: Fig. 2), these spots are present on the majority of individuals in this species… but why?

SPOT THE DIFFERENCE

 

Initial Project

I was interested to observe how the spots came to be present on H. atra.  I collected 10 specimens (with spots) from the inner-reef flat on Heron Island Reef and placed them in an aerated aquarium overnight.  With my hand, I brushed off the sand that covered the dorsal side of each animal so there were no spots visible and only the animal’s black epidermis was showing.  Presumably, by brushing the layer of sand off each individual sea cucumber, a layer of mucous (that the sand was stuck to) would have also been removed.  Sediment that was collected from beneath the animal at the time of collection was used to completely cover each the ‘naked’ (no sand) animals in the aquarium.

Observations

Within 30 seconds of being covered with sediment, all 10 individuals had some part of their body protruding from the sediment.  The part of the animal that usually broke through the sand first was the aboral end of the animal which was immediately followed by an expulsion of water from the anus.  Due to their unfortunate lack of limbs, sea cucumbers must use their mutable connective tissue (connective tissue that can change its structure under neural control (Motokawa 1984)) to manoeuvre out from the sediment.  The expulsion of water from the anus stiffens the animal (Tamori et al. 2010), allowing it to uncover itself.  After the initial protrusion from the sand, the point to where the whole animal was out from beneath the sand took a lot longer (approximately 20 minutes).  However, when the animals finally graced me with their presence, the sand had stuck to them again (indicating the presence of mucous on the epidermis) and the spots were also present again (Fig. 12).

 Figure 12: Uncovering of H. atra after it was covered in sediment

Follow-up Project

The results from the initial project suggested that there is mucous present on the areas of the animal where sand reattaches to, but more interestingly, that there is something occurring in the epidermal layer that is causing an absence of sand along the dorsal ambulacra.  Sea cucumbers differ to the other Echinoderms with regards to their water vascular system. Other Echinoderms (e.g. sea stars) essentially have a valve (madreporite) that regulates the water in their canal system that is used for locomotion and respiration  however the madreporite in sea cucumbers does have contact with the external environment (Ruppert et al. 2004).  Instead of water, the hydrostatic pressure of sea cucumbers is maintained by coelomic-fluid (Ruppert et al. 2004). 

                                                                        Methods    

A section of the body wall was removed from H. atra to investigate the epidermis of this animal (Fig. 13). The section of the body wall that was removed encompassed one entire spot of an animal and was placed in 4% paraformaldehyde (PFA) for 120 minutes. The PFA was then drained from the sample and it was placed in 70% ethanol for fixing. The samples were sectioned in Brisbane at the UQ campus and were stained with haematoxylin and eosin (H&E).



Figure 13: Removal of section of body wall from H. atra




                                                                    Observations

At 4x magnification, large gland-like structures were observed just below the epidermis of the animal (Fig. 14 – bottom left image). These glands all appeared to be positioned at the bottom of a pore which can be seen in all the images as a depression in the epidermal surface. Each of these glands had a heavily nucleated cap-like structure connecting the top of the large space to the bottom of the pore, and hence the outside of the animal (Fig. 14 – top left image). 


                                                                        Discussion

The function of these glands was not established during this project; however, it was hypothesised that these glands may have a secretory role due to their location in the epidermis and the large area of the dermis that they occupy. As a form of defence, most species of sea cucumbers are able to project sticky white threads through their anus to deter predators which are called cuvierian tubules (Ruppert et al. 2004). H. atra does not have cuvierian tubules. Instead, it employs a unique predator deterrence mechanism that involves releasing a dark maroon dye from its epidermis (Delia & Hurtel 1977) (Kropp 1982). The function of the glands is thought to be related to storage of this toxic dye in the epidermis layer of H. atra.

Further studies need to take epidermis samples from three places on the dorsal side of H. atra to understand the presence of the spots on this species. Samples should be taken from: 1) the spot (on the ambulacrum), 2) the area next to the spot (on the ambulacrum), and 3) from the inter-ambulacral zone (region between the two lines of spots). This will allow comparisons of the structure of the epidermis to be made and to investigate the function of the glands further.



Figure 14: H. atra epidermis sections stained with H & E.  Glands are visible in each of the four images as the large hole surrounded by blue or pink cells

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