The life history patterns of the polychaete, Terebrasabella heterouncinata, a pest of cultured abalone
- Authors: Simon, Carol Anne
- Date: 2005
- Subjects: Polychaeta -- Physiology Sabellidae -- Physiology Abalones -- Diseases Polychaeta -- Reproduction Sabellidae -- Reproduction Abalone culture -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5336 , http://hdl.handle.net/10962/d1005632
- Description: Terebrasabella heterouncinata is a small K-selected sabellid polychaete. It is a simultaneous hermaphrodite with a semi-continuous mode of reproduction, producing relatively few large eggs that are brooded within the parental burrow until the larvae emerge, to settle on the growing edge of the abalone shell. Despite its low fecundity, this worm has become problematic on abalone farms in South Africa. The present study was conducted to gain an understanding of the life history patterns of T. helerouncinata to determine how they contributed to the success of these worms under altered conditions. This study demonstrated that conditions prevalent on abalone farms were conducive to enhancing the reproductive success of this worm, and suggests that larger, more fecund worms may have been selected for in the decade that these worms have been present on the farms. Increased nutrient availability, and possibly the increased stability of the farm environment relative to its natural environment, has led to a 1.5-fold increase in the average size of the worms. Body size was found to be positively correlated with brood size, and this resulted in worms on farms brooding 3 to 4.5 times more offspring at a time than worms from wild abalone. The ability to increase the number of eggs produced at a time may have been limited by the fact that these worms have only two ovaries. Thus, the increase in fecundity may have been related primarily to the increase in the rate at which the eggs were laid by the worms on the farms, and the increase in the coelomic space available for the storage of these rapidly developing eggs. The ability to increase the rate at which oocytes develop may be related to the vitellogenic mechanisms employed by these worms. Vitellogenic oocytes are able to incorporate high molecular weight yolk precursors from the surrounding coelomic fluid through endocytotic activity. This may allow the oocytes to increase the rate at which they incorporate yolk material under conditions of nutrient enrichment. The increase in fecundity did not occur at the expense of offspring size and, presumably, quality. The increased reproductive output on the farms was compounded by a proportionate increase in the number of reproducing worms within the population. In addition, these worms are long-lived (worms from farmed abalone reached a maximum age of approximately 40 months) and exhibit negligible senescence. Thus, their reproductive output did not change significantly with an increase in age. Furthermore, the proportion of the reproductive worms did not decrease with an increase in age. Thus, within the age range tested, worms of all ages have the potential to make equal contributions to population growth. While diet and abalone stocking density could not be identified as having a significant effect on reproductive output and infestation rate under intensive culture conditions, it was demonstrated that in a naïve abalone population, the total intensity of infestation increased exponentially with time. This increase may be a consequence of an increase in fertilisation success. These worms continuously produce entaquasperm that are released into the water column. The sperm are collected by other individuals that then store the sperm in a single spermatheca. The ability to store sperm relieves individuals of a dependence on the synchronisation of spawning of eggs and sperm. As the population size and density increases, there could be more individuals releasing sperm into the water column, resulting in a continuous supply of sperm. The increased production of eggs would therefore not be constrained by a lack of sperm. The stored sperm are released into the brood chamber to fertilise eggs as they are laid, and this would probably increase the fertilisation success in the species. This study also provides evidence to suggest that reproduction in this worm has a seasonal component. Future studies should concentrate on measuring fertilisation success in greater detail, measuring the effect of season on reproduction, determining whether there are genetic differences between worms on farmed and wild abalone and determining whether wild worms have similar life-spans and age-related fecundity as worms on farms.
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- Date Issued: 2005
Extracellular digestion in two intertidal mussels and the role played by their gut bacteria
- Authors: Simon, Carol Anne
- Date: 1997
- Subjects: Perna -- Digestive organs , Mussels -- Digestive organs , Spirochaeta
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5692 , http://hdl.handle.net/10962/d1005378 , Perna -- Digestive organs , Mussels -- Digestive organs , Spirochaeta
- Description: The intertidal mussels. Perna perna and Choromytilus meridionalis co-occur on the southern coast of South Africa. Mussels ingest mixtures of bacteria. phytoplankton. zooplankton and detritus. with proportions varying according to availability. These bivalves filter similar-sized particles. which might result in interspecific competition. Carbohydrate-digesting enzymes of the mussels and their enteric bacteria. and bacteriolytic enzymes of the mussels were therefore examined to elucidate feeding ecology in these animals at an enzymatic level. Style enzymes of both species digested the storage carbohydrates amylose glycogen and laminarin, and the structural carbohydrate carboxymethyl cellulose (CMC). Differential rates of digestion of these carbohydrates suggests that Perna perna relies more on plankton (and possibly bacteria) than on detritus for food while Choromytilus meridionalis relies equally on all components of the seston. There may therefore be some degree of resource partitioning of the seston. The styles of P.perna had a lower specific enzyme activity. but higher protein content than those of C.meridionalis. P.perna could therefore release more glucose from a given concentration of substrate than C.meridionalis. The gut contents and tissue were examined microscopically to determine where the bacterial colonisation sites were. Bacteria were associated primarily with the gut contents but not the gut walls. of both species. The styles housed spirochaete bacteria (Crislispira sp), Perna perna housed large numbers of one species. while Choromytilus meridionalis had lower numbers of two species. Levels of infection differed between species and localities. Enteric (but not style) bacteria of Perna perna and Choromytilus meridionalis always digested the same carbohydrates as the mussels as well as the structural carbohydrates mannan and fucoidan. Activity was erratic on the structural compounds, carageenin and xylan, and absent on alginic acid or inulin. Activity on the storage carbohydrates by enteric bacteria from C.meridionalis was higher than by those from P.perna. This is probably related to the larger bacterial populations housed by C.meridionalis than by P.perna. Bacteriolytic activity by the digestive enzymes of Pema perna was higher than for Choromytilus. In P.perna it was due to a combination of different enzymes one of which is a true lysozyme. C.meridionalis did not produce a true lysozyme. Enzymes produced by the mussels and their enteric bacteria allow the mussels to utilise all components of the seston. Low endogenous enzyme activity by Choromytilus meridionalis, coupled with the high activity by its enteric bacteria, suggests that they rely more on bacterial activity to meet their dietary requirements than does Perna perna. The ability of enteric bacteria to digest carbohydrates which the mussels cannot indicates that the bacteria are endosymbiotic, although the sporadic nature of activity of some of the enzymes, and the fact that bacteria are associated with the gut contents, indicates that the relationship is only incidental.
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- Date Issued: 1997