Verification of ageing methods for the burrowing shrimp, Neotrypaea californiensis, using extractable lipofuscin and gastric mill cuticular features

Authors: BOSLEY, KATELYN - Oregon State University; COLEMAN, NATALIE - Oregon State University; Dumbauld, Brett

Submitted to: Journal of the Marine Biological Association of the United Kingdom
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2019
Publication Date: 11/1/2019
Citation: Bosley, K.M., Coleman, N., Dumbauld, B.R. 2019. Verification of ageing methods for the burrowing shrimp, Neotrypaea californiensis, using extractable lipofuscin and gastric mill cuticular features. Journal of the Marine Biological Association of the United Kingdom. 99(7):1627-1638. https://doi.org/10.1017/S0025315419000742.
DOI: https://doi.org/10.1017/S0025315419000742

Interpretive Summary: Burrowing shrimp are important native inhabitants of estuaries along the US Pacific Northwest coast where they are known as ecosystem engineers because their burrowing activity greatly changes the environment and community of other organisms that are able to coexist with them. The burrowing action of these shrimp is also a threat to oyster aquaculture because they cause young oysters to be buried by sediment and die. Although basic life history information about these shrimp is known, a viable technique to estimate how old they are in order to develop population models is lacking. Estimating age is difficult and size is often used instead because these crustaceans molt their hard parts and don’t retain structures with annual deposits, which are commonly used in aging other species like fish and shelled molluscs. We examined the viability of two aging methods for ghost shrimp (Neotrypaea californiensis), quantification of a pigment called iipofuscin which accumulates in their neural system, and analysis of ring deposits in the gastric milll, a hard structure food digestion. Animals of known age were held in outdoor saltwater mesocosms and sampled regularly to correlate these age metrics with size and true age. Lipofuscin accumulated at similar rates in known-age shrimp while size varied among these same aged shrimp, verifying that lipofuscin is useful in determining age of N. californiensis. The number of gastric mill rings in mesocosm shrimp correlated with both actual age and lipofuscin-based age more directly than with size. The number of gastric mill rings in shrimp collected in the estuary however was more directly correlated to size, suggesting that rings might not be a direct measure of age, but rather an indication of molting and growth, which can be greatly affected by environmental factors. Size is not a reliable indicator of age for this shrimp and quantification of lipofuscin appears to be the most reliable method for determining age.

Technical Abstract: The burrowing shrimp Neotrypaea californiensis is an important ecosystem engineer that inhabits estuaries along the US Pacific Northwest coast. The burrowing action of these shrimp is a threat to oyster aquaculture because they cause young oysters to be covered by sediment and die. Two techniques to determine age for these crustaceans were explored to assist in development of population models. Ageing crustaceans is difficult because they molt their exoskeletons and don’t retain structures with annual deposits, which are commonly used to age other marine organisms. A mesocosm growth experiment was combined with field surveys to investigate the viability and compare quantification of the autofluorescent pigment lipofuscin and counts of gastric mill rings to assess age in N. californiensis. Animals of known age were grown in outdoor mesocosms and sampled regularly to correlate age metrics with size and true age. Lipofuscin concentration increased with time across multiple cohorts at the rate of 1.430 ± 0.060 ng µg-1 yr -1. Gastric mill ring counts also increased with time (4.922 ± 0.337 rings yr -1 ), but were more variable than lipofuscin measurements in predicting age. While age estimates in mesocosm shrimp based on lipofuscin concentration and gastric mill rings generally agreed, size did not correlate to either age metric. Gastric mill ring number in shrimp from field collections correlated with size, suggesting that rings might not be a direct measure of linear age, but rather an indication of growth, which is highly variable and more influenced by environmental factors.