and 0 and 15% survival respectively at 16 ~ after 30 d. Time to morbidity was not ... Destructive grazing of kelp beds in St. Margaret's bay, .... 30-40 mm (12-28 g) and exhibiting initial symptoms of ..... data from Halifax Harbour, Table 6). In 1981 ...
Marine Biology 78, 153-164 (1984)
Marine .............Biology 9 Springer-Verlag 1984
Mass mortality of Strongylocentrotus droebachiensis (Echinodermata: Echinoidea) off Nova Scotia, Canada R. E. Scheibling and R. L. Stephenson Biology Department, Dalhousie University; Halifax, Nova Scotia B3H 4J1, Canada
Abstract
Introduction
A mass mortality of Strongylocentrotus droebachiensis, attributed to disease, was monitored in an echinoiddominated barren ground at Eagle Head on the southwestern coast of Nova Scotia, Canada, in 1982. Mortality was 70% in a shallow (3 m) nearshore area, resulting in a loss of echinoid biomass of 2 042 g fresh weight m-% and 6% in deeper (7 m, 10 m) offshore areas. Echinoid density, size and nutritional condition (gonad index) were highest in the nearshore area. Survivorship was higher in juveniles (< 15 mm diameter) than in adults resulting in the formation of a bimodal size distribution in the nearshore area. Mortality began around early October, near the peak of the annual cycle of seawater temperature ( ~ 15 ~ and was arrested by early December (seawater temperature 7~ when morbid echinoids appeared to recover. In laboratory experiments, time to morbidity of S. droebachiensis exposed to morbid conspecifics increased exponentially with decreasing temperature (20 ~ to 8 ~ There was no survival at 20 ~ and 16 ~ 20% survival at 12 ~ and 100% survival at 8 ~ after 60 d; suggesting a lower temperature limit (between 12 ~ and 8 ~ for possible transmission of a pathogenic agent. Morbid laboratory echinoids from experiments at 16~ and recovering echinoids collected in the nearshore area in early December, showed 100 and 85% survival respectively at - 30 ram) at Eagle Head Stations 1 and 2 and Sandy Cove, and of adults from Station 2, fed kelp or coralline algae for 90 d in the laboratory Date
Nov. 4, 1982 Jan. 27, 1 9 8 3 Mar. 7, 1 9 8 3
Source Eagle Head Station 1
Eagle Head Station 2
14.5_+4.1 (12) 18.8_+7.5 (15) 22.2+7.1 (15)
7.1+2.5 (15) 6.1__3.6 (15) 10.6_+4.0(15)
Sandy Cove
Laboratory kelp-fed
Laboratory coralline-fed
13.5+3.4 (15) 6.9_+2.6 (15) 18.2_+5.3 (10) 21.6__6.0(15)
Table 2. Strongylocentrotus droebachiensis. Time to morbidity and % survival of kelp-fed (high nutritional condition) and coralline-fed (low nutritional condition) individuals when exposed to morbid conspecifics at 20 ~ 16 ~ 12 ~ 8 ~ Nov. 9, 1983-Jan. 18, 1983 Source
Temp. (~
Treatment
Time to morbidity (d)
Survival (%)
Duration (d)
=>5% _->50% >_-95% Kelp-fed
20
Exposed Control
6.5 -
7.5 -
9.5 -
0 100
11 12
Coralline-fed
20
Exposed Control
5.5
6.5 -
8 -
0 100
9 12
Kelp-fed
16
Exposed Control
8.5 -
9.5 -
11.5
0 100
11.5 16
Coralline-fed
16
Exposed Control
8.5 -
9.5 -
11 -
0 100
12.5 16
Kelp-fed
12
Exposed Control
16 -
26.5 -
-
20 100
60 60
Coralline-fed
12
Exposed Control Exposed Control
20.5
29 -
-
20 100 100 100
60 60 60 60
Exposed Control
-
-
-
100 100
60 60
Kelp-fed
8
Coralline-fed
8
-
160
R.E. Scheibling and R. L. Stephenson: Mass mortality of echinoids
Strongylocentrotus droebachiensis. Time to morbidity and % survival of individuals from Eagle Head (Eli) and Sandy Cove (SC) when exposed to morbid conspecifics at 16~ (Feb. 2-22, 1983) and 12 ~ (March 14-May 6, 1983) Table 3.
Source
Temp. (~
Treatment
Time to morbitiy (d) ->5% ->50%
Field (EH)
16
Field (SC)
16
Field (EH)
Field (SC)
Survival (%)
Duration (d)
->95%
Exposed Control Exposed Control
8 8 -
9 9 -
11 11.5 -
0 100 0 100
13 13 12 13
12
Exposed Control
19 -
29.5 -
-
30 100
60 60
12
Exposed Control
18 -
35 -
-
25 95 *
60 60
* One individual developed darkly pigmented lesions around peristome within the first w6ek of experiment (did not exhibit disease symptoms) and was Cannibalized by conspecifics at 18 d
Strongylocentrotus droebachiensis. Time to morbidity and % survival of juveniles (< 10 mm) and adults (20-40 mm) when exposed to morbid conspecifics at 16 ~ Feb. 22-Mar. 7, 1983
T a b l e 4.
