Saturday, May 17, 2008

Abundance, Spatial Distribution, and Size Structure of the Sea Star Protoreaster nodosus in Palau, with Notes on Feeding and Reproduction

Scheibling, R. E. & Metaxas, A., 2008.

Bulletin of Marine Science 82(2): 221-235

Abstract

Protoreaster nodosus (Linnaeus, 1758) is a common sea star in shallow sand and seagrass habitats in the tropical Indo-pacific region. It has been extensively harvested as an ornament, but little is known of its biology and ecology. We quantitatively sampled populations at five sites in palau to obtain baseline information on abundance, spatial distribution, and population structure of this species in areas unaffected by human depredation. Density on sand bottoms ranged from 4 to 32 individuals 100 m-2 and biomass from 3.1 to 20.7 kg 100 m-2. Population size estimates at two locations were 100 and 630 individuals. Mean size (radius) of individuals ranged from 14 to 16 cm among populations, and was negatively related to density. Density was greatest in a seagrass bed (51 individuals 100 m-2, 12.3 kg 100 m-2) where the population was composed of juveniles (8-12 cm). Spatial distribution of sea stars in each population was random, as indicated by nearest-neighbor distance. Manipulations of density and spatial distribution, performed by removing or by adding and centrally aggregating individuals in circular plots (78.5 or 314 m2), showed that sea stars reestablish pre-manipulation densities and nearest-neighbor spacing within 2-5 d. Protoreaster nodosus feeds on meiobenthos and microbial/microalgal films by extra-oral ingestion of sediments and seagrass. The incidence of daytime feeding is generally high (> 60% of individuals), suggesting these sea stars are important consumers and bioturbators in sedimentary habitats. Synchronous spawning occurred at full moon (in May) in the laboratory, yielding planktotrophic larvae.

These photographs were captured after Tanjung Chek Jawa recovered from a mass death event that occurred in early 2007.Did other individuals survived this natural disaster? This question can be answered if photographs of individuals captured before and after the event are compared. If you have such photographs, please send them to chimck@yahoo.com.

Friday, May 16, 2008

Juveniles of the knobbly seastar (Protoreaster nodosus) are known to be rare in Singapore waters (Lane and Vandenspiegel 2003). The first visual documentation of a juvenile knobbly seastar is perhaps a posting in the blog of the Blue Waters Volunteers on 23 August 2005. The find happened during a ReefWalk activity at Kusu Island. Ever since juveniles were encountered at Cyrene Reef on 21 April 2007 by TeamSeagrass, recruitments of knobbly seastars were also observed at Pulau Semakau, Labrador Nature Reserve and Changi Beach.

The short population census conducted at Cyrene Reef on 8 and 9 May 2008 revealed that there are at least 62 individual knobbly seastars (Protoreaster nodosus) at the intertidal habitat. This is perhaps the largest Protoreaster population ever documented in Singapore. The population size is likely to be much larger than estimated as the two surveys were carried out on only a small (~20% of total area) intertidal area of Cyrene Reef. In addition, the population contained a wide range of body size (minor radius: 21-56 mm). There were at least 12 juveniles, which are seastars with minor radius less than 30 mm. The presence of juveniles, subadults and adults indicated that there is a healthy level of recruitment at Cyrene Reef. This habitat may be the only sustainable population of knobbly seastars left in Singapore today.

Tuesday, May 13, 2008

On 9 May 2008, a total of 45 knobbly seastar (Protoreaster nodosus) individuals were sighted at Cyrene Reef. Twelve of these were first encountered on 8 May 2008. Photographs of 33 'new' individuals are given below. Our recognition programme indicated that there are at least 62 individual seastars on this intertidal reef.