[spawner]

There is only one species of Lysmata that should require a settlement cue, it’s a peppermint shrimp and the it has not been proven as of yet. The notion that they would require a cue has come for people that are not equipped to determine if their larvae are competent to settle. Competency in Lysmata is determined by the full development of the pleopods, they must have fully formed pleopods with an appendix interna present before you can consider them competent. Other features are good clues to larval development, but as yet I can not find a structure that gives 100% confirmation that the larvae are competent.

1cm Lysmata amboinensis is not any where near ready to settle. Your larvae have to be fully developed and I have never seen any Lysmata amboinensis or L. grabhami near settlement, morphologically, that were less than 2cm (2.45cm is normal) in total length. You could speculate that larvae can grow a bit larger in the last larval stages then is totally necessary to metamorphosis, but growth between zoea stages is not something I would agree with (i.e. a zoea 8 being larger than a zoea 9).

Your assumptions as to why settlement does not occur by 43 days is incorrect, many larvae have not reached zoea 10 by this date, days are temperature dependant, thus lower temperatures will have a slower developmental time than higher temperatures and a longer larval duration. This is major problem with using days for rearing time. The problems with L. amb is that they increase their volume (biomass) in a huge way in relation to their total length in later stages, thus requiring a ton of energy to have grown enough during the intermolt period to achieve the next zoeal stage, there a few zoeal stages that have several multiple instars (5 or more, mark-time molting) this is normally at the points of rapid size increases (measured by biomass). These multiple instars (mark-time molting) in the mid zoeal stages are where the vast majority of developmental delays occur. Once larvae are at competence they will readily settle, with some delay up to 3-6 extra molts.

Questions as to why fire shrimp are smaller larvae, can be addressed by the phylogenetic position of the shrimp. Fire shrimp are the first branch out from the peppermint shrimps, thus they are closer related and have a similar sized larvae. L. amb/grabhami was formed later than fire shrimp and must have had some evolutionary pressure to settle at a large size and thus over time the larvae have become larger.

To address the prey capture issues, the reason you don’t see the larvae eating Artemia nauplii is primarily due to the rapid growth of the larvae, in conjunction with the faster handling time as they age. It can take over 10 minutes for a larvae to consume a nauplii in the first few stages and decreases to less than a minute after zoea 6. Consumption does not generally increase in conjunction with deceased time required to handle the prey. This is a results of the larvae developing more efficient digestive enzymes at around zoea 4-6.