Ecological interactions on a rocky shore : the control of macroalgal distribution by intertidal grazers
- Whittington-Jones, Kevin John
- Authors: Whittington-Jones, Kevin John
- Date: 1998
- Subjects: Limpets -- South Africa , Mollusks -- Ecology , Mollusks -- Feeding and feeds , Mollusks -- South Africa , Cryptogams , Cryptogams -- South Africa , Algae -- Control , Algae -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5665 , http://hdl.handle.net/10962/d1005350 , Limpets -- South Africa , Mollusks -- Ecology , Mollusks -- Feeding and feeds , Mollusks -- South Africa , Cryptogams , Cryptogams -- South Africa , Algae -- Control , Algae -- South Africa
- Description: The aim of the present study was to determine the potential impact of intertidal grazers on the distribution of macro algae on the south coast of South Africa. Particular attention was paid to the large patellid limpet, Patella oculus, which is found thoughout the intertidal zone. Studies of gut contents revealed that Patella oculus was capable of ingesting not only the thallus of foliose (eg. Ulva spp.) and encrusting coralline macroalgae, but also spores and diatoms. The inclusion of these relatively small particles in the diet was surprising, as electron micrographs of the radula of P.oculus revealed that it is typically docoglossan in structure. Such radulae are thought to be poorly suited for collecting small food particles. Sand made up a significantly higher proportion of the gut contents than other particles at all shore heights, which suggests that P.oculus might be capable of excavating the rocky substratum, or of sweeping up sand, while searching for food. Analysis of the gut contents of other local herbivorous molluscs, was also carried out. These species included the winkles, Oxystele variegata and O.sinensis, and the small pulmonate limpets, Siphonaria concinna, S.capensis, and S.serrata. The guts of all species contained mainly spores and diatoms, although small fragments of Ulva sp. were found. The population structure of Patella oculus was investigated at two sites, Cannon Rocks and Old Woman's River. At Cannon Rocks, mean shell length of low-shore animals was significantly lower than that of both mid- and high-shore animals, while at Old Woman's River, no significant difference was found among shore heights. A regression equation for In (shell length) vs In (dry weight) was calculated, and based on length data, the biomass density (g dry mass.m⁻²) of P.oculus at Old Woman's River was estimated. Values ranged from 2.8 on the low- and midshore to 0.37 on the high-shore. A manipulative field experiment was used to determine the impact of mesograzers and macrograzers (such as Patella oculus) on the distribution of intertidal macro algae on the mid- and low-shore at Old Woman's River. Grazers were excluded using mesh cages (mesh size = 3mm), in two separate experiments, one in winter and the other in spring. Percentage cover of macroalgal species and sessile invertebrates was estimated at approximately 6 week intervals for up to 3 months. MANOV A showed that treatments did not significantly affect cover of macroalgae or barnacles during winter. However, towards the end of the spring experiment (midshore only) cover of barnacles and green foliose turfs did increase in those plots from which mesograzers and/or macro grazers were excluded. The failure of the statistical tests to detect significant differences at some time intervals may have been caused by high levels of variation among replicates. This suggests that factors other than grazing are of overriding importance in determining the distribution of local macroalgae. The existence of a possible symbiotic relationship between Patella oculus and the red foliose alga, Gelidium pristoides, was investigated. The availability of various substratum types, including rock, limpet shells, barnacles etc., and the proportion of the total cover of G.pristoides on each, was calculated. It was shown that a significantly higher proportion of the alga grew on limpet shells, although the availability of this substratum type was low. It is thought that the aggressive behaviour of P.oculus prevents all but juvenile Patella longicosta from grazing on its shell, thus providing a refuge from grazing for G.pristoides.