Source
Temp. (~
Treatment
Time to morbidity (d) ->5% ->50%
->95%
Survival %
Duration (d)
Field juveniles
16
Exposed Control
6.5 -
8 -
11 -
0 100
12.5 13
Field adults
16
Exposed Control
7.5 -
9.5 -
12.5 -
0 100
12.5 13
Strongylocentrotus droebaehiensis. Time to morbidity and % survival when exposed to 7 morbid conspecifics for different periods, and to different numbers o f morbid conspecifics continuously, at 16 ~ (Feb. 2-22, 1983) and 12 ~ (Mar. 14-May 13, 1983)
T a b l e 5.
Source Temp. (~
Field
Treatment
Survival (%)
Duration (d)
->5%
->50% ->95%
10.5 8.5 8 8
14 10 9 9
15.5 12 11 11
0 0 0 0
16 16 17 12
3d 5d 7d
10 8.5 8 7 8
21 10 9.5 8.5 9
26 13 12 11 11
0 0 0 0 0
26 18 13.5 11 13
16
Control
-
-
-
100
26
t2
Continuous exposure 1 individual 3 individuals 5 individuals 7 individuals
20 19 19 19
36 36 30.5 29.5
56.5 -
20 5 25 30
60 60 60 60
19 22 17 17.5 19.5
46 29.5 24.5 28 28
-
1d 3d 5d 7d
45.5 -
15 20 10 0 10
60 60 60 48 60
Control
-
-
-
100
60
16
16
Continuous exposure 1 individual 3 individuals 5 individuals 7 individuals Varying exposure 1h 1d
Field
Time to morbidity (d)
12
Varying exposure 1h
12
R. E. Scheibling and R. L. Stephenson: Mass mortality of echinoids of cannibalism (Table 3)]: they remained well attached to aquaria and could right within 3 min at the end of each experiment. Echinoids exhibiting initial signs of morbidity at 20 ~ and 16 ~ and removed to observation tanks died within 1 and 2 d respectively. At 12 ~ removed individuals usually died within 4 d. However, on six occasions, removed individuals were able to right themselves within 2 d and were returned to experimental aquaria. Echinoids exposed to morbid conspecifics at 16 ~ and exhibiting initial signs of morbidity, showed no further progression of morbidity when transferred as a group to ambient temperature (5~176 Within 3wk all were attached and extended their tube feet and spines. After 4 wk, 35 % of recovering individuals righted within 20 rain; 90% righted within 12 h. Individuals treated similarly but maintained at 16 ~ died within 2 d. Echinoids with epidermal lesions and spine loss, collected from Station 1 on 10December (ambient temperature was 7 ~ showed 85% mortality at 16 ~ and 15% mortality at 8 ~ after 4 wk. There was no morbidity in echinoids exposed to lesioned individuals on 2 October (pre-epizootic) for 2 wk at 20~ or 16~ One was cannibalized in the 20~ control treatment and two were cannibalized in the 16 ~ exposed treatment.
Discussion
Experimental induction of morbidity in the laboratory The absence of morbidity/mortality of control individuals in laboratory experiments conducted at temperatures approximating (16~ or exceeding (20~ peak seawater temperatures in nature, suggests that experimental mortalities were pathogenic and not due to thermal stress. Li et al. (1982) found that gonadal and digestive tract tissues of moribund Strongylocentrotus droebachiensis in 1981 were heavily infected by an amoeboid protist, tentatively classified as Labyrinthomyxa sp. Li (personal communication) found this micro-organism in gonadal and digestive tract tissues of morbid echinoids obtained from laboratory experiments at 20 ~ and 16 ~ in the present study. The development of symptoms of morbidity was clearly temperature-dependent and relatively synchronous at 16 ~ and 20~ Variable-exposure experiments at 16 ~ and 12 ~ indicated that time to morbidity was greater when echinoids were exposed to only one or three morbid individuals continuously or to seven morbid individuals for only 1 h. Variable-exposure experiments were not strictly quantitative since actual levels of a potential pathogenic agent among treatments were unknown, although experimental individuals were exposed to the same pool of morbid individuals. In the 1-h exposure to morbid individuals at 16 ~ and 12 ~ morbidity occurred in two and three pulses respectively, at regular intervals (9-10 d at 16 ~ 19-21 d at 12 ~ suggesting that individuals in the first or second pulse infected those in the second or
161 third pulse respectively. In nature, echinoid disease may be transmitted by similar sequences of infection spreading from a point source(s). The experimental results also indicate that individuals may vary in susceptibility to brief exposures to a pathogenic agent. Miller and Colodey (1983) induced morbidity in apparently healthy Strongylocentrotus droebachiensis by exposing them to seawater flowing over morbid individuals. Median time to morbidity in their experiments at 17 ~ (9 and 12 d) was similar to that at 16 ~ (9 to 9.5 d) in this study. However, Miller and Colodey reported 23-37% mortality in controls. They suggested that these individuals were infected by the pathogen in the laboratory seawater supply pumped from Halifax Harbour, an area of high echinoid mortality in 1981. The absence of mortality in control individuals in the present study suggests that the pathogen was absent, or less abundant, in the same area in the absence of echinoids in 1982. Spatial and temporal patterns of mortality in nature If mass mortality of Strongylocentrotus droebachiensis is due to disease, local differences in the incidence of mortality in echinoids at Eagle Head provide insight into factors which may affect transmission of a disease and extent of mortality in nature. Station 1 was shallower than Stations 2 and 3 but bottom seawater temperature was similar at all stations. The shallow sill bounding the nearshore cobble trough at Station 1 contained the local population of S. droebachiensis and probably limited water mixing between inshore and offshore areas. A higher incidence of morbidity and mortality in nearshore, shallow areas (
