The objective of this study was to compare recruitment, growth, mortality, and reproduction of Amphiodia urtica in moderately-impacted and reference sites on the mainland shelf to help determine why the brittlestar does not succeed in sediments near wastewater outfalls.

Materials and Methods

The study was conducted in Santa Monica Bay and off Point Loma (Figure 1). In each area, one moderately-impacted and one reference station were selected based on analyses of existing monitoring data. Amphiodia urtica abundances were 100-200/m2 at moderately-impacted stations and 1200-2000/m2 at reference stations. In Santa Monica Bay, station C5 was the impacted station and station C3 was the reference station; the numbers correspond to designation in City of Los Angeles monitoring program. In this text, C5 is designated SMI and C3 is designated SMR. Off Point Loma, station A9 was the reference stations and B3 was the impacted station; the numbers correspond to designation in City of San Diego monitoring program. In this text, A9 is designated PLI and B3 is designated PLR.

Stations in Santa Monica Bay were sampled monthly from January to December 1990; stations off Point Loma were sampled monthly from April to December 1990. At each station, two sediment samples were collected with a 0.1 m2 van Veen grab. The samples were washed through 1.0 and 0.3 mm screens, fixed, and returned to the laboratory for processing. Amphiodia urtica were sorted and measured in the lab. The width of the oral frame, disk diameter, sex (determined microscopically), and evidence of disk regeneration were noted. The indicator of disk regeneration was the absence of the five primary plates formed at metamorphosis.

Size-frequency distributions were constructed from the data using a 0.05 mminterval for oral width (OW). A FORTRAN computer program developed for abalone (Fournier and Breen 1983, Breen and Fournier 1984) was used to calculate growth and mortality rates from the size-frequency data. This program assumes: 1) growth follows the von Bertalanffy growth model (von Bertalanffy 1938), 2) mortality is constant over age, and 3) there is a constant exponential decrease in abundance over time.

Reproductive condition was determined histologically in five adult females (OW 1.30-1.70 mm) in each sampling period. Sections of the disks were cut at 6µm, mounted on microscope slides, and stained with hematoxylin and eosin (Sheehan and Hrapchak 1980). The diameters of the first 50 oocytes with a visible nucleus encountered in transects through 3-5 sections were measured with an ocular micrometer to the nearest 0.01 mm.

Results

Population Structure and Recruitment
Amphiodia urtica was more abundant at the reference stations than at the moderately-impacted stations (Figures 2,3,4,5). The size frequency distribution of brittlestars collected at the reference station in Santa Monica Bay (SMR) had a peak at an oral width of about 0.6 mm (Figure 2). The size frequency distribution of brittlestars collected at the reference station off Point Loma (PLR) had a peak at about 1.6 mm (Figure 4). In both areas, large and small brittlestars were collected at the impacted and reference stations.

At PLR, the proportion of brittlestars with regenerated disks was 20-28%; at the other stations, 6-12% of the disks were regenerated (Figure 6a). At SMR and SMI, the proportion of regenerated disks appeared to have a seasonal component. Looking only at adults (1.3-1.6 mm OW), there was less divergence between PLR and SMR, and in August and October, brittlestars at SMR had a higher proportion of regenerated disks (Figure 6b). From April to December, the mean incidence (±1 SD) was 23.1% (±8.6) at PLR and 14.4% (±10.9) at SMR; the incidence was significantly different (t-test, p=0.04).

Growth and Mortality
The average growth rate (Brody growth coefficient; Brody 1945) was similar among all four stations (Figure 7; ANOVA, p=0.850). Mortality, however, was higher at the impacted sites than at the reference sites (ANOVA, p=0.002). The population at PLR had a negative mortality rate (Z) because the number of individuals increased with increasing size (the computer algorithm assumes that cohort size decreases with age). When mortality was calculated on the first three cohorts in April and July, Z was 0.078, a more realistic estimate. The average Z values corresponded to annual mortalities of 21.3% at SMR, 7.5% at PLR (based on three youngest cohorts), 42.8% at SMI, and 41.7% at PLI.

Reproduction
The smallest mature individuals had an oral width of 1.0 mm (Figure 8); 20-30% of individuals were mature at an oral width of 1.2 mm. About 30-50% of the larger individuals were mature. There was no difference in size at maturity or frequency of maturity between males and females. Reproductive condition could not be determined at the impacted sites because few mature specimens were collected.

The size-frequency distributions of oocytes were often bimodal (Figures 9 and );10 oocytes smaller than 5 µm were present every month. At SMR, the relative number of small and large oocytes fluctuated from month to month. At PLR, there were proportionately more small oocytes in the spring and summer and more large oocytes in the fall and winter.

Discussion

The size structure of the Amphiodia urtica populations were quite different between the reference sites. The large peak in abundance of juveniles at SMR did not occur at PLR suggesting that recruitment success varies between areas on the mainland shelf. At PLR, there was either no recruitment for several years prior to 1990, or most of the juveniles that recruited did not survive. The difference in population structure between SMR and PLR may be due to differential recruitment, survival of juveniles, predation, or some other disturbance.

