Do you know
if the arm damage occurred before or after you placed it in the tank? Virtually all octos we take in as pets are from the wild and will have lost parts of arms before being placed into a non-predatory environment.
Arm loss will generally occur in one of four ways (order of expected frequency).
- Predation - arm bitten off by any of a number of predators (pretty much any thing that swims and is larger than the octopus)
- Defense - arm discarded at a specialized section to avoid predation or entrapment.
- Autophagy - self immolation caused by damage and or stress
- Autophagy - self immolation caused a bacterial affliction (the bacteria not yet identified but anecdotally is considered contagious).
In cases 1 and 2, you do not need to be concerned if it occurred before you acquired the animal. The arm will grow back and there is no real harm to the animal (assuming the animal is not in the last stages of life. A senescent animal's tissue will not recover but the animal is in a dying state and is the least of the impacts on its health).
In case 3, sometimes the animal will recover if self consumption is stress related and the stressful condition is eliminated.
In case 4, the condition has always been observed as fatal but is unlikely in this instance.
From your pictures, I am guessing that it is NOT from the Atlantic, but rather imported from the Philippines general area and is likely in the Abdopus complex. Use the search term Abdopus in titles
for both journals and images of octopuses we have identified in this genus (many are assumed aculeatus as the species).
Aculeatus is a medium sized diurnal, small egg common octopus (sometimes referred to as a Bali octopus). However, there are an unknown number of similar looking animals in the Abdopus complex. Some are much smaller, some are nocturnal. We are not sure how many are not yet described.
Lastly, your PH is very low for an octopus. I would recommend slowly raising it to at least 8.2 through water changes (your new saltwater should be registering in that range if you are using RO water and a commercial salt). You may want to add buffering for awhile until your tank maintains the higher PH. DON'T try to raise it all in one day but work on it over a week's time the monitor it and add buffering to your NEW water if needed (not directly to the display). If you have a sump, you may want to add an air stone (again, not recommended for the display tank) to encourage additional release of CO2.