288 D
. Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
are flat making accurate estimates of the areas of the encrusting E . bellula colonies
possible. Photos of the colonies were taken with a Nikonos V 35-mm lens mounted with a 1:1 extension tube, using a flash directly behind the colonies. Measurements of colony
areas were obtained on scanned pictures, using a computer. The guts of the active zooids could easily be distinguished by their red colour, resulting from the captured
Rhodomonas sp. From this, the specific areas were estimated. To relate area to weight, areas of 36 colonies were measured. Under dissecting microscope these colonies were
carefully scraped off the Dictyopteris sp. and weighed. Dry weight was measured after . 48 h in 60–708C, and ash-free dry weight after ashing at 4758C for a minimum of 4 h.
3. Results
At 228C, the maximum colony clearance rates F as a function of total and
max
specific colony area are shown in Figs. 1 and 2. F varied linearly with both total and
max
specific area of the colony, but the correlation was more pronounced for the specific area
2 2
r 5 0.95 versus r 5 0.81, n 5 31. F for the specific colony area was 105
max
Fig. 1. Electra bellula. Maximum clearance rate F based on the total colony area at 228C n 5 31.
max
D . Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
289
Fig. 2. Electra bellula. Maximum clearance rate F based on the specific area of the colony active zooids
max
at 228C n 5 31.
21 22
ml h cm
. F for the total area was 47 of this, which is in agreement with the
max
active total area ratio given in Fig. 3 of 48 active zooids within colonies. Wet weight WW, dry weight DW and ash-free dry weight AFDW of 36 colonies
were related to the colony area, total area and specific area. Correlations were higher for the total area of the colony Table 1. Colony weight consists mainly of the calcified
parts which are non-variable in the activity cycling performed by individual zooids. Change in weight due to activity variation is therefore minimal and a regression using
the specific area does not enhance correlation between area and weight.
Based on the relation between area and F and area and weight it is possible to
max
calculate the weight-specific clearance rate. For the total colony area F at 228C was
max 21
21 21
21 21
21
9.5 l h g
DW, and 20 l h g
DW | 90 l h g
AFDW if only the weight of active zooids are considered. On the scanned pictures, the mean number of zooids per
22
area was found to be 1265 zooids cm . Knowing the area of the active part of the
colony, the number of feeding zooids was estimated. Thus the clearance rate was
21 21
calculated to 0.08 ml h zooid
.
21 22
F for the specific colony area varied from 69 ml h
cm at 168C to 107
max 21
22 21
22
ml h cm
at 248C, with a maximum of 115 ml h cm
at 208C. Clearance rates
290 D
. Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
Fig. 3. Electra bellula. Relation between area of active part of colony specific area and total area n 5 31.
including 95 confidence intervals are shown in Fig. 4. The Q value from 16 to 248C
10
is 1.7, whereas looking at the interval 16 to 208C Q is 3.6.
10
4. Discussion
