Swimming and Buoyancy in Ontogenetic Stages of the Cushion Star Pteraster tesselatus (Echinodermata: Asteroidea) and Their Implications for Distribution and Movement

Abstract

The eggs of some marine fish (1) and benthic invertebrates such as many corals (2. 3) and lecithotrophic echinoderms (4, 5) are positively buoyant at time of release front the parent, and density increases later in ontogeny. How these eggs and lan'ae are distributed in the water column and eventually reach suitable habitat for settlement will depend, in part, on their vertical velocity and on the turbulence in the water (i.e., the eddy diffusivity). For eggs and unhatched stages, vertical velocity is passive and depends on egg or embryonic volume and density relative to the seawater (6. 7). For motile stages, vertical velocity depends on relative density, swimming ability, and behavior of the lan'ae (8, 9). We have measured the vertical velocity of eggs and lan'ae of the sea star Pteraster tesselatus Ives, which spawns floating eggs (1.1 to 1.5 mm diameter) that develop into nonfeeding larvae and spend several weeks in flic plankton before settling to the benthos (10). Because of the simple shapes of eggs and lan'ae, we used force balance equations for drag and buoyant forces to determine the density of eggs and t\ro lan'al stages. Initially the eggs were positively buoyant and floated upwards at about I mm/s. Even formalin-fixed eggs floated in seawater, so concentrations of light ions were not responsible for the buoyancy. The density of the lan'ae increased in the first 10 to II days, but it varied considerably between the three larval cohorts examined. Ten-day-old lan'ae that were negatively buoyant swam downward at mean speeds as high as 1. 7 mm/s, while

Direct development of the brittle star Amphiodia occidentalis (Ophiuroidea, Amphiuridae) from the northeastern Pacific Ocean

“…Direct development is a category often used for developmental patterns that have lost virtually all features found in feeding larvae, and is here considered an extreme form of abbreviated larval development. After hatching from the fertilization envelope, planktonic stages swim and often feed with cilia (e.g., Strathmann 1971;Kelman & Emlet 1999); nonplanktonic development occurs on the benthos in tough envelopes (e.g., Fell 1941;Hendler 1977;Byrne 1995) or more often under (or inside) the protection of a parent (e.g., Chia 1968;David & Mooi 1990;Hendler 1991;Byrne 1996). When fertilization occurs in the water column, embryos develop rapidly and hatch as functional swimming forms (Staver & Strathmann 2002).…”

Novel method to concurrently sample the planktobenthos and benthos

“…Although eggs and early larval stages of many species are buoyant, some species or later larval stages are neutrally or negatively buoyant and can be detected only by benthic or planktobenthic sampling (Kelman and Emlet 1999). Benthic sampling is impractical for egg and larval collection, as the large amounts of sediment collected in the small mesh size needed to retain most larvae would make the sorting of any larvae collected extremely time-consuming and difficult.…”

Characterization of the lecithotrophic larval development of the temperate New Zealand asterinid Stegnaster inflatus

“…The buoyancy of fertilized and unfertilized eggs, as well as of unhatched larvae and early hatched brachiolaria larvae ( n = 10 per developmental stage from one female), were evaluated individually using a 1‐L graduated measuring cylinder (60 mm in diameter) filled with 1 μm filtered seawater (methods modified from Mann, Campos, & Luckenbach, and Kelman & Emlet, ). To reduce the effect of convection currents (Kelman & Emlet, ), all measurements were made within the environmentally controlled room using temperature‐equilibrated filtered seawater sourced from inside the room (i.e., the seawater within the cylinder was at 18–20°C in equilibrium with room temperature). The eggs or larvae were gently placed on the water surface in the center of the measuring cylinder, ensuring that there was no contact with the cylinder walls, and allowed to reach the starting point (the 800 ml mark).…”