Squid reproduction is quite a fascinating subject - something I tend to think about a little too much. At a mantle length of 2.5 metres this Mesonychoteuthis was nowhere near as large as the species can get (given the beak measurement, lower-rostral length of 37 mm, with max recorded for the species being 48 mm).
Of the ~ 90 species of squid that we get in New Zealand waters (quite high diversity, but our waters (our EEZ) extend from near tropical conditions in the north (off the Kermadec Islands) to near Antarctic conditions in the south, 55 degrees lat, off the Bounty, Auckland and Campbel Islands), so it should come as no surprise that we have so many species, and no surprise that more species turn up on a regular basis. (In the freezer back at work, picked up just last month, we have another large-bodied squid that represents another new record for our EEZ - probably ML of 0.5-0.75 metres.) If you want to work on weird and wonderful deep-sea squid systematics just come to NZ - there's enough work for everyone!
We've just submitted a paper to NZ Journal of Zoology (Kat, myself and a colleague from Massy University, Peter Ritchie) describing the egg massses of one commercially important species, for the first time. We haven't received the reviews back yet, but they shouldn't be that far away.
Now the egg masses for a handfull of these squid species are known from New Zealand waters (2 x Sepioteuthis spp., 3 x Sepioloidea spp. [2 of which are new species], Thysanoteuthis rhombus [yet to formally record from NZ waters], 1 species of Brachioteuthis and ~ 7 of enoploteuthid squid (Enoploteuthidae: Enoploteuthinae) - the enoploteuthid and brachioteuthid squids releasing eggs individually into the plankton - as in individual eggs spat out from the mantle/funnel and left to develop on their own, without being bound into any discrete/collective egg mass; the others all bind eggs into one of a mass that is either attached to the seafloor, or released as a free-floating gelatinous sphere or oblong structure. Some gonatid squid brood eggs within the arms; species of Todarodes (Ommastrephidae - like the Humboldt squid) also release eggs into the water column (in the form of a gelatinous sphere). BUT, and to cut a long story short, the egg masses of the numerous cranchiid squid (of which Mesonychoteuthis is an example) that we get in NZ waters (and that occur worldwide) are, to the best of my knowledge, completely unknown, or at least not reported in the literature. This is quite a shocking state of affairs/ignorance for 2003 don't you think! We need more people studying reproduction in squid, especially the exciting deep-sea fauna.
A partially intact male Mesonychoteuthis does exist - I believe recovered from the stomach contents of a sperm whale, alluded to in a paper by Nancy Voss (years ago .. I forget the details), and I believe accessioned into the collections of the USNM (Smithsonian Institution). I do not believe that this male has been described. To the best of my knowledge I am unaware of any fully mature female (given ours was only submature). Moreover, I am not aware of any mated female Mesonychoteuthis. NOW, if the USNM male specimen referred to earlier is just an arm crown (as is often the case), then we have no idea what sort of penis/terminal/reproductive organ it has. As no mated female is known/has been described, we don't know where the spermatophores are implanted on/in her. As a consequence we have no idea whether the female broods young at all, and as such, we don't know whether the darkened lining of her mantle wall would serve a function as you suggest Clem.
There is another paper by Rodhouse & Clarke (1985 I recall) that describes paralarval/larval Mesonychoteuthis specimens, and there was something in the description of the earliest stages of these paralarvae that lead me to believe that maybe the female released through her mantle or funnel individual eggs (with quite a bit of yolk); I don't know why - it is just a gut feeling that I have. It was this or that the eggs were bound in some gelatinous matrix, but hatched from this sphere/oblong structure at an early stage with much residual yolk.
So, I don't believe that Mesonychoteuthis broods embryos within the mantle, but really cannot justify why - it is just a 'feeling' that I have. The next 'major press release' someone will have will be the capture and reporting of either the mature male or fully mature female Mesonychoteuthis. When the male is reported we'll be in a better position to speculate on possible reproductive tactics (in mechanical terms); when the mature female is captured we will be able to determine whether fertilisation occurs within the mantle, possibly within the oviducts, or, alternatively, externally, either within the mantle or outside the mantle (maybe in the arms, with an egg mass being cradled as we propose for Architeuthis and is thought to occur in ommastrephid and some gonatid squid), with or without subsequent brooding (some gonatids are thought to carry the eggs around with them in their arms). In the interim we might learn an awful lot about studying the reproductive habits of smaller, more abundant cranchiid squid species, and from this extrapolate to possible reproductive/brooding behaviours of the larger colossal squid. We are looking into this now. If you are at all interested in deep-sea squid reproduction then come to New Zealand; I cannot promise you any answers but I can promise you some sensational material to work on, and some pretty interesting discussions - like this thread is proving to be.
All I can tell you is that when we opened up that colossal specimen that everything inside looked weird. I was not familiar with such anatomical organisation .... so believe something rather interesting might be happening.
I'd like to say thanks to everyone so far - you know we could have a similar discussion about almost any squid species.
Does that in any way address/answer your question?