Spatial and temporal variation in growth of the green sea urchin, Strongylocentrotus droebachiensis, in

The green sea urchin, Strongylocentrotus droebachiensis, is found primarily in subtidal habitats but also occurs in low intertidal pools in the Gulf of Maine. Previous studies of low intertidal populations on Swans Island, Maine, USA, found that this species is slow growing and long lived. However, these population parameter estimates may not be representative because low intertidal pools are at one extreme of the bathymetric range. To test whether these growth and age estimates are characteristic of the species across its bathymetric range and between habitats I compared growth from intertidal and subtidal habitats. Tanaka function growth parameter estimates indicate that temporal and spatial variation in growth exceeds variation associated with the intertidal versus subtidal habitat gradient. For green sea urchins in the Gulf of Maine, low intertidal pools are not different from shallow subtidal areas with respect to growth and age estimates. These results confirm that S. droebachiensis is a long-lived (> 30 years) and slow growing echinoderm. In: Echinoderms 2000 Proceedings of the 10 International Echinoderm Conference. pp. 533-538. Ed. M. Barker, A.A. Balkema. 2 MATERIALS AND METHODS Experiments were set up at two intertidal and two subtidal sites on Swans Island, Maine, USA (see Petraitis 1987, p.119, for a map of the island). I used four of the seven tidepools Russell et al. (1998) sampled (pools 1, 3, 4, and 5 from their study). Pool 1 is at Hockamock Head (44°8'2"N 68°26'53"W) and pools 3, 4, and 5 are within 2 m of each other at Hero Beach (44°8'1"N 68°24'30"W) on the other side of the island. I chose these pools because they yielded the highest percentage of recovered individuals tagged in the Russell et al. (1998) study. The two subtidal sites (depth ~ 7 m) were 10 m from each other, along a kelp dominated rock ledge that borders a sandy bottom (44°7'34"N 68°25'32"W). I chose this subtidal area because sea urchin divers had “fished it out” the previous season; the area produced sea urchins with exceptionally high quality roe (L. Ranquist, sea urchin buyer, pers. comm.). These observations indicated that sea urchin growth would be faster and be resource (or habitat) limited in this subtidal area. The fluorescent tagging method employed in this study has been used successfully in other sea urchin growth studies (e.g., Lamare & Mladenov 2000, Ebert & Russell 1993, Ebert & Russell 1992, Russell 1987, Pearse &Pearse 1975, Kobayashi & Taki 1969). Briefly, sea urchins were tagged internally with either tetracycline or calcein, which marked the skeletal structures and indicated the size at the time of tagging. Larger sea urchins (≥ 20 mm test diameter) were injected through the peristomal membrane with tetracycline (between 0.1 ml and 1 ml of a solution of 1g tetracycline in 100 ml of sea water) and smaller sea urchins were held for 24 hours in a bath of calcein (0.625 g calcein and 0.5 g sodium bicarbonate dissolved in 100 ml of tap water used for ~ 30 liters of seawater). The sea urchins were released in the field and then one year later all the individuals in the area of release were collected and processed. The test diameters were recorded and the soft tissue was removed with 5% sodium hypochlorite. The demipyramid of Aristotle’s lantern (jaw) was examined with UV light for the fluorescent mark and both the original and final size of the element was determined (yielding size specific growth). Finally, an allometric regression of test and jaw size was used to transform the jaw growth parameters to test growth. I thoroughly searched and removed any resident sea urchins before releasing tagged individuals at all four sites. Between June 3 – 6, 1996, sea urchins were collected from Duck Cove (44°7'50"N 68°25'43"W), tagged, and transplanted to pool 1 and subtidal site 1. Duck Cove is an area ignored by sea urchin divers because of poor quality roe (it lacks large brown kelps). On September 27, 1996, I accompanied sea urchin divers to Great Duck Island (44°9'30"N 68°15'0"W) to observe harvesting techniques. I collected the “cull” – smaller sea urchins (< 50 mm) usually returned after harvesting – to tag and transplant to the two other sites: Hero Beach on September 28, 1996, and subtidal site 2 on September 30, 1996. One year after tagging (June and September, 1997) all the sea urchins at the four sites were collected, processed, and examined for the fluorescent tag. Figure 1 summarizes the design of this study. Sea urchins were tagged and released at two times to encompass two summer seasons, the time of maximal somatic growth. At each tagging sea urchins were released at both subtidal and intertidal sites. The subtidal samples were released on the same rock reef (10 m apart) but were separated in time (subtidal site 1 for growth during summer 1996 and subtidal site 2 for summer 1997, Fig. 1). For the intertidal sites Russell et al. (1998) reported growth data between 1994 and 1995 so the results presented here for 1996 – 1997 provide the first estimate of sitespecific inter-annual growth variation for this species. Figure 1. Sampling design. The letters represent months in 1996 and 1997. Sea urchins were marked and recaptured in June and September. The Tanaka function was used to fit the jaw growth data (Tanaka 1982, Tanaka 1988). S f t c f t c fa d t f = − + − + + 1 2 2 2 ln ( ) ( ) (1) The Tanaka function can be expressed as a difference equation (Ebert 1998, Ebert & Russell 1993) where size of the jaw at one year ( J t+1 ) is a function of original jaw size ( Jt ): J f G G fa d t+ = + + + 1 2 1 2 2 ln (2)