Feeding and growth of Neomysis integer on
aggregates in the MTZ of the Scheldt estuary
N. Fockedey1, J. Mees² and M. Vincx1
1
Department of Biology, Marine Biology Section, Ghent University (Belgium); ²VLIZ, Flanders Marine Institute (Belgium)
Aim
Neomysis integer is present in high densities in the upper reaches of western European estuaries and plays an important role in the local food-web.
Estuarine aggregates showed to be abundantly present in the stomachs of N. integer living in the estuarine turbidity maximum (Fockedey and Mees,
1999). However, it was not clear if the mysids actively fed on the flocs, nor if they could survive or grow on this dietary item.
The aim of this study is to identify the survival and possible growth of Neomysis integer feeding on laboratory-made aggregates made of
natural water of the Scheldt estuary by means of a roller table (Shanks and Edmondson, 1989).
Fockedey N. and Mees J. (1999). Feeding of the hyperbenthic mysid Neomysis integer in the maximum turbidity zone of the Elbe, Westerschelde and Gironde estauries. J. Mar. Syst., 22:
207-228.
Shanks A.L. And Edmondson E.W. (1989). Laboratory-made artificial marine snow: a biological model of the real thing. Mar. Biol., 101: 463-470.
5 mm
1. Labory-made aggregates
3. Growth of Neomysis integer on a diet of
labory-made aggregates
A technique to simulate the flocculation process and form aggregates out of natural water of the
Scheldt estuary by means of a roller table had to be optimised. Estuarine water was collected by
bucket from a pontoon situated at Antwerpen (left bank) and filtered over a 250 µm sieve.
Neomysis integer individuals (SL: 3-5 mm) were fed laboratory-made aggregates for 7 weeks and their
survival and growth followed. Daily the water was renewed and the salinity, dissolved oxygen and
suspended matter (SPM, POM, C:N) monitored.
Ten plexiglass containers were filled and rotated at 10 rpm (at 15°C). Within 20-120 minutes, the
formation of macroflocs started and after 3 hours a stable situation was created. The counting
and measuring (surface area, length, width and perimeter) of the generated flocs was done by
means of photography and image analysis techniques (min. threshold 250 µm).
Over the course of the experiment the salinity ranged between 1.3 and 6.5 psu because of heavy rainfall.
The number of flocs per liter changed according to the semi-lunar cycle (500-33000.l-1) and was related to
the SPM concentration. Dissolved oxygen concentrations were 6-9 mg.l-1 in situ and minimally 4-7 mg.l-1
after 1 day in vitro.
# flocs per liter
SPM
40000
neap
35000
neap
spring
250
spring
neap
200
30000
25000
150
20000
100
15000
10000
mg per liter
Number per liter
According to Shanks and Edmondson (1989)
spring
50
5000
0
0
1
4
7
10 13 16 19 22 25 28 31 34 37 40 43
day
10
mm
The mysids survived well (70 %) and even grew significantly (0.059 mm.day-1) on a diet of flocs, though less
than the control individuals on a diet of Artemia nauplii (0.094 mm day-1). The intermoult period and the
growth factor on a diet of flocs are significantly lower than in the control individuals.
2. Growth performance
Mean intermoult period (per moult interval)
floc
Standard length
10
artemia
12
Neomysis integer was collected by short hauls with a hand net in a brackish pond at the left
bank of the Scheldt (Galgenweel). Individuals with a standard length of 3-5 mm were selected
and adapted to the experimental food (flocs or Artemia nauplii as control).
8
days
11
10
9
stagnant
8
The growth performance was
followed by the collection and
measuring of subsequent
moults and an endopod –
standard length regression.
mm
7
The mean growth rate,
intermoult period and growth
factor were tested between
treatments. The effect of
rotation
on the growth
performance
could
be
ignored.
6
5
4
3
0
5
10
15
20
25
30
35
N per liter
15000
10000
5000
Before
After
0
7
1-2
6
2-3
3-4
4-5
5-6
6-7
Mean growth factor (per moult interval)
5
4
3
0
10
20
30
40
50
60
day
20
18
16
14
12
10
8
6
4
2
0
1-2
2-3
3-4
4-5
5-6
6-7
5. Gut passage time
The feeding rate of Neomysis integer
on laboratory-made aggregates has
(roughly) been estimated as 38 flocs
per hour per individual. After 24 hour
feeding by 5 individuals of 7-10 mm
SL a significant decrease in the
number
of
flocs
could
be
demonstrated, especially in the 0.8 –
2.2 mm length class.
The gut passage time of Neomysis
integer fed on laboratory-made
aggregates (30 min) is significantly
shorter than fed with Artemia
nauplii (7h).
9
8
7
6
Such a short gut passage time can
indicate a low nutritional value of
the aggregates or refer to a partial
assimilation of the present food.
5
4
3
2
1
0
0
4
day
4. Feeding rate on aggregates
20000
6
2
8
%
rotation
h
Standard length
mm
9
flocs
Artemia