Aspects of the ecology and biology of the isopod, Exosphaeroma hylocoetes, (Barnard, 1940) in three temporarily open/closed southern African estuaries
- Authors: Henninger, Tony Oskar
- Date: 2009
- Subjects: Estuaries -- South Africa -- Eastern Cape Estuarine ecology -- South Africa -- Eastern Cape Isopoda -- South Africa -- Eastern Cape Estuarine biology -- South Africa -- Eastern Cape Ruppia maritima
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5640 , http://hdl.handle.net/10962/d1005322
- Description: Temporarily open/closed estuaries (TOCE’s) are the numerically dominant type of estuary accounting for ≈ 70 % of all estuaries along the South African coastline. Despite their numerical dominance, aspects of the biology of organisms within these systems, particularly macrocrustacea remain poorly understood. This study firstly assessed the abundance and biomass of the most common isopod, Exosphaeroma hylocoetes, in three Eastern Cape TOCE’s, (the West Kleinemonde, East Kleinemonde and Kasouga Estuaries), and their response to mouth breaching events. This study was followed by aspects of the biology of the isopod including their utilisation of submerged macrophytes as a refuge from predation and/or possible food source, as well as the growth rates of the isopods in the laboratory under different environmental conditions. Mean isopod abundances and biomasses ranged between 0 and 4 791 ind. m⁻² and 0 and 9.65 mg dwt. m⁻² in the West Kleinemonde Estuary and between 0 and 108 ind. m⁻² and 0 and 0.318 mg dwt. m⁻² in the nearby East Kleinemonde Estuary. In the Kasouga Estuary, the values ranged between 0 and 3 650 ind. m⁻² and 0 and 5.105 mg dwt. m⁻². Temporal and spatial changes in the abundance and biomass of E. hylocoetes within the three systems was primarily linked to mouth phase, with populations declining when the mouth was open and to a lesser extent, seasonality. In all three estuaries maximum isopod abundances and biomasses were recorded in their middle and upper reaches, which could be ascribed to the presence of submerged macrophytes particularly Ruppia maritima, in two of the estuaries. Males (5.71 ± 0.41 mm) were significantly larger than females (3.99 ± 0.26 mm), but the sex ratios were skewed in favour of females, (a common feature in many isopod populations). Females were found carrying brood throughout the study, releasing offspring directly into the water column, to allow recruitment to the populations. The larger the female, the larger the brood carried (up to a maximum of 72 embryo/mancas). Results of laboratory experiments indicate that the close association of Exosphaeroma hylocoetes with submerged macrophytes is a result of the plant stands providing a refuge against predation by selected ichthyofauna. However stable carbon isotope and fatty acid analyses indicate that E. hylocoetes made use of ephiphytic algae and detritus on the stems of R. maritima, rather than the submerged macrophyte itself. There were no significant differences in the growth rates of male and female isopods at combinations of temperature (15 and 25 °C) and salinity (15 and 35 ‰). Laboratory growth studies revealed that males lived longer than females (25.77 ± 3.40 weeks vs 21.52 ± 3.00 weeks), and therefore achieved larger overall size. Females, however, reached sexual maturity (at 2.5 mm) at a faster rate (two to four weeks) than males (5.5 mm after 6.5 to 11 weeks), dependent on temperature. Exosphaeroma hylocoetes with its association with submerged macrophytes, early maturity, its growth rates and longevity, female-biased sex ratios and year-round breeding, all contribute to its success in temporary open/closed estuaries.
