Why Attempt Sexual Reproduction (Spawning)?
Most people associate coral propagation with 'frags' (fragmentation), but frags are merely asexual reproduction—simply increasing genetically identical clones. Sexual reproduction, that is, raising new individuals through spawning (egg release) and planula larvae, is an extremely important technique for maintaining genetic diversity in captive settings.
This technique, which has gained attention in coral reef conservation efforts, is now achievable even in home reef tanks. This article explains in detail the practical procedures from inducing spawning in the tank, collecting planula larvae, settlement, to initial care of juvenile corals.
Environmental Conditions for Inducing Spawning
In the natural world where wild corals spawn, water temperature, lunar phase, photoperiod, and nutrient levels are all precisely coordinated. To induce spawning in a tank, it is necessary to intentionally recreate these environmental signals.
Temperature control is the most important factor. Many species experience a 'temperature drop phase' where water temperature decreases by 1–2°C from winter to spring, and then begin spawning when temperatures start rising again. By gradually manipulating temperature with heaters and coolers, you can control the spawning cycle to some extent. As a guideline, lower the temperature from 24–25°C to 22–23°C, maintain it for 2–3 weeks, then gradually return it to 27–28°C.
Lighting's lunar cycle is also an important trigger. By incorporating a program that reproduces light intensity changes every 12–14 days synchronized with the full moon and new moon, spawning timing becomes more predictable. Some LED light controllers (such as Apex and Hydros) have built-in lunar cycle simulation functions.
A slight increase in nutrient levels is also a pattern observed before spawning. Reports show that tanks with trace nutrient levels around NO₃ 1–3 ppm have more spawning success than completely zero-nutrient environments.
Collection of Planula Larvae
When spawning occurs, egg and sperm bundles rise to the water surface. Quickly collect these with a cup or siphon and transfer them to a small oxygenated tank (spawning tank). Within 2–5 days of fertilization, the larvae hatch and begin swimming as planulae (free-swimming larvae).
Planula larvae are about 0.1–0.3 mm in diameter and extremely delicate. At this stage, manage them without filtration or with only very gentle aeration, and absolutely avoid any intake into pumps. Perform small daily water changes (10–20% of total volume) to prevent ammonia accumulation.
They have weak positive phototaxis (tendency to be attracted to light), so directing gentle light from one direction makes it easier to collect and manage the larvae as they congregate.
Preparation of Settlement Substrate
The free-swimming period varies by species, but typically settlement readiness occurs within 5–10 days. Larvae select settlement sites by responding to specific chemical signals. The most proven effective stimulus is the presence of coralline algae (calcareous algae).
The following are effective as settlement substrates:
- Old live rock pieces with coralline algae attached
- Powder of red coralline algae called CRLAE (used in commercial aquaculture facilities)
- Small amounts of seawater with ground coralline algae added
Pre-culturing coralline algae on ceramic frag plugs or natural coral rock pieces substantially improves settlement rates. Once settlement is complete, the larvae flatten, and polyp structures begin to appear within days.
Initial Management of Juvenile Corals (1–6 Months After Settlement)
Newly settled juvenile corals (recruits) are about 1–2 mm in diameter. This period has the highest mortality rate.
Keep water flow to a minimum. Strong currents can dislodge newly settled juveniles from the substrate. It is recommended to set flow pumps to their lowest setting or manage the settlement substrate in an isolated small tank.
Start with light intensity at half or less of the parent colony and gradually increase it as they grow. Sudden high light levels can cause bleaching.
Feeding from the recruit stage onwards increases growth rate. Rotifers and ultra-fine phytoplankton are appropriate. Diluted commercial products such as 'Reef Roids' and 'BRS Phytoplankton' are also effective. Add small amounts near the water surface 1–2 times per day.
Checkpoints for Each Growth Stage
| Stage | Target Size | Management Notes |
|---|
| Planula Larvae | 0.1–0.3 mm | No filtration, small daily water changes |
| Post-Settlement (Recruit) | 1–3 mm | Minimal flow, start with low light |
| Early Juveniles | 5–15 mm | Begin feeding, prioritize water quality stability |
| Young Corals | 1–5 cm | Transition to parent colony-level care |
Significance of Sexual Reproduction
While frag propagation only increases identical clones, sexual reproduction creates genetically novel individuals. This is extremely valuable from the perspective of long-term disease resistance and adaptability to environmental change.
Coral aquaculture facilities worldwide (such as the Coral Restoration Foundation) also use this technique for selective breeding of 'super corals' with bleaching resistance. Small-scale sexual reproduction in home reef tanks is also a meaningful effort positioned within this conservation context.
If more breeders on Breeder Direct (br-choku) offer sexually reproduced juvenile corals, we can reach enthusiasts seeking genetically diverse individuals not found in the frag market. Try your hand at raising planula larvae and share your results with the community.
