No, remember the truism: "Absence of evidence is not evidence of absence."

You won't see any gametes if the animal is not in good condition or has been subject to the wrong environmental conditions (such as, I suspect, an environment that it too stable to trigger gamete development).  In these cases, it won't make gametes (either eggs or sperm).  And you won't see eggs.

There are three ways to determine gender:

1) Watch the animal spawn.

2) Externally examine the "gonads" and see their development.

3) Do a microscopic biopsy of the gonads and examine the cells.

I suggest you boost the temperature a few degrees on either end of that range.  You might get algal growth, though; I suspect your temperature is simply too cold for algal (and reef animal) growth during a large part of the time.  In the real world these creatures would likely never see any temperature that low.

Cheers, Ron

"The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind. The love for all living creatures is the most noble attribute of man." Charles Darwin (1809-1882)