The incidence of regenerated disks was significantly higher at PLR than at SMR. Since older brittlestars are more likely to have experienced a disturbance that would cause autonomy, it is reasonable to expect a population with proportionally more older animals to have a higher incidence of regenerated disks. When adults of the same size were compared, the magnitude of the difference was reduced but not eliminated. At least some of the difference in the incidence of regenerated disks can be attributed to differences in size structure of the populations.

The estimated mortality rates were significantly higher at impacted sites than at the reference sites, but there were no differences in growth rates. At the impacted sites, the size structure of the populations indicates that recruitment of Amphiodia urtica occurs, however there were few adults in either population.

The reproductive data suggest that Amphiodia urtica reproduces throughout the year. Individuals with large and small oocytes were found in every month, and individuals with ripe gonads were found throughout the year. Males and females mature at the same size (1.1-1.2 mm OW).

Conclusions

Differences in the size structure of Amphiodia urtica populations between reference stations in Santa Monica Bay and off Point Loma suggest spatial and temporal differences in recruitment and survival on the mainland shelf. Fewer juveniles were collected off Point Loma and the incidence of regenerated disks was higher. Amphiodia urtica may reproduce throughout the year.

Amphiodia urtica recruited to both the reference and impacted stations. There were no differences in growth rates between the stations, but mortality was higher at the impacted sites. The fact that brittlestars recruit to impacted sites, coupled with the lack of effects of contaminated sediments on adult growth and survival in the laboratory (see Response of the brittlestar, Amphiodia urtica, to an outfall gradient in this volume), suggests that the mechanism resulting in low brittlestar abundance near sewage outfalls acts during the early juvenile stage.

References

•Breen, P.A. and D.A. Fournier. 1984. A user’s guide to estimating total mortality rates from length frequency data with the method of Fournier and Breen. Canadian Tech. Rep. Fish. Aquat. Sci. 1239.

•Brody, S. 1945. Bioenergetics and growth. Reinhold, New York.

•City of Los Angeles. 1990. Marine monitoring in Santa Monica Bay: Annual assessment report for the period July 1988 through June 1989. Environmental Monitoring Division, City of Los Angeles. 215 pp.

•City of San Diego. 1990. City of San Diego ocean monitoring program, 1990 annual benthic monitoring report. Water Utilities Department, City of San Diego. 175 pp.

•County Sanitation Districts of Orange County. 1992. 1991 Annual Report. 3 volumes. County Sanitation Districts of Orange County, Fountain Valley, CA.

•Fournier, D.A. and P.A. Breen. 1983. Estimation of abalone mortality rates with growth analysis. Trans. Am. Fish. Soc. 112:403-411.

•Jones, G.F. 1969. The benthic macrofauna of the mainland shelf of southern California. Allan Hancock Monog. Mar. Biol. 4:1-219.

•Sheehan, D.C. and B.B. Hrapchak. 1980. Theory and practice of histotechnology. Battelle Press, Columbus, OH. 491 pp.

•von Bertalanffy, L. 1938. A quantitative theory of organic growth. Human Biol. 10:181-213.

Acknowledgements

Authors Bruce Thompson (present address: Aquatic Habitat Institute, Richmond, CA) and Mary Bergen thank the skippers and crews of the R/V La Mer and the M/V Monitor IV. H. Stubbs and D. Diehl took the grab samples. D. O’Donohue and F. Cummings measured innumerable specimens. D. Tsukada measured oocytes and assisted with data analyses and graphics.

Figure 2.
Mean number of Amphiodia urtica per size class per grab by month at the reference station in Santa Monica Bay (SMR).

Figure 3.
Mean number of Amphiodia urtica per size class per grab by month at the moderately-impacted station in Santa Monica Bay (SMI).

Figure 4.
Mean number of Amphiodia urtica per size class per grab by month at the reference station off Point Loma (PLR).

Figure 5.
Mean number of Amphiodia urtica per size class per grab by month at the moderately-impacted station off Point Loma (PLR).

Figure 6.
Percent (±1 SD) regenerated disks by month for A) all Amphiodia urtica and B) adult brittlestars (1.3-1.6 mm oral width) at the reference (SMR, PLR) and moderately-impacted stations (SMI, PLI).

Figure 7.
A) Mean (±95% CI) instaneous growth rates (K) of Amphiodia urtica at reference (SMR, PLR) and moderately-impacted stations (SMI, PLI). B) Mean (±95% CI) instantaneous mortality rates (Z) of Amphiodia urtica at reference and moderately-impacted stations.

Figure 8.
Percent of Amphiodia urtica with observable gametes in each size class at reference stations in A) Santa Monica Bay (SMR) and B) off Point Loma (PLR).

Figure 9.
Mean size-frequency distribution of Amphiodia urtica oocyte diameter by month at reference station in Santa Monica Bay (SMR).

Figure 10.
Mean size-frequency distributions of Amphiodia urtica oocyte diameter by month of reference station off Point Loma (PLR).