- Full Text:
- Date Issued: 2009
- Authors: Henninger, Tony Oskar
- Date: 2009
- Subjects: Estuaries -- South Africa -- Eastern Cape Estuarine ecology -- South Africa -- Eastern Cape Isopoda -- South Africa -- Eastern Cape Estuarine biology -- South Africa -- Eastern Cape Ruppia maritima
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5640 , http://hdl.handle.net/10962/d1005322
- Description: Temporarily open/closed estuaries (TOCE’s) are the numerically dominant type of estuary accounting for ≈ 70 % of all estuaries along the South African coastline. Despite their numerical dominance, aspects of the biology of organisms within these systems, particularly macrocrustacea remain poorly understood. This study firstly assessed the abundance and biomass of the most common isopod, Exosphaeroma hylocoetes, in three Eastern Cape TOCE’s, (the West Kleinemonde, East Kleinemonde and Kasouga Estuaries), and their response to mouth breaching events. This study was followed by aspects of the biology of the isopod including their utilisation of submerged macrophytes as a refuge from predation and/or possible food source, as well as the growth rates of the isopods in the laboratory under different environmental conditions. Mean isopod abundances and biomasses ranged between 0 and 4 791 ind. m⁻² and 0 and 9.65 mg dwt. m⁻² in the West Kleinemonde Estuary and between 0 and 108 ind. m⁻² and 0 and 0.318 mg dwt. m⁻² in the nearby East Kleinemonde Estuary. In the Kasouga Estuary, the values ranged between 0 and 3 650 ind. m⁻² and 0 and 5.105 mg dwt. m⁻². Temporal and spatial changes in the abundance and biomass of E. hylocoetes within the three systems was primarily linked to mouth phase, with populations declining when the mouth was open and to a lesser extent, seasonality. In all three estuaries maximum isopod abundances and biomasses were recorded in their middle and upper reaches, which could be ascribed to the presence of submerged macrophytes particularly Ruppia maritima, in two of the estuaries. Males (5.71 ± 0.41 mm) were significantly larger than females (3.99 ± 0.26 mm), but the sex ratios were skewed in favour of females, (a common feature in many isopod populations). Females were found carrying brood throughout the study, releasing offspring directly into the water column, to allow recruitment to the populations. The larger the female, the larger the brood carried (up to a maximum of 72 embryo/mancas). Results of laboratory experiments indicate that the close association of Exosphaeroma hylocoetes with submerged macrophytes is a result of the plant stands providing a refuge against predation by selected ichthyofauna. However stable carbon isotope and fatty acid analyses indicate that E. hylocoetes made use of ephiphytic algae and detritus on the stems of R. maritima, rather than the submerged macrophyte itself. There were no significant differences in the growth rates of male and female isopods at combinations of temperature (15 and 25 °C) and salinity (15 and 35 ‰). Laboratory growth studies revealed that males lived longer than females (25.77 ± 3.40 weeks vs 21.52 ± 3.00 weeks), and therefore achieved larger overall size. Females, however, reached sexual maturity (at 2.5 mm) at a faster rate (two to four weeks) than males (5.5 mm after 6.5 to 11 weeks), dependent on temperature. Exosphaeroma hylocoetes with its association with submerged macrophytes, early maturity, its growth rates and longevity, female-biased sex ratios and year-round breeding, all contribute to its success in temporary open/closed estuaries.
- Full Text:
- Date Issued: 2009
Aspects of the ecology and reproductive biology of the limpet, Helcion pruinosus (Gastropoda : prosobranchia)
- Authors: Henninger, Tony Oskar
- Date: 2000
- Subjects: Prosobranchia , Limpets -- Reproduction , Limpets -- Ecology , Prosobranchia -- Ecology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5641 , http://hdl.handle.net/10962/d1005323 , Prosobranchia , Limpets -- Reproduction , Limpets -- Ecology , Prosobranchia -- Ecology
- Description: Helcion pruinosus, the rayed limpet, is found in the midshore region of the intertidal zone, (i.e. the Balanoid zone) of rocky shores, from Saldanha Bay to Sodwana Bay along the coast of South Africa. At Gonubie (32°57'S/25°01'E) this species of limpet was found under boulders, (when not feeding), and could reach average densities of 85/ m². The limpets prefer smooth boulders, avoiding those encrusted by coralline algae or boulders with a rough texture. There was no habitat segregation between adult and juvenile H. pruinosus, i.e. there was no sign of up-shore migration by larger individuals. Males out-numbered females by, on average 1.4 : 1, which was a similar sex ratio to that of the congeneric species, H. pectunculus (Gray, 996). Unlike H. pectunculus, there were no differences in shell lengths between males and females (p = 0.946 at Gonubie, south-east coast and p = 0.961 at Kommetjie, south-west coast; t-test). Shell height could also not be used as a criterion to differentiate between the sexes. The average maximum shell length of H. pruinosus at Gonubie and Kommetjie was 25 mm, but the maximum shell lengths found were 30 mm, (one individual in each case), on both the southeast coast and west coast. Growth occurred most rapidly in the first year of life, with individuals reaching 15 mm after 1 year. In the second year limpet growth slowed to only 7 mm. Sexual maturity was reached at a shell length of between 11 to 14 mm (at an age of 8 months to 1 year). The life-span of H. pruinosus was determined at 2.55 years on the south-east coast and 2.9 years on the south-west coast. Micro-growth bands were produced tidally. Growth occurred allometrically, i.e. shell length increased at a faster rate than shell height. The limpets foraged during low tide, at night, presumably feeding on epilithic algae on the boulders. A second smaller peak of limpet activity was often recorded during the day, when low tide coincided with dawn). More limpets (up to ten times more) were active at spring tides compared to neap tides. Limpet activity was greatest during the spring tides of autumn. During all seasons limpet activity peaked 30 minutes before low water, after which numbers decreased rapidly. All limpets had retreated beneath the boulders before being covered by the flooding tide. It was concluded that the limpets were responding to both endogenous and exogenous cues, but the actual stimulus for retreat could not be determined. The limpets did not return to a fixed scar nor did they return rigidly to a home site. H. pruinosus was found to be gonochoristic. Sexes could be separated, on dissection, according to the colour of their gonads (males had white gonads, whilst that of the females was olive-green). Gonads were present throughout the year in most individuals, i.e. totally spent animals were never observed. Reproductive seasonality was similar in west coast and south-east coast animals. The gonad indices of the west coast limpets were higher, (maximum of 35%), compared to that on the southeast coast, (25%). The animals were probably trickle spawners, with some individuals liberating gametes throughout the year. There was however synchrony in gametogenesis between male and female limpets on both the west coast and south-east coast lines. Gonad indices peaked in the summer months (October to December) with a second minor peak in April/May. These peaks occurred before the release of the gametes in spawning events. The gonad index was lowest in winter, (July). The ovaries were full of mature eggs (oocytes having a diameter of 200 to 250 ~) prior to spawning. The acinal wall thickness had been decreasing prior to spawning. After spawning there was an increase in previtellogenic eggs « 100 μm) in females, and an increase in the acinal wall thickness to indicate spermatogenesis. The two populations are subjected to different environmental conditions and the actual cue for stimulating the release of gametes could not be determined. Finally, a table of differences between H pruinosus and its sister species, H pectunculus was created to show the life-style of H pruinosus.
