An early study at UC Berkeley, California provides detail on larval development of Corymorpha palma. Unlike in many other hydroids, the medusa is not liberated.  Eggs are apparently released throughout the year.  The eggs sink to the bottom and are sticky.   Development is to an undifferentiated planula-like stage, which attaches to the sea bottom shortly after hatching, and is never free-swimming (see drawing upper Right).  Within a short time (the author does not provide a detailed time-line for development), the polyp elongates, and produces several distal and proximal tentacles (see drawing lower Right).  The author provides much detail on histological changes in the polyp during embryological development, but little on its behaviour.  Torrey 1907 U Calif Publ Zool 3: 253. Photograph courtesy Kevin Lee, Fullerton, California diverkevin.

Solitary hydroid Corymorpha palma being eaten
by several aeolid nudibranchs Flabellina sp. 2X

Large swarms of medusae of Clytia gregaria frequent the waters around San Juan Island, Washington throughout the summer. Observation in Friday Harbor Laboratories of gonad condition, and counts of sperm in water containing males, provide information on sexual reproduction. Rough counts from individual males indicate that 3 million sperm may be released each day, some as continuous dribble, but with larger releases at the same time as release of eggs from females.  Females release eggs readily in laboratory conditions, up to 50-70 at a time from fully grown and well-fed specimens.  Egg release mostly occurs 1-3h after sunset and/or 1h before sunrise, suggesting that change in light conditions may be a stimulating factor.  Cleavage begins about an hour after fertililsation (at 12^oC) and the 32-cell stage is reached within 4h.  Roosen-Runge 1962 Pac Sci 16: 15.

NOTE  the author does not rear the embryos past this stage

Life cycles are known for only a few species of west-coast hydroids.  One of these is Clytia hemisphaerica (Phialidium hemisphaericum), collected as medusae from the dock of Friday Harbor Laboratories, Washington and reared in plastic carboys in the laboratory at 12.5^oC.  Settled planulae are collected on glass microscope slides placed at the bottoms of the carboys.  Most planulae settle within a few days and, by 4-5wk after fertilisation, growth of the colonies by stolons is vigorous (see photos on Left).   The first gonangia appear by 3wk and the first medusae are liberated by 4-5wk. 

The newly released medusae are about 1.3mm in diameter and have 4 tentacles (see photo on Right).  The medusae are capable of feeding while still attached to the blastostyle.  Within a few days the medusae have 8 tentacles and by 4wk they are sexually mature, with about 2 dozen tentacles and a diameter of 1cm).  A female in laboratory culture may produce 50 eggs per day for 60d, or about 3000 eggs in its lifetime.  Roosen-Runge 1970 Biol Bull 139: 203.

NOTE  the author uses a modified “Kreisel” apparatus in the carboys to ensure good water circulation, and self-siphoning to maintain constant water volume