- Full Text:
- Authors: Whittington-Jones, Kevin John
- Date: 1998
- Subjects: Limpets -- South Africa , Mollusks -- Ecology , Mollusks -- Feeding and feeds , Mollusks -- South Africa , Cryptogams , Cryptogams -- South Africa , Algae -- Control , Algae -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5665 , http://hdl.handle.net/10962/d1005350 , Limpets -- South Africa , Mollusks -- Ecology , Mollusks -- Feeding and feeds , Mollusks -- South Africa , Cryptogams , Cryptogams -- South Africa , Algae -- Control , Algae -- South Africa
- Description: The aim of the present study was to determine the potential impact of intertidal grazers on the distribution of macro algae on the south coast of South Africa. Particular attention was paid to the large patellid limpet, Patella oculus, which is found thoughout the intertidal zone. Studies of gut contents revealed that Patella oculus was capable of ingesting not only the thallus of foliose (eg. Ulva spp.) and encrusting coralline macroalgae, but also spores and diatoms. The inclusion of these relatively small particles in the diet was surprising, as electron micrographs of the radula of P.oculus revealed that it is typically docoglossan in structure. Such radulae are thought to be poorly suited for collecting small food particles. Sand made up a significantly higher proportion of the gut contents than other particles at all shore heights, which suggests that P.oculus might be capable of excavating the rocky substratum, or of sweeping up sand, while searching for food. Analysis of the gut contents of other local herbivorous molluscs, was also carried out. These species included the winkles, Oxystele variegata and O.sinensis, and the small pulmonate limpets, Siphonaria concinna, S.capensis, and S.serrata. The guts of all species contained mainly spores and diatoms, although small fragments of Ulva sp. were found. The population structure of Patella oculus was investigated at two sites, Cannon Rocks and Old Woman's River. At Cannon Rocks, mean shell length of low-shore animals was significantly lower than that of both mid- and high-shore animals, while at Old Woman's River, no significant difference was found among shore heights. A regression equation for In (shell length) vs In (dry weight) was calculated, and based on length data, the biomass density (g dry mass.m⁻²) of P.oculus at Old Woman's River was estimated. Values ranged from 2.8 on the low- and midshore to 0.37 on the high-shore. A manipulative field experiment was used to determine the impact of mesograzers and macrograzers (such as Patella oculus) on the distribution of intertidal macro algae on the mid- and low-shore at Old Woman's River. Grazers were excluded using mesh cages (mesh size = 3mm), in two separate experiments, one in winter and the other in spring. Percentage cover of macroalgal species and sessile invertebrates was estimated at approximately 6 week intervals for up to 3 months. MANOV A showed that treatments did not significantly affect cover of macroalgae or barnacles during winter. However, towards the end of the spring experiment (midshore only) cover of barnacles and green foliose turfs did increase in those plots from which mesograzers and/or macro grazers were excluded. The failure of the statistical tests to detect significant differences at some time intervals may have been caused by high levels of variation among replicates. This suggests that factors other than grazing are of overriding importance in determining the distribution of local macroalgae. The existence of a possible symbiotic relationship between Patella oculus and the red foliose alga, Gelidium pristoides, was investigated. The availability of various substratum types, including rock, limpet shells, barnacles etc., and the proportion of the total cover of G.pristoides on each, was calculated. It was shown that a significantly higher proportion of the alga grew on limpet shells, although the availability of this substratum type was low. It is thought that the aggressive behaviour of P.oculus prevents all but juvenile Patella longicosta from grazing on its shell, thus providing a refuge from grazing for G.pristoides.
- Full Text:
Morphometrics and preliminary biology of the caridean shrimp Nauticaris marionis Bate, 1888, at the Prince Edward Islands (South Ocean), 37° 50'E, 46° 45'S
- Authors: Kuun, Patrick John
- Date: 1998
- Subjects: Shrimps -- Prince Edward Islands
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5733 , http://hdl.handle.net/10962/d1005419 , Shrimps -- Prince Edward Islands
- Description: Carapace length, as the best measure of N. marionis body size, is precisely defined. It is shown that N. marionis is a partially protandric hermaphrodite. N. marionis appear to hatch just before April each year, with a little hatching persisting until May. The vast majority of juveniles develop into males. The majority of males transmutate into females in their third year. By April/May the transmutation is probably complete. Reproduction can occur before all male secondary characteristics have been lost. A small minority of individuals develop directly into females without passing through a male phase. At least some of these females can be initially recognized after they have developed mature ovaries by the presence of appendices internae on their first pleopods, a male copulatory structure which all juvenile N. marionis possess. Too few gravid females were recovered to make any statement on whether spawning can occur before this structure is lost. Such females may lose their first pleopod appendices internae in one moult, possibly just before spawning, which may be in late April/early May. Such individuals seem to mature into ovigerous females at a slightly smaller carapace length than do the