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  Physiological ecology
  Topics considered here include gas exchange, light perception, and balance reception.
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  Gas exchange
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Research study 1
 

drawing showing location of ctenidium in X-sectional view of body of sea hare Aplysia vaccariaphotograph of a male crab Opisthopus transversus courtesy U Washington LibrariesGas exchange in sea hares, including Aplysia vaccaria, takes place via a single ctenidium or gill located in the mantle cavity (= pallial cavity).  A collection of A. vaccaria at Corona del Mar State Beach Park, California reveals the presence of 2 parasitic crabs Opisthopus transversus, one of each sex, in 2 of the sea hares.  The crabs appear to have the run of the pallial cavity, but whether they interfere with ctenidial function is not known.  Opisthopus is found also in a variety of other mollusks, including snails, chitons, and bivalves, and also in certain sea cucumbers.  Beondé 1968 Veliger 10: 375. Photograph courtesy Freshwater & Marine Image Bank, University of Washington Libraries, Seattle.

 
Research study 2
 

photograph of dorid nudibranch Onchidoris bilamellatamellatadiagram showin hemolymph flow in a single pinnule of the ctenidia of the dorid nudibranch Onchidoris bilamellata
Gas exchange in the intertidal-inhabiting dorid nudibranch Onchidoris bilamellata is via secondary gills located around the anus.  The gills consist of a circlet of individual pinnules, each supplied with an afferent (outward-flowing) and efferent (inward-flowing) vessel (see drawing above Right). Note the vascular loops in each pinnule that ensure that most circulating hemolymph contacts the surface epithelium.  Flow of hemolymph within the pinnules tends to be in the opposite direction to the flow of seawater past them, creating a counter-current exchange system.  From the gills the oxygenated blood flows to the heart for circulation to the rest of the body. 

drawing of water flow over the body and ctenidia of the dorid nudibranch Onchidoris bliamellataCilia covering the external surface of the pinnules beat to drive water through the gill pinnules and away from the body, and this flow carries fecal pellets away from the anus (see drawing lower Right). On the body, water is moved outwards from the midline, and this may additionally function to distribute mucus over the body surface.  

graph showing number of gill pinnules with age in the nudibranch Onchidoris bilamellataAnother gas-exchange adaptation in nudibranchs is that the number of gills increases with increasing body size, as shown here for Onchidoris bilamellata (see graph lower Left). Thus, as the surface area-to-volume relationship becomes less favourable for skin diffusion (see Research Study 2 below), the number of pinnules goes up.  This occurs only to a size of about 0.2g live mass, however, after which the number of gills becomes stable at around 25 throughout the remainder of the juvenile portion of life through to the adult. Potts 1981 J Mar Biol Assn UK 61: 959. Photo courtesy Paul
Young, Massachusetts and seaslugforum
.

NOTE during development the veligers of dorid nudibranchs undergo torsion (a 180o twist of head/velum region on the foot) as do other marine gastropods. However, shortly after this and prior to metamorphosis, they detort by 180o and, during this process, the primary gas-exchange organs, or ctenidia, are lost. The new set of gas-exhange organs that grow at the posterior end are therefore known as secondary gills

 
Research study 3
 

graph showing comparative oxygen uptake by the dorid nudibranch Onchidoris bilamellata with gills normal and ligaturedgraph of oxygen uptake in air and seawater in the dorid nudibranch Onchidoris bilamellatalamellata
Gills in Onchodoris bilamellata are contractile but not fully retractile (as they are in species such as Doris montereyensis).  When in water the gills are extended by hydrostatic pressure, but when in air the gills either collapse onto the body surface (as in Onchidoris) or are retracted into a branchial pocket (Doris).  It has long been thought that the skin of nudibranchs, if moist, would enable oxygen uptake in air.  Measurements on Onchidoris bilamellata in Plymouth, England show this to be true, but uptake is 40% less in air than in water (see graph on Left).

If the gills are ligatured and confined within the branchial pocket in Doris, an estimate can be made of the contribution of skin diffusion to overall oxygen consumption in seawater.  The results show that skin diffusion is responsible for a considerable portion of overall gas exchange, especially in smaller individuals (see graph on Right).  Smaller individuals, with their more favourable surface area-to-volume relationship, are less dependent upon their gills than larger ones. Potts 1981 J Mar Biol Assn UK 61: 959.

