Some observations on gametogenesis,larval development and substratum selection of the sea pen Ptilosarcus guerneyi

Ptilosarcus (Leioptilus) guerneyi (Gray) maintained in the laboratory, were observed to spawn in late March, 1972. Gametes, developed in the leaf proper, discharged through the mouths of feeding polyps and were fertilized externally in the sea water. The sea pen's eggs are 500 to 600 in diameter; a large female is capable of producing over 200,000 eggs in one season. A pear-shaped and free-swimming planula larva developed 4 days after fertilization, at a temperature of 12 °C. The larvae were ready to settle and metamorphose when 7 days old if favorable substratum was available, but would remain as planulae for at least 30 days if kept in glass dishes only. The 30-day-old larvae would metamorphose if a suitable substratum (coarse sand, for example) was presented. The larvae do not feed and, hence, development is lecithotrophic. Studies of histogenesis showed that metamorphosis greatly enhanced the rate of cellular differentiation. The high fecundity, lecithotrophic development, and the ability of substratum selection by the larvae explain the success of this species in maintaining a high-density population in many areas of sandy substratum in the shallow waters of Puget Sound (USA), despite the fact that it is preyed upon by 7 species of predators.