"How different does a fish really feel from one day to the next?" Zebrafish larvae become protected animals at the age of 5 days post fertilisation. At 4 days, they are not. Why is this?

“[Before 5 days] you can do what you want to it, you can chop its head off, you can do what you like. And then the next day it’s protected, you can’t do anything, you can’t even inject it or anything. And how different does a fish really feel from that one day to the next? Probably not much different.” (Interview with experienced Named Veterinary Surgeon, 2018)

 

A recent article in the Guardian by the marine biologist Carl Safina discusses an ongoing source of disagreement amongst biologists: the question “do fish feel pain?”

From the point of view of laboratory animal welfare law however this question is arguaby somewhat moot. Fish in labs in the UK are protected animals by the singular virtue of having a spine, and what they are protected from is experiencing unnecessary “pain, suffering, distress or lasting harm” caused in the course of an experimental procedure licenced under the Animals (Scientific Procedures) Act (1986). Whether or not all scientists agree on the capacity of fish to feel pain as a matter of fact, the law effectively assumes that they do and requires people to treat them as such.

I say arguably for a reason however. There is one area in particular where this question can arise and is of some significance. This has less to do with the capacity of fish to feel pain per se than it does the capacity of organisms with larval forms (like fish and amphibians) to experience pain at particular stages of their early development.

The legislation defines the critical point as that at which the organism becomes capable of “independent feeding”. Prior to this stage, the embryos and larvae of fish don’t count as animals, as comprehended by the Act. Thus, as a consequence of having a spine and being considered capable of independent feeding, these forms become legal animals and as such garner a new status that commands special ethical recognition.

It is easy to see that the crux of this is the connection between the legal construction “independent feeding” and the subjective experience of pain. “Independent feeding” is indeed really a more-or-less useful proxy for the development of relevant neuroanatomical features thought to be consistent with the capacity to experience pain or suffer.

However, development is obviously a continuous process. It is very hard to find a singular point defined by a clear and obviously relevant biological feature in the developing embryo or larva that everyone agrees is indicative of the capacity for experiencing pain. Whatever you choose, given the complexity of the phenomenon called “pain” in general (and pain in fish in particular, and pain in early developmental forms even more so) this is most likely always going to be open to dispute.

It’s thus probable that wherever you choose to draw a line in the sand, there will be a whiff of the arbitrary about it. It is this sentiment which underpins the doubts of the experienced Named Veterinary Surgeon quoted in the epigraph to this blog post.

The subject in this case is zebrafish – a hugely important model organism in the UK and around the world. The zebrafish is also a major focus of my research in the Animal Nexus Project. For zebrafish, the consensus is that the capacity for independent or exogenous feeding occurs at 5 days’ post fertilisation (or “5 dpf”) when the embryos are reared in normal husbandry conditions with a temperature of 28.5 degrees Celsius. The “5 dpf” line thus divides protected and non-protected zebrafish forms. This is why the question arises of whether or not the different potential experience of a fish from one day to the next is really significant or not?

An obvious question that arises is: What is the rationale for choosing “independent feeding” as the threshold dividing those development forms that are included as a part of our moral community – as constructed by the legislation –  and those that are not? In other words, how good a proxy is it?

This is a matter of expert opinion, and the whole area suffers under a relative dearth of experimental evidence and consensus. But the reasoning, whatever its shortcomings, is conveniently spelled out in a Scientific Report produced by a panel of expert’s commissions by the European Food and Safety Authority who were tasked with reviewing the question of larval sentience in the mid-2000s (EFSA 2005). They argued that – in the absence of detailed and commanding evidence into the mental lives of larval and embryonic forms – it was judicious to look to the capacity of an organism to lead an independent existence as a sign of their capacity to experience pain. The reasoning assumes that pain is an ancient evolutionary adaptation whose benefit comes from it teaching us not to make the same mistake twice. Look at it this way: if you lead an independent existence, it is likely that you are in touch with the opportunities (food) and dangers (predators) that surround you in your environment. You are moving around and, most importantly, you are capable of avoiding danger – and if you are capable of doing this, then you most likely have the capacity to learn from your experiences. Ergo, the neural circuits that are necessarily for generating the experience of pain could be adequately developed at this point.

No doubt, the reasoning is not watertight –  indeed it is presented in the spirit of “this is the best we can manage under the circumstances”. But what is of equal importance here, however, and is not always clearly spelled out, is that the choice of “independent feeding” is above all else practical – or at least, more practical from the point of view of public policy than attempting to base the definition of protected larval or embryonic form directly on the appearance or formation of some neuroanatomical feature believed to be indicative of the capacity for experiencing pain.

In sum, the issue here is not so much whether or not fish feel pain or not, but rather of defining the point at which the law will begin treating the larval stages of animals such as fishes and frogs as though they are capable of having subjective experiences including pain. Scientific assessment and experimental evidence are clearly an indispensable input. But on their own they cannot resolve the question as it is presently posed. There are also irreducibly pragmatic and ethical dimensions that require the exercise of judgment.

I find myself no closer to a firm opinion on the matter, despite my ongoing fieldwork in zebrafish facilities around the country. I’ve observed for example that there is also no clear consensus amongst zebrafish technicians and scientists about whether the technical definition of a sub-5dpf fish as a 3Rs “replacement” for adult vertebrate forms is ethically acceptable or not.

Influencing this is, perhaps, the embodied experience of encountering these forms at first hand. If you were to look into a petri dish of 5-day old zebrafish, you’d typically see between, perhaps, 25 and 50 small, black dots. Just a few millimetres long, they appear even smaller from above because the ends of their tails are transparent. Their swim bladders should by now be inflated, allowing them move through the short water column in only irregular, sporadic little jerks. Their movements would seem random – they are not yet social beings that school collectively the way adults do. At this point you will be probably be trying, with some difficulty, to count them as they’ve just become legal animals and, in a sense, individuals whose trajectory must from now on be traced and recorded. A few will probably not be moving though. They’ll being lying still on the bottom, either malformed and ill, as yet without swim bladders, or already dead – it’s usually impossible to tell which without microscopic examination. These you’ll need to suck out with a pipette and dispose of – if alive at all, they will not make it, and there are no veterinary treatments available. The natural attrition rate for these youngsters is significant, commensurate with the vast numbers of eggs that zebrafish produce and fertilise. Quite often fish of this age and size just disappear, digested into the water or culture medium.

It’s hard to form an affective bond with such animals of the kind that helps remind one of their newly awarded ethical status, never mind what we may beleive about their subjective experiences. No doubt, it is not forms like these that members of the public have in mind when they consider the legitimate claims that laboratory animals make on us. Only by observing them through a microscope is one reminded that they are, however small and imperfect, really little fishes at all. How does one adjust ones personal moral compass in the light of such knowledge and experience, let alone develop enforceable public policy to which most people would be prepared to assent most of the time?

 

EFSA. “Scientific Report ‘Aspects of the Biology and Welfare of Animals Used for Experimental and Other Purposes’ (EFSA-Q-2004-105).” EFSA Journal, (3)12, (2005): 1–136. 

Safina, Carl. "Are we wrong to assume fish can't feel pain?" Guardian. 30 October 2018.

Photo credit: NICHD, "Zebrafish blood vessels".