IN SITU MONITORING OF LONGFIN INSHORE SQUID EGG DEPOSITION AND EMBRYONIC DEVELOPMENT.
Despite the importance of environmental effects on cephalopod life history and population dynamics (Rodhouse et al. 2014), the understanding of such effects on cephalopod early development is limited to relatively few laboratory-based studies (Boletzky 2003, Robin et al. 2014). Recent research on environmental effects on loliginid squid embryonic development has focused on laboratory studies aimed at understanding the effects of ocean warming (Pecl & Jackson 2008), ocean acidification (Kaplan et al. 2013), and oxygen depletion (Navarro et al. 2016). Opportunities to observe and document squid egg deposition and embryonic development outside of laboratory environments are rare.
The longfin inshore squid Doryteuthis pealeii (Lesueur, 1821) is distributed in continental shelf and slope waters of the northwest Atlantic Ocean from Newfoundland to the Gulf of Venezuela (Jereb et al. 2010). The species is harvested extensively along the northeastern United States continental shelf, where the species is considered a single unit stock (Shaw et al. 2010, Northeast Fisheries Science Center 2011). Spawning activity of D. pealeii occurs at the seafloor, culminating in the placement of egg masses or "mops" on the bottom (Griswold & Prezioso 1981).
Each mass is composed of a cluster of gelatinous egg capsules or "fingers," each of which contains approximately 180 eggs wound in a spiral array (Arnold et al. 1974). Egg masses may consist of capsules deposited by multiple females and fertilized by multiple males (Shashar & Hanlon 2013), and average 12-15 cm in diameter at deposition (Griswold & Prezioso 1981).
Longfin squid spawn in shelf waters off southern New England, including the inshore waters of Nantucket Sound (Hatfield & Cadrin 2002), which is a site of a seasonal inshore fishery (McKiernan & Pierce 1995). Fishermen working in and around Nantucket Sound have expressed concern that egg masses laid on the bottom could be damaged by mobile fishing gear before egg hatching (Mid-Atlantic Fishery Management Council 2016). Although no research on the effects of fishing gear on Doryteuthis pealeii eggs has been conducted, it has been hypothesized that damage to squid egg casings by mobile gear could cause mortality (O'Shea et al. 2004). Knowledge of hatch timing in inshore waters, combined with an ability to predict time to hatching should egg masses be discovered, could be useful for localized, dynamic spatiotemporal management efforts conducted among fishermen wishing to reduce potential impacts to egg masses. Environmental effects on time to hatching of D. pealeii has been studied in laboratory environments (e.g., McMahon & Summers 1971, Kaplan et al. 2013, Long et al. 2016), but no in situ studies have been attempted as have been for some other loliginid species (e.g., Oosthuizen et al. 2002, Arkhipkin & Middleton 2003). The objectives of this study were to monitor egg mass deposition and embryonic development within individual egg masses under natural conditions, document hatch timing, and record the associated seawater temperatures.
MATERIALS AND METHODS
Longfin inshore squid egg masses were collected from commercial fish weirs in northeastern Nantucket Sound (41[degrees] 40' N, 70[degrees] 05' W) in spring and early summer 2008 to 2016. Fish weirs in the Sound consist of nets strung from hickory poles driven into the substrate and set perpendicularly to shore in shallow water (depth ca. 8 m) along the shore (McKiernan &
Pierce 1995). Spawning behavior has been observed within the weirs (Hanlon et al. 1997, Shashar & Hanlon 2013), and egg masses are frequently found attached to or drifting along the mesh at the bottom of the traps. During fishing operations, a dip net was used to transfer the catch, including squid egg masses, into the boat.
Logbooks were maintained from 2008 to 2016 in which catch (including the presence of egg masses) and effort data for each weir were recorded daily. Starting in 2009, newly deposited egg mops were transferred to a mesh enclosure on an opportunistic basis. Egg masses were considered newly deposited if they were not observed during the previous day's fishing operations. The enclosures were made from 3.3-cm plastic-coated wire mesh and featured 22 x 45 x 22 cm compartments, two sides and the bottom of which consisted of 1-cm plastic mesh, suspended ca. 7 cm off the seafloor. The mesh size was chosen to minimize loss of water flow due to fouling while excluding spawning squid that might add eggs to the masses within. The enclosures were placed at a depth of ca. 7 m immediately adjacent to the weir to permit semiweekly retrieval and sampling. Samples of squid egg masses (five randomly chosen "fingers" excised from a mass) were chilled and transported to a nearby wet laboratory. The samples were examined using a dissecting microscope and the embryonic development stage of eggs within each of five randomly chosen fingers was classified following Arnold (1965).
Sampling frequency was adjusted based on observations of embryonic development to accurately document hatch timing.
Hatch timing was determined via direct observation of paralarvae emerging from egg masses. In the event that hatching was not observed directly (i.e., an egg mass was found to have hatched before sampling), hatch timing was inferred based on the embryonic development stage observed during the previous sample.
Development time was calculated as the number of days between egg mass deposition and hatching. A temperature data logger (VEMCO 8-bit MiniLog or Onset Computers TidBit v.2) was attached to the weir or the mesh enclosure and recorded seawater temperature every 15 min. Mean seawater temperature was calculated for the development time of each egg mass.
Egg mass presence in fish weirs was first observed in the month of May during 2008 to 2016, with the exception of April 25, 2012 (Fig. I). The longest period of egg mass presence (May 4--August 28) was in 2014 (Fig. 1.). The latest date that an egg mass was observed was August 28, 2014 (Fig. 1). The number of days in a season with egg mass presence ranged from 6 in 2013 to 32 in 2009 (Table I). Each year the egg mass presence was observed and egg mass samples were collected if possible (n = 15). An average of two egg mass samples were collected each year, and the only year that egg mass samples were not collected was in 2013 (Table 1).
