Spotted ratfish: building a viable aquarium breeding cycle

Spawning in an aquarium is not the same as completing a sustainable breeding cycle. Adult spotted ratfish (Hydrolagus colliei) have survived in managed collections for decades, yet embryos often fail before hatching and early juveniles remain challenging to rear. A husbandry report from Planet Ocean Montpellier brings together 15 years of experience, the first longitudinal biometric records from hatchling to subadult stages, and the conditions associated with both progress and loss.

The paper is not a universal protocol. Its practical value lies in the structure of the programme: separate life stages, retain individual identity, monitor environmental context, investigate failure points and postpone invasive research until survival is more reliable. That approach is relevant to any aquarium managing a slow-maturing or poorly documented aquatic species.

A familiar exhibit species with an incomplete husbandry evidence base

The spotted ratfish is a chimaera from the north-eastern Pacific. Like sharks and rays, it is a cartilaginous fish, but it belongs to the holocephalans. Its large head, prominent eyes, broad pectoral fins and tapering tail form a distinctive body plan that must not be treated as a generic shark shape. The species lives near the bottom from coastal water to depths beyond 1,000 metres and has been exhibited for more than three decades.

An European Union of Aquarium Curators monitoring programme has gathered captive information from 13 facilities since 2014. Planet Ocean Montpellier recorded its first captive-bred hatchlings in 2016 and regular hatching from 2018. Berio and colleagues describe one institution and a limited number of animals. Their work is a longitudinal husbandry report, not a controlled comparison that identifies an optimal temperature, tank or feeding regime.

This distinction matters internationally. A condition associated with success in one system may interact with flow, tank volume, source-water microbiology, animal history, stocking, diet and staff coverage. Transferring one number without those relationships can fail even when the original observation is valid.

Separate life stages and preserve individual identity

The Montpellier programme uses four systems for successive stages: eggs and neonates, hatchlings and early juveniles, intermediate juveniles, and older animals in the public display. Separating stages reduces egg predation, matches current and volume to animal size, and makes individual feeding and observation more manageable.

The scale of change is substantial. Published animals under six months ranged from approximately 6 to 17 cm total length. Intermediate juveniles measured 23 to 27 cm, while specimens older than three years measured 35 to 54 cm. Mean hatchling measurements were 9.3 cm and 5.7 g, but these values came from animals that died shortly after hatching. They should not be converted into clinical reference intervals.

Traceability begins with the egg. Staff attach a small coloured and numbered marker to the egg-case pedicle. After hatching, natural colour patterns help distinguish animals of similar size. This continuity can link an egg-laying event, incubation duration, growth trajectory, transfer and health event. Without individual identity, a visible success may obscure recurrent loss at a specific developmental stage.

Observe development without sacrificing the embryo

Egg cases are checked daily and decayed cases removed. Once a month, staff use candling: light is passed through the case to view the yolk and developing embryo without opening the capsule. The method can document the emergence of the head, eye, fins and dorsal spine while limiting disturbance.

The decision to remain non-invasive supports both welfare and scientific quality. Cutting a window into the egg case might produce more detailed images, but it would increase risk to an already vulnerable embryo. The authors therefore prioritised reliable survival before more invasive developmental research. Better-looking data are not better evidence when collecting them changes the outcome under study.

Approximately half of monitored eggs were unfertilised, empty or decayed. The report does not force these outcomes into one diagnosis. Developmental abnormalities may contribute, while pathogens could also cross the mucous plug. Rapid decomposition then makes investigation difficult. Aquarium teams should record the stage at which development stopped, egg-case appearance, water data and recent handling, with a pre-agreed sampling plan. A cloudy or deteriorating case is not, by itself, proof of infection.

Recreate a coherent environment, not a single target value

Juvenile and adult systems in Montpellier follow a seasonal temperature cycle, with cooler water in winter and warmer water in summer. The photoperiod lasts ten hours and light changes gradually at the start and end of the day. Natural nursery conditions for this species remain poorly documented, however, and embryonic development has been reported across a relatively broad temperature range. Mortality may still differ within that range.

Temperature is likely to influence incubation duration, as it does in other egg-laying cartilaginous fishes, but the report cannot define a transferable set point. Any adjustment should be evaluated alongside thermal stability, oxygenation, flow around the cases, water quality and observation capacity. Changing one variable without a defined monitoring plan makes the result hard to interpret.

Adults receive a varied marine diet. In one mature female, eggs were found on average every two weeks, with seasonal and year-to-year variation. Mating was rarely seen and paternity was not established. The findings support a combined approach: systematic daily observations, genetic work when ethically obtained samples become available, and collaboration across institutions rather than reconstruction of the cycle from recovered egg cases alone.

A practical framework for aquarium teams

Before aiming to maximise hatch numbers, a facility can establish a shared register for breeders, observed mating, laying events, egg-case identity, development, transfers, feeding, growth, water variables and health events. Terms must remain stable. “Empty case”, “no visible development” and “decayed case” are observations with different evidential meanings, not interchangeable diagnoses.

Housing plans should account for suitable volume and flow at each stage as well as staff capacity for observation, feeding and low-stress transfers. Movement criteria can combine size, feeding competence, behaviour and predation risk rather than age alone. Candling, restraint and clinical procedures should each have a defined frequency, stop criteria and contingency plan.

Comparisons between institutions must preserve context. The same temperature does not represent the same exposure when flow, light, substrate, tank mates or diet differ. Shared protocols should standardise definitions and units while retaining the features needed to interpret each facility’s results.

Turn husbandry success into reusable evidence

The strongest contribution from Montpellier is not one husbandry value. It is a chain of evidence connecting long-term observation, identified animals, separated stages and explicitly stated uncertainty. This framework helps teams locate losses, decide which changes deserve a controlled trial, and identify questions that should be investigated collaboratively.

Vetofish can support reproductive registers, critical-point reviews, welfare and health indicators, and inter-aquarium protocols designed for meaningful comparison without presenting one institution’s experience as an automatic prescription.