Table 1 Histological appearance of six gonad stages in M. barbatus

Inactive

Abundant connective tissue. Acini wall empty and collapsed. Few undifferentiated germinative cells. Phagocytes present. Sex determination hard

Abundant dark purple connective tissue. Acini wall empty and collapsed. Few undifferentiated germinative cells. Phagocytes present. Sex determination hard

Early active

Disseminated wide acini with monolayer of developing, small oogonia and germinative cells attached to the wall. Abundant connective tissue

Dense connective tissue with islets of small compact acini, filled with dark and large germinative cells and first spermatogonia

Late active

Elongated or oval small acini. Oogonia in less number attached to the acini wall, among large previtellogenic and few vitellogenic oocytes

Larger acini filled with spermatogonia on periphery and spermatocytes centrally. Lumen pink from tails of first spermatozoa

Ripe

Contracted interstitial connective tissue with flattened cells and few phagocytes. Acini rounded, large filled with attached and detached mostly vitellogenic oocytes

Light pink interstitial connective tissue stretched between large elliptical acini, filled with basally attached spermatogonia and spermatocytes. Acini lumen filled with small spermatozoa, with distinguishable pink tails in the acini center

Partially spawned

Only few layers of interstitial connective tissue. Most postvitellogenic oocytes free in the acini lumen, with large, light nucleus and small basophilic nucleolus

Loose and abundant connective tissue surrounding partially emptied acini, mainly filled with mature spermatozoa

Spent

Loose and abundant connective tissue around empty acini. Only few degenerating oocytes present. Acini wall layered with phagocytes

Collapsed and emptied acini throughout more compact connective tissue. Remains of basally located spermatozoa present with phagocytes