Studies on the biology and ecology of the free swimming larval stages of Lepeophtheirus Salmonis (Kroyer, 1838) and Caligus Elongatus Nordmann, 1832 (Copepoda: Caligidae) - PhDData

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Studies on the biology and ecology of the free swimming larval stages of Lepeophtheirus Salmonis (Kroyer, 1838) and Caligus Elongatus Nordmann, 1832 (Copepoda: Caligidae)

The thesis was published by Gravil, Helen Ruth, in September 2022, University of Stirling.


The study investigated biological and ecological parameters controlling and
influencing the production and distribution of the free-swimming larval stages of
Lepeophtheirus salmonis (Kroyer, 1838), and to a lesser extent Caligus elongatus
Nordmann 1832, in the natural environment
The reproductive output of L. salmonis was influenced by seasonal effects. The
number of eggs produced per brood showed an inverse relationship with increasing
temperature. The number of eggs per brood was also influenced by adult female body
size (cephalothorax length), which in itself exhibited an inverse relationship with
increasing temperature. Photoperiod had no significant effect upon the number of eggs
produced or on adult female size. Mean egg size of L. salmonis varied significantly
over the year; larger eggs were produced during the summer months and smaller eggs
over the winter. However, factors controlling the size of the eggs were not elucidated.
The proportion of viable eggs per L. salmonis ovisac remained constant throughout the
year. Large variations in egg number per egg string were found in both L. salmonis and
C elongatus populations sampled at one point in time. These were attributed in part to
phenotypic variation in adult female size and also the number of broods individual
females had produced. Egg viability was not correlated with brood size, but mean egg
size was related to the number of eggs per brood.
Experimental studies indicated that hatching and development of L. salmonis
was highly variable. The percentage of eggs hatched and the time period over which
hatching occurred varied markedly, even when held under constant and optimal
environmental conditions. Temperature did not affect hatching success or viability of
the nauplius I stage, although at higher temperatures the period over which hatching occurred was reduced. Low and medium salinities caused a significant decrease in both
hatching success and nauplius viability. Photoperiod had no effect on initiation of
hatching. Hatching occurred in a manner similar to that observed in free-living
copepods. The nauplii were enclosed by two egg membranes, the outer one bursting
within the ovisac, the inner one after the ovisac membrane has split. Swelling of the
egg and its subsequent hatching was attributed to osmotic effects, with water being
taken up from the external environment.
Development was also highly dependent upon both temperature and salinity.
At 5’C, nauplius 11 stages failed to enter the moult to the copepodid stage. At 7.5’C,
although moulting was initiated, in a large proportion of cases it was not successfully
completed. At I O’C, development to the copepodid stage was successful. Nauplii only
developed successfully to the copepodid stage at salinities of 25%o or greater.
Copepodids raised under optimal conditions then exposed to a range of salinities had a
greater salinity tolerance than nauplii.
Biochemical analysis of the eggs of L. salmonis revealed that lipids constituted
a large proportion of their dry weight. Naupliar stages contained a discrete area
containing lipid which decreased in size over time, suggesting that the free-swimming
larval stages utilised this as an energy reserve. Rate of depletion was faster in nauplii
held at higher temperatures. Longevity, activity and infectivity of the infective stage
decreased with age. However, both spontaneous and stimulus dependent activity
ceased many hours before death and both activity and longevity were affected by
temperature. Infectivity of I day old L. salmonis copepodids was higher than 7 day old
larvae, and was considered to be related to the size of the energy reserves. The settlement and distribution pattern of copepodids did not change with age of copepodid,
the majority being recorded from the fins.
All three L. salmonis free-swimming larval stages demonstrated a “hop and
sink” swimming pattern. The velocity and duration of both passive sinking and active
swimming was recorded for both nauplii and copepodids. Although greater periods of
time were spent passively sinking, the speeds obtained during both upward spontaneous
and stimulated swimming meant that a net upward movement of larvae in the water
column occurred. At higher temperatures spontaneous swimming activity increased,
whilst low salinities caused a cessation of such ability. L. salmonis larvae were
positively phototactic and negatively geotactic. As well as their positive responses to
light intensity, the nauplius 11 and copepodid stages reacted positively to blue-green
spectral wavelengths. Moulting times were relatively short, although the larvae were
not able to swim during such periods. No relationship was found between the level of
lipid reserves and the overall buoyancy of the larvae.
Naupliar stages of both L. salmonis and C. elongalus were obtained from the
water column as a result of a plankton sampling programme at a commercial Atlantic
salmon farm. No copepodid stages of either species were found. There was no
difference in the vertical distribution of the two L. salmonis naupliar stages. Live
larvae tended to aggregate between 0 and 5m in depth, with no diurnal vertical
migration. Dead nauplii, and those with low lipid reserves, were found deeper in the
water column. Naupliar stages, and in particular the first larval stage, were
concentrated in number within cages indicating that the cages have a retentive
characteristic. A novel control method in the form of a commercially available light
lure was tested. Though increasing the numbers of free-living copepods captured, it had no effect on
the numbers of L. salmonis naupliar or copepodid stages obtained in plankton samples.
The present study has therefore provided valuable data concerning the biology
and ecology of the free-swimming larval stages of sea lice, in what was a comparatively
poorly understood area.

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