NOTE  it is not possible to do this with Onchidoris because a branchial pocket is absent.  The experiments are done on Doris pseudoargus, a species similar in morphology to the west-coast Doris montereyensis

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Research study 4
 

graph comparing costs of swimming and crawling in Melibe leonina with other invertebratesphotograph of hooded nudibranch Melibe leonina swimmingHooded nudibranchs Melibe leonina locomote by crawling and swimming.  Costs of each activity, expressed initially as oxygen consumed, can be measured directly in a respirometer.  Data from studies at the Shannon Point Marine Center, Washington show that swimming is about 12 times more metabolically costly than crawling.  The difference is actually less than this because swimming speeds are about 5 times greater than crawling speeds. Overall, difference in cost between the 2 locomotory activities is not as great as has been found generally for other invertebrates (see graph). Note in the graph that while Melibe expends about as much energy crawling as do other crawling gastropods, its swimming costs are significantly greater than those of other swimming invertebrates The authors discuss possible explanations for this, and conclude that it may largely owe to the apparently ineffective and, thus, inefficient, non-directional side-to-side thrashing mode of swimming style of Melibe in comparison with the sculling and jet-propulsing modes exhibited by other invertebrates represented in the graph.  Caldwell & Donovan 2003 Veliger 46: 355.

NOTE  the authors remark that Melibe, though confined in a respirometer flask, will readily swim, sometimes for as long as 1.5h without stop

NOTE  the data set for Melibe is unique, as no other studies appear to have been done on any other west-coast invertebrate that both crawls and swims

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  Light perception
 
Research study 1
 

drawing of a single receptor cell in the eye of the nudibranch Hermissenda crassicornisdrawing of eye and statocyst in relation to the optic ganglion in the nudibranch Hermissenda crassicornis
Most nudibranchs have small paired eyes, but what sort of vision do they have?   Each eye of Hermissenda crassicornis has a lens and 5 photoreceptor cells, and each of these cells connects via a nerve fibre to an optic ganglion, which itself has neural neural connections to the nearest cerebral ganglion. Neurophysiological studies disclose 2 different photoreceptor cell types, based on the size of depolarisation spike produced.  The 2 types have different sensitivities to light, whether dim or bright, or flashing or constant, and there is inhibition between the different cell types.  Although fine discriminations are unlikely, the eyes could respond to light intensity, direction, and movement.  Eakin et al. 1967 J Cell Sci 2 (3): 349; Stensaas et al. 1969 J Ultrastr Res 27: 510; Alkon & Fuortes 1972 J Gen Physiol 60: 631.

 

One of 5 photoreceptor cells in the eye of
Hermissenda crassicornis
, here shown with
2 pigment (supporting) cells on either side

 

 

 
Research study 2
 

drawing showing mucous tracks of the nudibranch Hermissenda crassicornis in daylight and nighttime
photograph of an aeolid nudibranch Hermissenda crassicornisExperiments on light-responses in Hermissenda crassicornis reveal phototactic behaviours that are on a diurnal cycle.  During the day, test individuals placed in a light gradient tend to move towards the brighter region of the field and increase their crawling speed in the darker areas.  During the night, test animals move away from brighter regions and crawl more quickly in the brighter regions.  Lederhendler et al. 1980 Behav Neural Biol 28: 218.

Eyes of Hermissenda are located at the
bases of the rhinophores, but are usually
difficult to see owing to their small size 1X

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  Balance reception
 
Research study 1
 

diagram of nerve connections between visual and balance pathways in the nudibranch Hermissenda crassicornisPaired balance receptors or statocysts in Hermissenda crassicornis are fluid-filled spherical structures containing masses of small stones or statoconia.  The statoconia impinge upon the tips of cilia projecting from sensory cells1 lining the statocyst lumen and give information on spatial orientation. Interestingly, in cultures of eggs obtained from California specimens of Hermissenda, slow-growing post-metamorphic individuals tend to have statocysts2 with a single statoconium, whereas fast-growing ones tend to have the more typical number of 150-200 statoconia.  The cause of reduced growth appears to be treatment with certain antibiotics used to control bacterial populations in the culture dishes.  Untreated specimens grow faster and are more likely to develop statocysts with a normal complement of statoconia.  The single-statoconium animals exhibit behavioural differences from the multi-statoconia ones, reflected in weaker conditioned-learning abilities.  For example, if both groups are trained not to crawl into a lighted area3 and then re-tested a day or two later, the single-statoconium individuals tend to re-enter the lighted area more quickly than the multi-statoconia individuals.  The authors suggest that reduced mechano-transduction (the small lumen is postulated to restrict the movement of the stone), may be responsible for the difference in behaviour.  Harrigan et al. 1986 Biol Bull 170: 305.

NOTE1  also known as hair cells

NOTE2  the statocysts of antibiotic-treated animals also differ from typical statocysts in that they are smaller (with smaller lumina), have thicker hair cells, and have more closely packed sensory cilia

NOTE3 for background studies leading up to these experiments on Hermissenda, see Alkon 1974 J Gen Physiol 64: 70

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