Embryonic development stage of each sample was classified from I to 30 following the system of Arnold (1965). Resolution of the dissecting microscope was insufficient to differentiate between the earliest stages without staining or dechorionation (1-12; classified by cell division/cleavage), all of which may occur during the first 1-3 days of development (Arnold 1967, McMahon & Summers 1971). During examination of the final sample of each egg mop, egg fingers were entirely empty or contained eggs that were in the latest stages of development (29-30) and actively shedding yolk sacs and hatching. Although hatching of the late stage embryos may have been a result of the physical disturbance associated with examination, Arnold (1965) also observed hatching at stages 29-30, so it was assumed that hatching would have taken place at the same time in the field. Little to no heterogeneity was observed in developmental rate between randomly selected egg capsules as evidenced by stage classification.
Development time of egg masses ranged from 12 to 34 days
(Fig. 2). The mean seawater temperature recorded during egg mass monitoring from 2009 to 2016 was 18.0[degrees]C, and most egg samples developed at a seawater temperature between 16[degrees]C and 18[degrees]C (Fig. 2). The two time periods that had the highest mean seawater temperatures in 2009 (19.6[degrees]) and 2015 (18.6[degrees]) also encompassed the shortest egg mass development times (Fig. 2).
An inverse relationship was observed between average seawater temperature and development time (r = 0.69; Fig. 2).
Although no other in situ studies of Doryteuthis pealeii embryonic development have been conducted, the relatively few such studies of other loliginids have yielded similar results.
Naturally incubated embryos of Loligo vulgaris reynaudii grew slightly faster at higher mean temperatures (Oosthuizen et al. 2002), as did those of Loligo gahi (Arkhipkin & Middleton 2003). Laboratory studies of embryonic development of D. pealeii (McMahon & Summers 1971) and other loliginids (Laptikhovsky 1999, Pecl & Jackson 2008) indicated similar relationships. A larger sample size would yield a more quantitative relationship between embryonic development duration and seawater temperature based on natural conditions, which would likely be an even better predictor of development time than laboratory-derived relationships. Measurements of developing embryos and hatchlings would facilitate better comparison with length-based growth models (Laptikhovsky 1999) and provide insights into temperature effects on hatchling size and fitness (Peel & Jackson 2008). Measurements of other environmental variables (e.g., pH and dissolved oxygen) would also be useful to compare with laboratory studies (e.g., Kaplan et al. 2013, Long et al. 2016). Anoxic conditions at fish weirs were recorded periodically during the 2008 to 2016 field study and may have increased the length of some of the development times observed in this study, as observed in laboratory environments on Doryteuthis opalescens by Navarro et al. (2016), although experimental evidence suggests that low dissolved oxygen may occur naturally in D. pealeii egg masses (Long et al. 2016).
Known temperature development time relationships may be a useful predictor of hatch timing of eggs discovered in the field, which in turn could be used as a local management tool by fishermen wishing to minimize potential impact on squid eggs on the bottom. For example, should an egg mass be discovered while deploying fishing gear, it could be brought in for examination and the stage of the eggs combined with seawater temperature could be used to predict hatch timing and allow decisions regarding gear deployment to be made based on that information. The ability to sample newly deposited eggs and monitor them under natural conditions provided new insights into the biology and ecology of Doryteuthis pealeii, with potential implications for the understanding of population dynamics and fisheries management.
This research was supported in part by the Massachusetts Marine Fisheries Institute, the Quebec-Labrador Foundation Sounds Conservancy Grants Program, the American Museum of Natural History Lerner-Gray Memorial Fund, and the Nancy Spofford Yerkes Foundation. Assistance with sample transport and laboratory work was provided by Sarah Fortune, Christy Hudak, Charles "Stormy" Mayo, Dana Padgett, Josh Raia, Emily Sgarlat, and Karen Stamieszkin, and the E. E. family provided invaluable support in the field. The authors wish to thank Steve Cadrin, Jon Hare, and Sandy Shumway for their thoughtful reviews, and Geoff Cowles, Roger Hanlon, Pingguo He, and Mike Vecchione for stimulating discussion.
Squid egg samples were collected under Massachusetts Division of Marine Fisheries Scientific Collection Permits 136315 and 169784.
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OWEN C. NICHOLS, (1,2*) KATIE GROGLIO (3) AND ERNIE ELDREDGE (4)
(1) Center for Coastal Studies, 5 Holway Avenue, Provincetown, MA 02657; (2) School for Marine Science and Technology, University of Massachusetts-Dartmouth, 836 South Rodney French Boulevard, New Bedford, MA 02744; (3) Harvard University, Northwest Building, Room B239, 52 Oxford Street, Cambridge, MA 02138; (4) Chatham Fish Weirs Enterprises, 3 Champlain Road, Chatham, MA 02633
(*) Corresponding author. E-mail: firstname.lastname@example.org
TABLE 1. Frequency of egg mass presence and the number of collected egg masses that were sampled per year from 2008 to 2016. Year Egg mass presence (days) Samples 2008 26 N/A 2009 32 2 2010 22 1 2011 26 2 2012 31 2 2013 6 0 2014 25 1 2015 13 5 2016 20 2
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|Author:||Nichols, Owen C.; Groglio, Katie; Eldredge, Ernie|
|Publication:||Journal of Shellfish Research|
|Date:||Aug 1, 2019|
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