Abstract
M.Sc.
The larvae of gnathiid isopods are known to parasitise a large variety of intertidal fish
worldwide. In South Africa, the larvae of Gnathia africana Barnard, 1914, have been
recorded from various intertidal fish hosts along the West and South Coasts. These
regions are influenced by the cold waters of the Benguela Current and share similar
intertidal fish species. However, the fauna of the East Coast is influenced by the warm
waters of the Agulhas Current and the species composition differs dramatically from the
South and West Coasts. It was thus hypothesised that the gnathiid species infesting the
East Coast intertidal fishes would not be G. africana, but possibly a new species.
To test this hypothesis, sampling for gnathiids was done at different times from
March 2006 to February 2007, including all four seasons to determine seasonal
variation of the infestations. Intertidal fishes were collected using hand held nets and
kept in aerated tanks until the gnathiids completed their feeding. Once fed, the gnathiids
were kept alive in sample bottles with fresh sea water until moulting occurred. A total
of 316 fishes belonging to 17 species were collected. Of these, 15 species were
parasitised by gnathiids with a mean of four gnathiids per fish. Although the East Coast
gnathiids parasitised a large variety of hosts, the preferred hosts were Scartella
emarginata (maned blenny), Antennablennis bifilum (horned rockskipper), and
Istiblennius dussumieri (streaky rockskipper) with a prevalence of 89.6%, 77.2% and
74.5% respectively.
Light and scanning electron microscopy were used to assist in comparing the
characteristics of the East Coast gnathiid to other known gnathiids. Since the taxonomy
of gnathiids is based on male morphology, these experiments included the moulting of
final stage larvae into adults. Results indicated that all the East Coast intertidal gnathiids
were from the same species and are new to science. The descriptions for the male,
female and praniza larva were done to ensure identification of all the life stages of this
species.
Aspects of the ecology of this new gnathiid species were analysed to determine
host preference, gnathiid prevalence and intensity, as well as the possible correlation
between the host size and gnathiid abundance. Univariate and multivariate statistical
tests (CANOCO and SPSS packages respectively), were used to analyse the data sets
Abstract
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collected during the four seasons. The three preferred hosts had the highest prevalences
and a positive correlation was detected between host size and gnathiid abundance.
Laboratory experiments were also done to elucidate the life cycle and to study
the feeding ecology of the East Coast gnathiid. Through life cycle experiments it was
determined that the second and third unfed larval stages (zuphea larvae 2 and 3) took an
average of 3 h 52 min and 4 h 19 min to feed respectively. After feeding, the second
stage fed larvae (praniza 2) took 35 days to moult into the third zuphea form, and the
third stage fed larvae (praniza 3) took 38 days to moult into an adult. Male and female
praniza 3 larvae moulted into adults approximately 42 and 48 days respectively after
their blood meals. Fertilisation occurred within 24 hours after the female had completed
her moult. It was thus estimated that the complete life cycle from the first larval stage to
adult took between 134 to 140 days in water temperatures ranging between 20°C and
25°C. This is unexpectedly long for gnathiids living in subtropical waters and indicates
that water temperature is not always the main factor in determining the duration of
gnathiid life cycles.
Thus, the hypothesis that a new gnathiid species would be found on the warmer
East Coast of South Africa was confirmed. The new Gnathia sp. and all of its life forms
were described, analysed using various ecological and statistical techniques, and its life
cycle was determined in the laboratory in order to construct a thorough review of this
new gnathiid.