- Full Text:
- Date Issued: 2000
- Authors: Henninger, Tony Oskar
- Date: 2000
- Subjects: Prosobranchia , Limpets -- Reproduction , Limpets -- Ecology , Prosobranchia -- Ecology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5641 , http://hdl.handle.net/10962/d1005323 , Prosobranchia , Limpets -- Reproduction , Limpets -- Ecology , Prosobranchia -- Ecology
- Description: Helcion pruinosus, the rayed limpet, is found in the midshore region of the intertidal zone, (i.e. the Balanoid zone) of rocky shores, from Saldanha Bay to Sodwana Bay along the coast of South Africa. At Gonubie (32°57'S/25°01'E) this species of limpet was found under boulders, (when not feeding), and could reach average densities of 85/ m². The limpets prefer smooth boulders, avoiding those encrusted by coralline algae or boulders with a rough texture. There was no habitat segregation between adult and juvenile H. pruinosus, i.e. there was no sign of up-shore migration by larger individuals. Males out-numbered females by, on average 1.4 : 1, which was a similar sex ratio to that of the congeneric species, H. pectunculus (Gray, 996). Unlike H. pectunculus, there were no differences in shell lengths between males and females (p = 0.946 at Gonubie, south-east coast and p = 0.961 at Kommetjie, south-west coast; t-test). Shell height could also not be used as a criterion to differentiate between the sexes. The average maximum shell length of H. pruinosus at Gonubie and Kommetjie was 25 mm, but the maximum shell lengths found were 30 mm, (one individual in each case), on both the southeast coast and west coast. Growth occurred most rapidly in the first year of life, with individuals reaching 15 mm after 1 year. In the second year limpet growth slowed to only 7 mm. Sexual maturity was reached at a shell length of between 11 to 14 mm (at an age of 8 months to 1 year). The life-span of H. pruinosus was determined at 2.55 years on the south-east coast and 2.9 years on the south-west coast. Micro-growth bands were produced tidally. Growth occurred allometrically, i.e. shell length increased at a faster rate than shell height. The limpets foraged during low tide, at night, presumably feeding on epilithic algae on the boulders. A second smaller peak of limpet activity was often recorded during the day, when low tide coincided with dawn). More limpets (up to ten times more) were active at spring tides compared to neap tides. Limpet activity was greatest during the spring tides of autumn. During all seasons limpet activity peaked 30 minutes before low water, after which numbers decreased rapidly. All limpets had retreated beneath the boulders before being covered by the flooding tide. It was concluded that the limpets were responding to both endogenous and exogenous cues, but the actual stimulus for retreat could not be determined. The limpets did not return to a fixed scar nor did they return rigidly to a home site. H. pruinosus was found to be gonochoristic. Sexes could be separated, on dissection, according to the colour of their gonads (males had white gonads, whilst that of the females was olive-green). Gonads were present throughout the year in most individuals, i.e. totally spent animals were never observed. Reproductive seasonality was similar in west coast and south-east coast animals. The gonad indices of the west coast limpets were higher, (maximum of 35%), compared to that on the southeast coast, (25%). The animals were probably trickle spawners, with some individuals liberating gametes throughout the year. There was however synchrony in gametogenesis between male and female limpets on both the west coast and south-east coast lines. Gonad indices peaked in the summer months (October to December) with a second minor peak in April/May. These peaks occurred before the release of the gametes in spawning events. The gonad index was lowest in winter, (July). The ovaries were full of mature eggs (oocytes having a diameter of 200 to 250 ~) prior to spawning. The acinal wall thickness had been decreasing prior to spawning. After spawning there was an increase in previtellogenic eggs « 100 μm) in females, and an increase in the acinal wall thickness to indicate spermatogenesis. The two populations are subjected to different environmental conditions and the actual cue for stimulating the release of gametes could not be determined. Finally, a table of differences between H pruinosus and its sister species, H pectunculus was created to show the life-style of H pruinosus.
- Full Text:
- Date Issued: 2000
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