The main stems of Nemertesia ramosa branch occasionally whereas those of Nemertesia antennina do not. The size at maturity for Nemertesia ramosa (a smaller species) may be less than that for Nemertesia antennina. Growth rates for Nemertesia ramosa may also be lower than those recorded for Nemertesia antennina. Growth rates are highest in the summer and lowest in the winter. An individual planula larva gives rise to a colony (sometimes referred to as an individual). These colonies (individuals) are gregarious. The feeding polyps of this species are too large to be withdrawn into the protective theca. Nemertesia ramosa is fed on by a variety of sea slugs including Doto fragilis, Doto cuspidata, Lomanotus genei, and by the sea spider Endeis spinosa.

• Males and females are separate but similar, differentiation being possible through the colour of the reproductive tissues, females being orange (yolk) and males white.
• Allocation of reproductive frequency is difficult. An individual colony will only reproduce once during its 4-5 month lifespan but this reproductive effort is probably spread over an extended period rather than a short episode. In Nemertesia ramosa, gonothecae have been observed in all months of the year with the exception of January, October, November and December (Ansín Agíl et al., 2001).
• Information on fecundity is sparse and has only been recorded for Nemertesia antennina as mean length of reproductive areas in relation to total length. Recorded values are only an estimate.
• The planula larvae are released from the gonothecae and drop off the end of the hydrocladium. They settle and metamorphose at between 12-24 hours. This is the only mobile stage in the life cycle of Nemertesia antennina and therefore very important for dispersal.
• Dispersal distance is dependent on current speed, turbulence and the height at which the larvae are released but in Torbay, the distance is thought to be between 5 and 50m.
• The dense larva reduces sinking rates by producing a mucous thread (without the thread the larvae sink at 5mm per second in still water).
• Once the larva lands on the seabed, further dispersal is limited to crawling although this probably last for no more than 1-2 hours. Crawling speeds may reach up to 5mm per minute on smooth surfaces so the planula larvae will probably not move further than 1-2 m before settlement.
• Some regeneration may occur from broken stems but this is generally found in few individuals.