majority of females which have had a male-phase past. A few females which have passed through a male phase seem to begin developing ovaries at about this small carapace length as well. Once the appendices internae have been lost there appears to be no way of identifying any given female's past life-history. It would seem that during the first year of life N. marionis survive in undetected localities, moult into juveniles, and then settle amongst the benthos from the plankton. Diurnal vertical migration then occurs up to an unknown larger size. It is not known whether the larvae are initially planktonic or not. It is possible that settling of small N. marionis onto the benthos only begins after November. Whether the appendices masculinae of some males only begin growing after they have settled Abstract XIX onto the benthos is unknown, but for the majority at least this begins whilst they are planktonic juveniles. Individuals older than five years are undetectable using samples of the sizes analyzed in this thesis, but they may well persist until quite an advanced age. Niche separation between smaller and larger N. marionis individuals may occur. Diel vertical migration may occur to some extent amongst large N. marionis. Itinerant euphausiids may contribute substantially to the maintenance of top predator populations at the archipelago, either through direct predation by those predators or via predation by N. marionis, which in turn are consumed by those predators. N. marionis itself is an opportunistic feeder, although the majority of its prey seem to be suspension feeders, both benthic and pelagic. In multisample situations, ageing of N. marionis cohorts is made less subjective if one utilizes the phenomenon of synchronized sexual inversion. The von Bertalanffy growth parameters for N. marionis are tentatively identified as k = 0.2353/year, L_ = 12.69mm, to = -0.2828 years and WW_ = 2.03g. The programme FiSAT is discussed, having been found to be extremely useful, but having also been found to have certain faults. Various hypotheses are proposed and are put forward as suggestions for future studies.
- Full Text:
- Authors: Kuun, Patrick John
- Date: 1998
- Subjects: Shrimps -- Prince Edward Islands
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5733 , http://hdl.handle.net/10962/d1005419 , Shrimps -- Prince Edward Islands
- Description: Carapace length, as the best measure of N. marionis body size, is precisely defined. It is shown that N. marionis is a partially protandric hermaphrodite. N. marionis appear to hatch just before April each year, with a little hatching persisting until May. The vast majority of juveniles develop into males. The majority of males transmutate into females in their third year. By April/May the transmutation is probably complete. Reproduction can occur before all male secondary characteristics have been lost. A small minority of individuals develop directly into females without passing through a male phase. At least some of these females can be initially recognized after they have developed mature ovaries by the presence of appendices internae on their first pleopods, a male copulatory structure which all juvenile N. marionis possess. Too few gravid females were recovered to make any statement on whether spawning can occur before this structure is lost. Such females may lose their first pleopod appendices internae in one moult, possibly just before spawning, which may be in late April/early May. Such individuals seem to mature into ovigerous females at a slightly smaller carapace length than do the majority of females which have had a male-phase past. A few females which have passed through a male phase seem to begin developing ovaries at about this small carapace length as well. Once the appendices internae have been lost there appears to be no way of identifying any given female's past life-history. It would seem that during the first year of life N. marionis survive in undetected localities, moult into juveniles, and then settle amongst the benthos from the plankton. Diurnal vertical migration then occurs up to an unknown larger size. It is not known whether the larvae are initially planktonic or not. It is possible that settling of small N. marionis onto the benthos only begins after November. Whether the appendices masculinae of some males only begin growing after they have settled Abstract XIX onto the benthos is unknown, but for the majority at least this begins whilst they are planktonic juveniles. Individuals older than five years are undetectable using samples of the sizes analyzed in this thesis, but they may well persist until quite an advanced age. Niche separation between smaller and larger N. marionis individuals may occur. Diel vertical migration may occur to some extent amongst large N. marionis. Itinerant euphausiids may contribute substantially to the maintenance of top predator populations at the archipelago, either through direct predation by those predators or via predation by N. marionis, which in turn are consumed by those predators. N. marionis itself is an opportunistic feeder, although the majority of its prey seem to be suspension feeders, both benthic and pelagic. In multisample situations, ageing of N. marionis cohorts is made less subjective if one utilizes the phenomenon of synchronized sexual inversion. The von Bertalanffy growth parameters for N. marionis are tentatively identified as k = 0.2353/year, L_ = 12.69mm, to = -0.2828 years and WW_ = 2.03g. The programme FiSAT is discussed, having been found to be extremely useful, but having also been found to have certain faults. Various hypotheses are proposed and are put forward as suggestions for future studies.
- Full Text:
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